This article describes severe weather terminology used by the
National Weather Service
The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
(NWS) in the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
. The NWS, a
government agency
A government or state agency, sometimes an appointed commission, is a permanent or semi-permanent organization in the machinery of government that is responsible for the oversight and administration of specific functions, such as an administrati ...
operating as an arm of the
National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...
(NOAA) branch of the
United States Department of Commerce
The United States Department of Commerce is an executive department of the U.S. federal government concerned with creating the conditions for economic growth and opportunity. Among its tasks are gathering economic and demographic data for bu ...
(DoC), defines precise meanings for nearly all of its weather terms.
This article describes NWS terminology and related weather scales used by the agency. Some terms may be specific to certain cities or regions.
Specific Area Message Encoding
Specific Area Message Encoding (SAME) is a protocol used for framing and classification of broadcast emergency warning messages. It was developed by the United States National Weather Service for use on its NOAA Weather Radio (NWR) network, and w ...
(SAME) product codes assigned to each term for
NOAA Weather Radio
NOAA Weather Radio NWR; also known as NOAA Weather Radio All Hazards is an automated 24-hour network of VHF FM weather radio stations in the United States (U.S.) that broadcast weather information directly from a nearby National Weather Servi ...
(NWR) broadcasts are included in parentheses following the title of the described alert type if used; products that do not have a specified code are identified where applicable as Non-Precipitation Warnings/Watches/Advisories (NPW), Coastal Flood Warnings/Watches/Advisories (CFW), Marine Weather Statement (MWS), Surf Discussion (SRD) or Winter Weather Warnings/Watches/Advisories (WSW) as defined by NOAA.
Definitions of severe weather alerts
The NWS divides severe weather alerts into several types of hazardous/hydrologic events:
:# Severe local storms – Short-fused, small-scale hazardous weather or hydrologic events produced by
thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are someti ...
s (including large
hail, damaging
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
s,
tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
es, and
flash flood
A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing o ...
s).
:# Winter storms – Weather hazards associated with freezing or frozen precipitation (
freezing rain
Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The rain ...
,
sleet, and/or
snow
Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.
It consists of frozen crystalline water throughout ...
), or combined effects of winter precipitation and strong winds.
:# Fire weather – Weather conditions that contribute to an increased risk and help cause the spread of
wildfire
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...
s.
:# Flooding – Hazardous hydrological events resulting in temporary inundation of land areas not normally covered by water, often caused by excessive rainfall.
:# Coastal/lakeshore hazards – Hydrological hazards that may affect property, marine or leisure activities in areas near ocean and lake waters including high surf and coastal or lakeshore flooding, as well as rip currents.
:# Marine hazards – Hazardous events that may affect marine travel, fishing and shipping interests along large bodies of water, including hazardous seas and freezing spray.
:# Tropical cyclone hazards – Hazardous tropical cyclone events that may affect property in inland areas or marine activities in coastal waters, resulting in wind damage,
storm surge
A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low-pressure weather systems, such as cyclones. It is measured as the rise in water level above the n ...
, tornadoes and flooding rain.
:# Non-precipitation hazards – Weather hazards not directly associated with any of the above including extreme heat or cold, dense
fog, high winds, and river or lakeshore flooding.
Severe local storms
![NWS_Weather_Map_Example_3](https://upload.wikimedia.org/wikipedia/commons/2/28/NWS_Weather_Map_Example_3.jpg)
*
Tornado watch
A tornado watch ( SAME code: TOA) is a severe weather watch product issued by national weather forecasting agencies when meteorological conditions are favorable for the development of severe thunderstorms capable of producing tornadoes. In addit ...
(TOA; yellow box in some select NWS documentation, red box in most other media) – Conditions are favorable for the development of
severe thunderstorms
Severity or Severely may refer to:
* ''Severity'' (video game), a canceled video game
* "Severely" (song), by South Korean band F.T. Island
See also
*
*
{{disambig ...
and
tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
es in and close to the watch area. These watches are issued for large areas by the
Storm Prediction Center
The Storm Prediction Center (SPC) is a US government agency
A government or state agency, sometimes an appointed commission, is a permanent or semi-permanent organization in the machinery of government that is responsible for the oversight a ...
in
Norman, Oklahoma
Norman () is the third-largest city in the U.S. state of Oklahoma, with a population of 128,097 as of 2021. It is the largest city and the county seat of Cleveland County, Oklahoma, Cleveland County, and the second-largest city in the Oklahoma C ...
, and are usually valid for five to eight hours.
*:
Particularly dangerous situation tornado watch (TOA) – Conditions are favorable for the development of severe thunderstorms capable of producing destructive tornadoes in and close to the watch area. These watches are occasionally issued, and usually mean that a major
tornado outbreak
__NOTOC__
A tornado outbreak is the occurrence of multiple tornadoes spawned by the same synoptic scale weather system. The number of tornadoes required to qualify as an outbreak typically are at least six to ten, with at least two rotational l ...
is possible, where the potential for multiple violent (EF-4 and EF-5) tornadoes exists. Usually only reserved for forecast "high-end" severe weather events, this type of watch is usually valid for a longer period of time and issued for a larger area by the Storm Prediction Center in Norman, Oklahoma, than a typical tornado watch.
*
Tornado warning
A tornado warning ( SAME code: TOR) is a severe weather warning product issued by regional offices of weather forecasting agencies throughout the world to alert the public when a tornado has been reported or indicated by weather radar within the ...
(TOR) – Strong rotation in a thunderstorm has been indicated by
Doppler weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation (meteorology), precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern wea ...
or a tornado has been sighted by
Skywarn
Skywarn (sometimes stylized as SKYWARN) is a program of the National Weather Service (NWS). Its mission is to collect reports of localized severe weather in the United States. These reports are used to aid forecasters in issuing and verifying sev ...
spotters or other persons (such as local law enforcement). These warnings are currently issued on a
polygon
In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two toge ...
al basis, are usually issued for a duration of 30 minutes (but can run for as long as one hour), and can be issued without either a tornado watch or a severe thunderstorm watch being already in effect. Tornado warnings may be issued in combination with a Special Marine Warning if the storm affects nearshore or coastal waters.
*:Particularly dangerous situation tornado warning (TOR) – A large tornado has been confirmed to be producing damage and is moving into and through the warned area. It is usually issued as the initial tornado warning or as a complete re-issuance of the previous tornado warning. These may also include wording for a tornado emergency.
*:
Tornado emergency
A tornado emergency is an enhanced version of a tornado warning, which is used by the National Weather Service (NWS) in the United States during imminent, significant tornado occurrences in highly populated areas. Although it is not a new warning ...
(SVS) – Sent as a "severe weather statement" or a complete re-issuance of the tornado warning, this is an unofficial, high-end tornado warning issued when a violent tornado is expected to impact a heavily populated area. Such warnings have been issued for, among other significant tornado events, the
F5-rated tornado that destroyed much of
Bridge Creek,
Moore and portions of southeastern
Oklahoma City, Oklahoma
Oklahoma City (), officially the City of Oklahoma City, and often shortened to OKC, is the capital and largest city of the U.S. state of Oklahoma. The county seat of Oklahoma County, it ranks 20th among United States cities in population, and ...
on May 3, 1999 (during which the product was conceived by the
NWS Forecast Office in
Norman
Norman or Normans may refer to:
Ethnic and cultural identity
* The Normans, a people partly descended from Norse Vikings who settled in the territory of Normandy in France in the 10th and 11th centuries
** People or things connected with the Norm ...
); the
EF5 tornado
The Enhanced Fujita scale (abbreviated as EF-Scale) rates tornado intensity based on the severity of the damage they cause. It is used in some countries, including the United States, Canada, China, and Mongolia.
The Enhanced Fujita scale repl ...
that destroyed much of
Greensburg, Kansas
Greensburg is a city in, and the county seat of, Kiowa County, Kansas, United States. As of the 2020 United States census, the population of the city was 740. It is home to the world's largest hand-dug well. On the evening of May 4, 2007, Greens ...
on May 4, 2007; the
EF4 tornado
The Enhanced Fujita scale (abbreviated as EF-Scale) rates tornado intensity based on the severity of the damage they cause. It is used in some countries, including the United States, Canada, China, and Mongolia.
The Enhanced Fujita scale repla ...
that hit
Tuscaloosa, Alabama
Tuscaloosa ( ) is a city in and the seat of Tuscaloosa County in west-central Alabama, United States, on the Black Warrior River where the Gulf Coastal and Piedmont plains meet. Alabama's fifth-largest city, it had an estimated population o ...
on April 27, 2011; the
EF5 tornado
The Enhanced Fujita scale (abbreviated as EF-Scale) rates tornado intensity based on the severity of the damage they cause. It is used in some countries, including the United States, Canada, China, and Mongolia.
The Enhanced Fujita scale repl ...
that affected Moore and adjacent southern portions of the Oklahoma City area on May 20, 2013; the
May 31, 2013 tornado system that went over the Oklahoma City area through an extremely densely populated area of
El Reno
El Reno is a city in and county seat of Canadian County, Oklahoma, United States. As of the 2020 census, the city population was 16,989, marking a change of 1.55% from 16,729, recorded in the 2010 census. The city was begun shortly after the 18 ...
in
Canadian County
Canadian County is a county located in the U.S. state of Oklahoma. As of the 2020 census, the population was 154,405, making it the fifth most populous county in Oklahoma. Its county seat is El Reno.
The county is named for the Canadian Ri ...
; the EF4 tornado that hit
Beauregard, Alabama
Beauregard is an unincorporated area, unincorporated community located in central Lee County, Alabama, Lee County, Alabama, United States. It is located east of Auburn, Alabama, Auburn and south of Opelika, Alabama, Opelika.
History
Beauregard ...
on
March 3, 2019, the EF4 tornado that affected western portions of the
Kansas City area on
May 28, 2019 and the EF4 tornado that affected several areas in
Mississippi
Mississippi () is a state in the Southeastern region of the United States, bordered to the north by Tennessee; to the east by Alabama; to the south by the Gulf of Mexico; to the southwest by Louisiana; and to the northwest by Arkansas. Miss ...
on
April 12, 2020. This enhanced form of a tornado warning is issued mainly by Weather Forecast Offices within the National Weather Service's Central and Southern Region Headquarters; a tornado emergency is the highest level of a three-tiered Impact Based Warning system for tornadoes used by all WFOs within the Central Region Headquarters, and eight others within the Eastern, Southern and Western Regions.
*Severe thunderstorm watch
A severe thunderstorm watch ( SAME code: SVA) is a severe weather watch product issued by regional offices of weather forecasting agencies throughout the world when meteorological conditions are favorable for the development of severe thunders ...
(SVA; pink box or blue box in NWS documents, typically yellow box in other media) – Conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm contains large damaging hail of diameter or larger, and/or damaging winds greater than 58 mph (95 km/h or 50 knot
A knot is an intentional complication in cordage which may be practical or decorative, or both. Practical knots are classified by function, including hitches, bends, loop knots, and splices: a ''hitch'' fastens a rope to another object; a ' ...
s) or greater. Isolated tornadoes are also possible (depending on whether atmospheric conditions are sufficient to produce tornadic activity) but not expected to be the dominant severe weather event. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for five to eight hours.
*:Particularly dangerous situation severe thunderstorm watch (SVA) – Conditions are favorable for the development of severe thunderstorms in and close to the watch area. Isolated tornadoes are possible but not expected to be the dominant severe weather event, hence these watches are very rarely issued. An expected severe wind event (derecho
A ''derecho'' (, from es, derecho, link=no , 'straight') is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of severe thunderstorms known as a mesoscale convective system.
Derechos can cause hurri ...
) is the mostly likely reason for a PDS severe thunderstorm watch to be issued, with widespread winds greater than 90 mph (150 km/h or 80 knots) possible. These watches are usually valid for a longer period of time and are issued for a larger area by the Storm Prediction Center in Norman, Oklahoma than a typical severe thunderstorm watch. This type of watch is usually only reserved for forecast "high-end" severe weather events. If, however, tornadoes are expected to be a major weather threat in addition to the preceding criteria, then a standard tornado watch (generally not a PDS watch) would be issued instead.
*Severe thunderstorm warning
A severe thunderstorm warning ( SAME code: SVR) is a severe weather warning product issued by regional offices of weather forecasting agencies throughout the world to alert the public that severe thunderstorms are imminent or occurring.
A sev ...
(SVR) – A severe thunderstorm is indicated by Doppler weather radar or sighted by Skywarn spotters or other persons, such as local law enforcement. A severe thunderstorm contains large damaging hail of in diameter or larger, and/or damaging winds of or greater. These warnings are currently issued on a polygonal basis, are usually issued for a duration of 30 minutes to one hour, and can be issued without a severe thunderstorm watch or a tornado watch being already in effect. Because severe thunderstorms can produce tornadoes with little or no advance warning, a severe thunderstorm warning may be upgraded to a tornado warning if strong rotation is indicated or a tornado is sighted. Lightning
Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
frequency is not a criterion for issuing a severe thunderstorm warning. Severe thunderstorm warnings may be issued in combination with a special marine warning if the storm affects nearshore or coastal waters.
*:Particularly dangerous situation severe thunderstorm warning (SVR) – Issued within the initial severe thunderstorm warning or as a complete re-issuance of the previously issued warning, a PDS severe thunderstorm warning indicates a significant severe thunderstorm has been detected by Doppler weather radar and/or observed by Skywarn spotters or other civil defense personnel, and is moving into and through the warned area with the capability of causing significant property damage, and severe injury or fatality to people or animals caught in the storm's path. These may be issued for an ongoing severe wind event (either by a derecho, or from an intense wind core or downburst within a squall line or supercell) containing winds greater than 80 mph (124 km/h or 70 knot
A knot is an intentional complication in cordage which may be practical or decorative, or both. Practical knots are classified by function, including hitches, bends, loop knots, and splices: a ''hitch'' fastens a rope to another object; a ' ...
s), and/or extreme hail of diameter or larger over a wide area. Specific to this product, the boilerplate PDS wording ("this is a particularly dangerous situation") placed between the storm summary and hazard information in the warning text was replaced in July 2021 with the notation "this is a destructive storm for .." highlighting specific locations under greatest threat from extreme winds or hail accompanying the warned storm.
*:Severe thunderstorm emergency (SVS) – Sent as a "severe weather statement" or a complete re-issuance of the severe thunderstorm warning, this is an unofficial, high-end severe thunderstorm warning that is occasionally issued when a significant severe thunderstorm is imminent or impacting a highly populated area. First issued by the National Weather Service office in Cheyenne, Wyoming
Cheyenne ( or ) is the capital and most populous city of the U.S. state of Wyoming, as well as the county seat of Laramie County, with 65,132 residents, per the 2020 US Census. It is the principal city of the Cheyenne metropolitan statistical ...
on August 16, 2019, the National Weather Service does not currently maintain a concrete criterion for issuance, although Weather Forecast Offices that issue or have issued such warnings may use the same criteria meriting a PDS Severe Thunderstorm Warning to indicate life-threatening severe weather conditions that will impact a densely populated area.
* Severe weather statement (SVS) – A statement issued to provide updated information for active severe thunderstorm and tornado warnings to the public and emergency managers, with revised information including reports of observed severe weather and the approximate location of the storm cell (as indicated by the most recently updated NEXRAD
NEXRAD or Nexrad (Next-Generation Radar) is a network of 160 high-resolution S-band Doppler weather radars operated by the National Weather Service (NWS), an agency of the National Oceanic and Atmospheric Administration (NOAA) within the United S ...
data) at the time of the statement's issuance. Severe Weather Statements may include notices of cancellation (if the warning is being discontinued entirely, or if sections of counties or county-equivalent jurisdictions are being removed from the continuing warning), or notices of a warning being allowed to expire because the prompting storm has weakened below severe criteria.
*Flash flood watch
A flash flood watch ( SAME code: FFA; also referred as a "green box" by meteorologists) is severe weather watch product of the National Weather Service that is issued when conditions are favorable for flash flooding in flood-prone areas, usually ...
(FFA; green box) – Conditions are favorable for flash flood
A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing o ...
ing or urban flooding
Urban flooding is the inundation of land or property in a built environment, particularly in more densely populated areas, caused by rainfall overwhelming the capacity of drainage systems, such as storm sewers. Although sometimes triggered by event ...
in and close to the watch area within the next 36 hours. These watches are issued by the Weather Forecast Office and are usually issued six to 24 hours in advance of expected flood potential. (The Heavy Rainfall Warning product issued by the Meteorological Service of Canada
The Meteorological Service of Canada (MSC; french: Service météorologique du Canada – SMC) is a division of Environment and Climate Change Canada, which primarily provides public meteorological information and weather forecasts and warnings o ...
has a similar meaning as an NWS Flash Flood Watch.)
*:Particularly dangerous situation flash flood watch (FFA) – Conditions are favorable for an extremely elevated level of severe and life-threatening flash flooding beyond the level of a normal flash flood watch in and close to the watch area. These watches are usually issued for a smaller area by the local WFOs than typical flash flood watches, which often span multiple county warning areas, and are usually valid for a longer period of time. This type of watch is usually only reserved for forecast "high-end" flash flood events.
*Flash flood warning
A flash flood warning ( SAME code: FFW) is an hazardous weather statement issued by national weather forecasting agencies throughout the world to alert the public that a flash flood is imminent or occurring in the warned area. A flash flood is a ...
(FFW) – Flash flooding—either indicated by Doppler weather radar or stream gauges, or reported by Skywarn spotters or local emergency officials—is occurring, imminent, or highly likely in the short term. A flash flood is a flood that occurs within six hours of excessive rainfall and poses a threat to life and/or property, especially in low-lying or flood prone areas (including urban areas with poor drainage, rivers and streams); ice jams and dam failures can also cause flash floods. These warnings are issued on a county by county (or equivalent thereof) basis by the local Weather Forecast Office and are generally in effect for two to six hours, although particularly during tropical cyclones a warning may last for a longer period of time, and occasionally last shorter than two hours.
*:Particularly dangerous situation flash flood warning (FFW) – Issued within the initial flash flood warning or as a complete re-issuance of the previously issued warning, this indicates widespread, life-threatening flash flooding—caused by excessive rainfall, ice jams or imminent dam failures—has been indicated by Doppler weather radar, emergency personnel or trained observers over a broadly populated area.
*:Flash flood emergency (FFS) – A flash flood emergency is a high-end usage of the flash flood warning. The flash flood emergency term is used when widespread flooding is occurring, and either, multiple water rescues have been reported in the past few hours, or if highly populated regions are undergoing significant flash flooding likely to cause loss of life and property. This terminology is more widely used during hurricanes with high amounts of moisture (an example is Hurricane Harvey
Hurricane Harvey was a devastating Category 4 hurricane that made landfall on Texas and Louisiana in August 2017, causing catastrophic flooding and more than 100 deaths. It is tied with 2005's Hurricane Katrina as the costliest t ...
in 2017, during which multiple flash flood emergencies were issued for portions of southeast Texas and southern Louisiana, including the Houston
Houston (; ) is the most populous city in Texas, the most populous city in the Southern United States, the fourth-most populous city in the United States, and the sixth-most populous city in North America, with a population of 2,304,580 in ...
area).
* Flash flood statement (FFS) – A hydrological weather statement issued to provide updated information on active flash flood watches and warnings to the public and emergency managers.
Deprecated
*Significant weather advisory
A significant weather advisory was a hazardous weather statement issued by certain Weather Forecast Offices (WFO) of the National Weather Service (NWS) in the United States to alert the public of thunderstorm activity that is below designated sever ...
(SPS; alt.: significant weather alert) – A strong thunderstorm below severe criteria, containing small hail below diameter, and/or strong winds of , is indicated by Doppler weather radar and may create some adverse impacts on travel. These advisories are issued as special weather statements written in the style of severe thunderstorm and other short-fused warnings, usually on a county by county (or equivalent
Equivalence or Equivalent may refer to:
Arts and entertainment
*Album-equivalent unit, a measurement unit in the music industry
* Equivalence class (music)
*'' Equivalent VIII'', or ''The Bricks'', a minimalist sculpture by Carl Andre
*''Equiva ...
thereof) basis. Some areas use an entirely different format (most notably WFOs in the Northeast
The points of the compass are a set of horizontal, radially arrayed compass directions (or azimuths) used in navigation and cartography. A compass rose is primarily composed of four cardinal directions—north, east, south, and west—each se ...
and Mid-Atlantic), denoting which locations in each county will be affected by the thunderstorm. The NWS ceased use of the "significant weather advisory" titling in July 2021; special weather statements for non-severe thunderstorms concurrently adopted product language following the Impact Based Warning format used for severe convective storms.
Winter precipitation
*Blizzard warning
A blizzard warning ( SAME code: BZW) is a hazardous weather statement issued by Weather Forecast Offices (WFO) of the National Weather Service (NWS) in the United States, which indicates heavy snowfall accompanied by sustained winds or frequent gu ...
(BZW) – Sustained winds or frequent gusts of or greater, accompanying heavy snowfall, frequently reducing visibilities to or less is forecast to occur over a period of at least three hours. There are no temperature criteria in the definition of a blizzard, but freezing temperatures of at least and winds will create wind chills of at least . A winter storm watch for blizzard conditions is issued when conditions meeting criteria are forecast to occur within 12 to 48 hours.
*Winter storm warning
A winter storm warning ( SAME code: WSW) is a hazardous weather statement issued by Weather Forecast Offices (WFO) of the National Weather Service (NWS) in the United States to alert the public that a winter storm is occurring or is about to occur ...
(WSW) – Hazardous winter weather
A winter storm is an event in which wind coincides with varieties of precipitation that only occur at freezing temperatures, such as snow, mixed snow and rain, or freezing rain. In temperate continental climates, these storms are not necessar ...
conditions that pose a threat to life and/or property are occurring, imminent, or highly likely within 12 to 48 hours. The generic term, Winter Storm Warning, is used for a combination of two or more of the following winter weather events: heavy snow, freezing rain, sleet, and strong winds. The forecast accumulation criteria for each frozen precipitation type vary significantly over different county warning areas. The National Weather Service has deprecated the Heavy Snow Warning and Sleet Warning products in favor of issuing a Winter Storm Warning for Heavy Snow or a Winter Storm Warning for Heavy Sleet, respectively.
*Ice storm warning
An ice storm warning is a hazardous weather statement issued by the Weather Forecast Offices of the National Weather Service in
the United States which indicates that freezing rain produces a significant and possibly damaging accumulation of ice ...
(WSW) – Heavy ice accumulations that may cause significant disruptions to travel and public utilities, and damage to trees and utility infrastructure impacting life and property are imminent. The criteria for amounts vary over different county warning areas; accumulations range from or more of freezing rain on elevated horizontal flat surfaces. (Hazardous icing conditions are covered and similar warning criteria are used by the Meteorological Service of Canada for their Freezing Rain Warning product.)
* Winter storm watch (WSA) – Hazardous winter weather conditions including significant accumulations of snow and/or freezing rain and/or sleet are possible generally within 24 to 48 hours. The generic term, Winter Storm Watch, is used for a combination of two or more of the following winter weather events: heavy snow, freezing rain, sleet, and strong winds. The forecast accumulation criteria for each frozen precipitation type vary significantly over different county warning areas. These watches are issued by local National Weather Service Forecast Offices.
*Winter weather advisory
A winter weather advisory (originally identified as a Traveler's Advisory until the 2002-03 climatological winter when officially renamed, and informally as such by some local television stations thereafter) is a hazardous weather statement issued ...
(WSW) – Hazardous winter weather conditions are occurring, imminent, or likely. Conditions will cause a significant inconvenience and if caution is not exercised, may result in a potential threat to life and/or property. The generic term, Winter Weather Advisory (which replaced the previously used "Travelers Advisory"), is used for a combination of two or more of the following events; snow, freezing rain or freezing drizzle, sleet, and blowing snow. The forecast accumulation criteria—defined below locally designated warning criteria or based on NWS impact-driven advisory criteria—for at least one of the frozen precipitation elements within a defined 12 or 24 hour period vary significantly over different county warning areas. The National Weather Service has deprecated the snow advisory and blowing snow advisory in favor of issuing a winter weather advisory for snow or a winter weather advisory for snow and blowing snow, respectively.
*Lake effect snow warning
A lake effect snow warning is a bulletin issued by the National Weather Service in the United States to warn of heavy snowfall accumulations that are imminent from convective snow generated by very cold airmass passing over unfrozen lakes (lake ef ...
(WSW) – Very heavy lake-effect snowfall amounts of generally in 12 hours or less or in 24 hours or less are imminent or highly likely. Lake-effect snow squalls can significantly reduce visibilities with little notice. Although discontinued on October 15, 2018 and replaced with a winter storm warning for heavy lake-effect snow, it was reinstated beginning with the 2019-2020 winter storm season.
*Snow squall warning
A snow squall warning (Specific Area Message Encoding, SAME code: SQW) is a bulletin issued by Environment Canada's Meteorological Service of Canada and the National Weather Service in the United States to warn population of two types of snow eve ...
(SQW) – An intense, generally limited duration, period of moderate to heavy snowfall has been observed by radar. Usually accompanied by strong, gusty surface winds, significantly reduced visibility up to or less (which may reach levels creating whiteout conditions), and possibly lightning; temperature drops behind an arctic front that are sufficient to produce flash freezes, in conjunction with a significant reduction in visibility from falling and/or blowing snow, may also serve as a warning criteria factor. Snow accumulation may be significant.
Deprecated
:*Heavy snow warning
A heavy snow warning was a hazardous weather statement issued by the Weather Forecast Offices of the National Weather Service (NWS) in the United States which indicates a high rate of snowfall was occurring or was forecast to occur. Generally, th ...
(WSW) – Heavy snowfall amounts are imminent; the criteria for amounts (based on 12-hour and 24-hour minimum accumulations) vary significantly over different county warning areas. Discontinued beginning with the 2008-2009 winter storm season and replaced with the winter storm warning for heavy snow.
:*Sleet warning (WSW; alt.: heavy sleet warning) – Heavy sleet accumulations of or more, which may cause significant disruptions to travel or utilities, are imminent or expected to occur within in 12 hours. Discontinued beginning with the 2008-2009 winter storm season and replaced by the winter storm warning for heavy sleet.
:*Sleet advisory (WSW) – Moderate sleet accumulations of are imminent or expected to occur within 12 hours. Because sleet usually occurs with other precipitation types, a Winter Weather Advisory will almost always be used in such cases. Discontinued beginning with the 2008-2009 winter storm season and replaced with the winter weather advisory for sleet.
:* Snow advisory (WSW) – Moderate snowfall amounts are imminent; the criteria for amounts vary significantly over different county warning areas. Under the former definition, a snow advisory could be warranted if lesser snowfall accumulations were forecast to produce travel difficulties, especially early in the winter season. Discontinued beginning with the 2008-2009 winter storm season and replaced by the winter weather advisory for snow.
:* Blowing snow advisory (WSW) – Sustained winds or frequent gusts of accompanied by falling and blowing snow, occasionally reducing visibilities to or less, will occur for at least three hours. Discontinued beginning with the 2008-2009 winter storm season and replaced by the Winter Weather Advisory for Blowing Snow.
:*Snow and blowing snow advisory (WSW) – Sustained winds of are expected to be accompanied by falling and blowing snow, occasionally reducing visibilities to or less for at least three hours. Discontinued beginning with the 2008-2009 winter storm season and replaced by the winter weather advisory for snow and blowing snow.
:*Extreme cold watch (WSW) – Dangerously low temperatures are possible for a prolonged period of time. Frostbite
Frostbite is a skin injury that occurs when exposed to extreme low temperatures, causing the freezing of the skin or other tissues, commonly affecting the fingers, toes, nose, ears, cheeks and chin areas. Most often, frostbite occurs in the ha ...
and hypothermia
Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe ...
are likely if exposed to these temperatures.
:* Extreme cold warning (WSW) – Dangerously low temperatures are expected for a prolonged period of time. Frostbite and hypothermia are likely if exposed to these temperatures. Initially used as an experiment in 2011 and discontinued beginning with the 2011-2012 winter storm season, it was reinstated in 2018 as a merger of the extreme cold warning and wind chill warning.
:* Lake effect snow watch (WSW) – Significant amounts of lake-effect snow (generally within 12 hours or within 24 hours) are possible in the next 12 to 48 hours. Discontinued on October 2, 2017; a winter storm watch is now issued instead.
:* Lake effect snow advisory (WSW) – Moderate amounts of lake-effect snow (generally ) are expected or occurring. Discontinued on October 2, 2017; a winter weather advisory for lake-effect snow is now issued instead.
:*Freezing rain advisory A freezing rain advisory was an advisory issued by the National Weather Service in the United States when freezing rain or freezing drizzle was expected to cause significant inconveniences, but did not meet warning criteria (typically greater than ...
(WSW; alt.: freezing drizzle advisory) – Freezing rain or freezing drizzle producing ice accretion of up to that may cause significant travel impairments is expected or occurring. Discontinued on October 2, 2017; a winter weather advisory for freezing rain is now issued instead.
:* Blizzard watch (WSW) – Sustained winds or frequent gusts of or greater, accompanying considerable falling and/or blowing snow, frequently reducing visibilities to or less for a period of three hours or more are possible generally within 12 to 48 hours. The NWS deprecated issuance of blizzard watches in October 2017; a winter storm watch is now issued in its place.
Fire weather
*Fire warning
A fire warning ( SAME code: FRW) is a warning issued through the Emergency Alert System (EAS) in the United States by civil authorities to inform the public of major, uncontrolled fires (usually wildfires) threatening populated areas and/or major ...
(FRW) – A major, uncontrolled wildfire or structural fire is currently burning in a populated area and/or near major roadways, and evacuation is recommended by local civil authorities.
*Red flag warning
A red flag warning is a forecast warning issued by the National Weather Service in the United States to inform the public, firefighters, and land management agencies that conditions are ideal for wildland fire combustion, and rapid spread. After d ...
(FWA) – Extreme burning conditions favor the combustion and rapid spread of wildfires within 12 to 24 hours in areas that have not received precipitation for a short period or are experiencing drought conditions, and which the National Fire Danger Rating System National Fire Danger Rating System (NFDRS) is used in the United States to provide a measure of the relative seriousness of burning conditions and threat of wildfires.
Background
John J. Keetch, a fire researcher in the southeast, wrote that "One ...
(NFDRS) designates as having a high to extreme fire danger. It is issued primarily to alert fire department officials and firefighters of conditions ideal for the combustion and spread of wildfires. The surface relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
(RH) and 10-hour fuel moisture criteria meriting a Fire Weather Watch may vary by state based on local vegetation, topography and distance from major water sources, though forecast sustained winds are usually expected to be or greater.
*:Extreme red flag warning (FWA) – Conditions for wildland fire combustion, rapid growth and behavior are extremely dangerous, due to very dry fuels, very low humidity levels, and strong winds. Introduced in October 2019, it is analogous to particularly dangerous situation wording for severe weather watches and warnings and superseded such wording occasionally applied to red flag warnings previously.
*Fire weather watch (FWA) – Conditions expected to become favorable for the rapid spread of wildfires in areas of dry vegetation within 12 to 48 hours or up to 72 hours if the NWS office is reasonably confident that red flag fire conditions will occur. It is issued primarily to alert fire and land management agencies of conditions ideal for the combustion and spread of wildfires. The surface relative humidity and 10-hour fuel moisture criteria meriting a fire weather watch may vary depending by state based on local vegetation, topography and distance from major water sources, though forecast sustained winds are usually expected to be or greater.
Flooding
* Areal flood warning (FLW) – General or areal flooding of streets, low-lying areas, urban storm drain
A storm drain, storm sewer (United Kingdom, United States, U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain (Australia and New Zealand) is infrastructure designed to Drainage, drain excess rain and ground water ...
s, creeks, and small streams in which there is a serious threat to life or property is occurring, imminent, or highly likely. Flood warnings are usually issued for flooding that occurs more than six hours after the excessive rainfall, or when flooding is imminent/occurring but is not rapid enough to prompt a flash flood warning. These warnings are issued on a polygonal basis by the local Weather Forecast Office and are generally in effect for six to twelve hours.
*Areal flood watch (FLA) – Hydrometeorological conditions favor a threat of areal flooding affecting area streets, rivers, streams, and/or urban storm drains within six to 24 hours. It is intended to inform the public and cooperating agencies (including ambulance services and fire departments that may assist in water rescues) in advance of expected flood potential, even though the occurrence of flooding is neither certain nor imminent.
*Areal flood advisory (FLS) – Minor general or areal flooding of streets, low-lying areas, urban storm drains, creeks, and small streams is occurring, imminent, or highly likely within the next one to three hours, but is not expected to substantially threaten life and property. These advisories are issued on a polygonal basis by the local Weather Forecast Office and are generally in effect for three to six hours.
* River flood warning (FLS; formerly flood warning for river forecast point) – Flooding of stream
A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a stream ...
s or river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
s is occurring, imminent, or highly likely. These warnings are issued either by the River Forecast Centers for the area surrounding affected streams or rivers or on a county by county basis by the local Weather Forecast Office and are generally in effect for an indeterminate period until river gauges in the affected areas record water levels receding below the water body's predetermined flood stage.
* River flood advisory (FLS) – Minor flooding of streams or rivers reaching action stage is occurring, imminent, or highly likely. These advisories are issued on a county by county basis by the local Weather Forecast Office and are generally in effect for a couple of days or longer.
*Urban and small stream flood advisory
An urban and small stream flood advisory is issued when ponding of water of streets, low-lying areas, highways, underpasses, urban storm drains, and elevation of creek and small stream levels is occurring or imminent. Urban and small stream flood a ...
– Ponding Ponding is the (typically) unwanted pooling of water, typically on a flat roof or roadway. Ponding water accelerates the deterioration of many materials, including seam adhesives in single-ply roof systems, steel equipment supports, and particularly ...
of water on streets, low-lying areas, highway
A highway is any public or private road or other public way on land. It is used for major roads, but also includes other public roads and public tracks. In some areas of the United States, it is used as an equivalent term to controlled-access ...
s, underpass
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube constr ...
es, urban storm drains, and elevation of creek and small stream levels is occurring or imminent. Urban and small stream flood advisories are issued for flooding that occurs within three hours after the excessive rainfall. These advisories are issued on a polygonal basis by the local Weather Forecast Office and are generally in effect for three to four hours. (The watch/warning/advisory display map on the NWS's national and local Forecast Office websites does not display the advisory as an area-specific polygon, instead indicating the affected counties and/or county-equivalent subdivisions.)
* Flood statement (FLS) – A hydrological weather statement issued to provide updated information on ongoing flooding along major streams in which there is not a serious threat to life and/or property active or flood warnings to the public and emergency managers; information will be provided on observed (or if a dam is at risk of breaking, updates on the ongoing threat) flooding within the warning area, and current and forecast flood stages for lakes and rivers at the time of the statement's issuance.
Coastal/lakeshore hazards
*Coastal flood warning
A coastal flood warning is an hazardous weather statement issued by the Weather Forecast Offices of the National Weather Service in the United States which indicates coastal flooding is either imminent or occurring along the coast of the Atlantic ...
(CFW) – Coastal flooding
Coastal flooding normally occurs when dry and low-lying land is submerged by seawater. The range of a coastal flooding is a result of the elevation of floodwater that penetrates the inland which is controlled by the topography of the coastal land ...
from ocean water being forced inland from the nearby body (caused by either nor'easter
A nor'easter (also northeaster; see below), or an East Coast low is a synoptic-scale extratropical cyclone in the western North Atlantic Ocean. The name derives from the direction of the winds that blow from the northeast. The original use ...
s, tropical cyclones or thunderstorms) is occurring, imminent or highly likely within 12 to 24 hours, and poses a serious threat to life and/or property. WFOs may occasionally issue warnings valid after the second forecast period of the date of validity if a strong likelihood of the event exists or when a longer advance notice is needed for public response.
*Coastal flood watch
A coastal flood watch is issued by the National Weather Service of the United States when coastal flooding along the coast of the Atlantic Ocean, Pacific Ocean, or the Gulf of Mexico is possible. The flooding must be due to water being forced from ...
(CFA) – Coastal flooding from ocean water being forced inland from the nearby body is possible within 12 to 48 hours, creating significant impacts to life and/or property.
* Coastal flood advisory (CFW) – Minor coastal flooding or tidal overflow is occurring or is imminent within 12 hours. Flooding will not pose a serious threat to lives or property, but will create a nuisance for pedestrian and vehicle travel in the affected area.
*Storm surge warning (SSW) – Localized heavy flooding associated with a tropical cyclone from storm surge waves moving inland from the shoreline is occurring or is imminent in the specified area within the next 36 hours, posing a significant threat to life and/or property. Similar criteria included in a Storm Surge Watch for conditions that would limit lead time for undertaking safety precautions and inclusion of areas that may experience isolation from surge inundation also apply to this product. Issued in collaboration between the NHC and local WFOs, Storm Surge Warnings are only issued for regions in the Atlantic and Gulf Coasts of the conterminous United States.
*Storm surge watch (SSA) – Life-threatening inundation from rising water being forced inland by an ongoing or potential tropical, subtropical or post-tropical cyclone is possible within the specified area, generally within 48 hours. A watch may be issued in advance of the cyclone's landfall if other hazardous tropical conditions (such as the onset of tropical storm-force winds) are expected to limit enactment of evacuations and other safety precautions, and may also be issued for adjacent locations that could potentially be isolated by surge inundation. Issued in collaboration between the National Hurricane Center
The National Hurricane Center (NHC) is the division of the United States' NOAA/National Weather Service responsible for tracking and predicting tropical weather systems between the Prime Meridian and the 140th meridian west poleward to the 3 ...
(NHC) and local Weather Forecast Offices, storm surge watches are only issued for regions in the Atlantic and Gulf Coasts of the conterminous United States.
*Lakeshore flood warning (CFW) – Lakeshore flooding that poses a serious threat to life and/or property is occurring or is imminent in the next 12 to 24 hours.
*:Seiche
A seiche ( ) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors, caves and seas. The key requirement for formation of ...
warning (CFW) – Rapid, large fluctuations in water level in the Great Lakes
The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...
(similar to the sloshing in a bath tub) caused by storms or high winds, resulting in both lakeshore flooding and critically low water levels at different times. Issued as a lakeshore flood warning with reference of being a seiche warning.
*Lakeshore flood watch (CFW) – Lakeshore flooding that could pose a serious threat to life and/or property is possible within the next 12 to 48 hours.
*Lakeshore flood advisory (CFW) – Minor lakeshore flooding that may pose a threat to life and/or property is occurring or is imminent in the next 12 hours.
*Lake wind advisory A lake wind advisory is issued by the United States National Weather Service local forecast offices when windy conditions on area lakes are expected to be hazardous for boaters and other recreational events on or around lakes.
Example
URGENT - WEA ...
(NPW) – Sustained wind speeds of are forecast to persist for one hour or longer on area lakes that may cause hazards for maritime travel; wind speeds meeting advisory criteria may vary depending on the county warning area. The usage of this product is locally determined by each Weather Forecast Office.
*High surf warning (SRD) – Destructive, pounding surf poses an especially heightened danger to life, boating and other marine property in and near the surf zone, and may damage property near the shoreline; high surf criteria may vary depending on the region.
*High surf advisory (SRD) – Pounding surf poses a danger to life, boating and other marine property within the surf zone; high surf criteria may vary depending on the region.
*High surf watch (SRD) – A high surf event due to significant breaking wave action is possible in the next few days.
*Rip current statement (SRD) – Describes a risk of rip currents present in the specified area; may be issued as a Beach Hazards Statement.
*Beach hazards statement (SRD) – Issued for rip or longshore currents, or other hazards (including chemical or biological hazards) may create life-threatening conditions in lake or ocean waters. Beachgoers will be advised to exercise caution when in or near the water.
Marine hazards
* Heavy freezing spray warning (MWS) – Usually issued for shipping interests when conditions are favorable for the rapid freezing of sea spray on vessels at a rate of more than per hour, caused by some appropriate combination of cold water, wind, air temperature and vessel movement.
*Heavy freezing spray watch (MWS) – Usually issued for shipping interests when conditions are favorable for a heavy freezing spray event meeting Heavy Freezing Spray Warning criteria but its occurrence, location, and/or timing is still uncertain.
* Freezing spray advisory (MWS) – Usually issued for shipping interests when conditions are probability of accumulating frozen sea spray on vessels of less than 2 cm per hour caused by some combination of cold water, wind, air temperature, and vessel movement.
* Hazardous seas warning (MWS) – Significant wave heights and/or wave steepness values—independent of strong winds—are forecast to cause rough surf in the warned coastal area and adjacent waters, which could pose a serious threat to vessels that do not move to stable waters or dock. Warning criteria may vary depending on the NWS Weather Forecast Office.
* Hazardous seas watch (MWS) – A rough surf event—regardless of forecast wind speeds that may contribute to such an occurrence—is possible in the warned coastal area and adjacent waters, but its occurrence, location, and/or timing remains uncertain. Warning criteria may vary depending on the NWS Weather Forecast Office.
*Low water advisory (MWS) – Critically below average water levels over the Great Lakes, coastal marine zones or other tidal marine area, waterway, or river inlet within or adjacent to a marine zone have been observed, and potentially present a hazard to maritime navigation.
*Marine weather statement (MWS) – The equivalent of a special weather statement at sea, indicating potentially hazardous marine conditions.
*Special marine warning A special marine warning ( SAME code SMW) is a warning issued by the U.S. National Weather Service
The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warning ...
(SMW) – A warning to mariners of short-duration hazardous weather conditions (lasting up to two hours) including thunderstorms or squall
A squall is a sudden, sharp increase in wind speed lasting minutes, as opposed to a wind gust, which lasts for only seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the ...
s with wind gusts of or more, hail diameter or larger, or waterspout
A waterspout is an intense columnar vortex (usually appearing as a funnel cloud, funnel-shaped cloud) that occurs over a body of water. Some are connected to a cumulus congestus cloud, some to a cumuliform cloud and some to a cumulonimbus clou ...
s affecting coastal areas not adequately covered by existing marine warnings. Short-duration mesoscale events (such as a strong cold front, gravity wave or squall line
A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front (which often are accom ...
) expected to last two hours or less and produce criteria wind speeds, or volcanic ashfall may also warrant issuance.
*:Particularly dangerous situation special marine warning (SMW) – A warning to mariners of hazardous weather conditions that present a considerable threat to life and property.
*Small craft advisory
A small craft advisory is a type of wind warning issued by the National Weather Service in the United States. In Canada a similar warning is issued by Environment Canada. It is issued when winds have reached, or are expected to reach within 12 ho ...
(SCA) – A warning to mariners of high sustained winds or frequent gusts and/or significant sea or wave heights that may damage or capsize small boats. Only issued by WFOs in coastal regions and the Great Lakes for areas included in the Coastal Waters Forecast or Nearshore Marine Forecast products. Thresholds necessitating the issuance of small craft advisories are determined by the accordant NWS Regional Headquarters.
*Small craft advisory for hazardous seas (SCAH) – An advisory to mariners of wave or sea conditions that are potentially hazardous to small boats because of wave height, wave period, steepness, or swell direction, even if wind speeds are expected to fall below locally defined small craft advisory criteria. Only issued by WFOs in coastal regions and the Great Lakes for areas included in the Coastal Waters Forecast or Nearshore Marine Forecast products. Thresholds necessitating the issuance of this type of small craft advisory vary among geographic areas within each NWS Regional Headquarters.
*:Small craft advisory for rough bar (SCARB) – Waves in or near harbor or river entrances are expected to be especially hazardous to mariners due to the interaction of swell, tidal and/or river currents in relatively shallow water. Thresholds governing the issuance of this type of small craft advisory vary among local geographic areas, and are based upon locally determined parameters such as wave steepness, wind speed and direction, and local bathymetry
Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...
.
*:Small craft advisory for winds (SCAW) – Wind speeds may pose potential hazards for maritime activity. Wave heights must remain below standard small craft advisory criteria to merit the issuance of a wind-based advisory. Thresholds necessitating the issuance of wind-based small craft advisories are determined by the accordant NWS Regional Headquarters.
*:Brisk wind advisory (NPW) – Sea or lake ice is expected and may be hazardous to small marine craft. May be issued as a Small Craft Advisory with reference of being a brisk wind advisory. This advisory is not issued by WFOs serving the Great Lakes region due to the prevalence of ice floes during winter in the lake bodies.
Temperature
''See also Windchill
Wind chill or windchill (popularly wind chill factor) is the lowering of body temperature due to the passing-flow of lower-temperature air.
Wind chill numbers are always lower than the air temperature for values where the formula is valid. When ...
section below.''
*Excessive heat warning
An excessive heat warning is a notice issued by the National Weather Service of the United States within 12 hours of the heat index reaching one of two criteria levels. In most areas, a warning will be issued if there is a heat index of at least f ...
(NPW) – Maximum Heat Index
The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is als ...
(HI) values are forecast to meet or exceed locally defined warning criteria for more than three hours over at least two consecutive days. Excessive Heat Warning are issued within 12 hours of the onset of the following Heat Index (HI) values criteria. Specific criteria vary among local Weather Forecast Offices, due to climate variability and the effect of excessive heat on the local population. Typical HI values are maximum daytime temperatures above for up to three hours per day, with minimum nighttime temperatures above (criteria may vary slightly by county warning area) for two consecutive days. Warning criteria may be lowered by the issuing WFO if the heat event occurs early in the season, during a multi-day heat wave or during a widespread power outage occurs during a heat event due to severe weather phenomena (such as high winds, severe thunderstorms, or a derecho).
**Excessive heat watch (NPW) – Conditions are favorable for an excessive heat event with extreme Heat Index values during the day, combined with nighttime low temperatures of or higher that limit perspiration recovery, are forecast to occur to meet or exceed local Excessive Heat Warning criteria in the next 24 to 72 hours.
* Extreme cold warning (NPW) – Forecast shelter
Shelter is a small building giving temporary protection from bad weather or danger.
Shelter may also refer to:
Places
* Port Shelter, Hong Kong
* Shelter Bay (disambiguation), various locations
* Shelter Cove (disambiguation), various locatio ...
temperature of or colder and air temperature remains below up to the level for three or more consecutive days. Only issued by Weather Forecast Offices in Alaska
Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...
. Elsewhere, an Extreme Cold Warning can be issued by local Weather Forecast Offices as an experimental product using locally appropriate thresholds, usually with little or no wind. (The product was first used experimentally by Weather Forecast Offices in North Dakota
North Dakota () is a U.S. state in the Upper Midwest, named after the Native Americans in the United States, indigenous Dakota people, Dakota Sioux. North Dakota is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north a ...
, South Dakota
South Dakota (; Sioux language, Sioux: , ) is a U.S. state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Lakota people, Lakota and Dakota peo ...
and Minnesota
Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over to ...
from January until April 2011.) The Extreme Cold Warning and the Wind Chill Warning products will be consolidated into a singular Extreme Cold Warning product for use by WFOs in the rest of the conterminious United States in 2021, accounting for NWS software upgrades.
**Extreme cold watch (NPW) – Conditions are favorable for an extreme cold event to meet or exceed local Extreme Cold Warning criteria. Only issued by Weather Forecast Offices in Alaska.
* Freeze warning (NPW/FZW) – Minimum shelter temperatures are forecast to be or below over a widespread area within 12 to 36 hours, regardless of whether frost
Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a gas) ...
concurrently develops on plants, vegetation and other exposed surfaces. Freeze Warnings are usually issued to highlight freezes occurring during the locally defined starting and ending periods of the growing season (as determined by the average dates of the first freeze of Autumn and the last freeze of Spring), and will be intermediately issued when appropriate until the end of the growing season based on criteria set by the Weather Forecast Office.
** Freeze watch (NPW) – Issued during the locally defined growing season when conditions are favorable for a freeze event that may damage or kill crops to meet or exceed Freeze Warning criteria within 24 to 48 hours.
*Frost advisory (NPW) – Issued during the locally defined growing season when minimum sheltered temperatures are forecast to be near or slightly above freezing () over an extensive area on nights with good radiational cooling conditions (e.g., light winds and clear skies). Widespread frost can be expected. Frost Advisories are usually issued to highlight frost events occurring during the locally defined starting and ending periods of the growing season, and will be intermediately issued when appropriate until the end of the growing season based on criteria set by the Weather Forecast Office.
*Hard freeze warning (NPW) – Minimum shelter temperatures are forecast to be or below over a widespread area during the growing season. A hard freeze may occur with or without frost. Temperature criteria may vary slightly in some county warning areas.
*Hard freeze watch (NPW) – Conditions are favorable for widespread sub-freezing temperatures that would meet or exceed Hard Freeze Warning criteria within the next 12 to 48 hours during the locally defined growing season.
*Heat advisory
A heat advisory is a notice issued by the National Weather Service of the United States. Local offices often have their own criteria. High values of the heat index are caused by temperatures being significantly above normal and high Relative humid ...
(NPW) – High Heat Index (HI) values are forecast to meet or exceed locally defined warning criteria for one or two days. Specific criteria vary over different county warning areas, due to climate variability and the effect of excessive heat on the local population. Usually issued within 12 hours of the onset of conditional advisory criteria, typical HI values are maximum daytime temperatures above for up to three hours per day, with minimum nighttime temperatures falling within or above the range of (depending on county warning area criteria) for two consecutive days. Warning criteria may be lowered by the issuing WFO if the heat event occurs early in the season, during a multi-day heat wave or during a widespread power outage occurring during a heat event due to severe weather phenomena (such as high winds, severe thunderstorms, or a derecho).
Windchill
*Wind chill warning
A wind chill warning is a hazardous weather statement issued by local forecast offices of the National Weather Service (NWS) in the United States and previously by the Meteorological Service of Environment and Climate Change Canada (ECCC) when w ...
(NPW) – Extreme wind chills, capable of causing life-threatening medical conditions (such as severe frostbite
Frostbite is a skin injury that occurs when exposed to extreme low temperatures, causing the freezing of the skin or other tissues, commonly affecting the fingers, toes, nose, ears, cheeks and chin areas. Most often, frostbite occurs in the ha ...
and hypothermia
Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe ...
) or death associated with accelerated heat loss from exposed skin, are imminent or occurring. The apparent temperature and wind speed criteria vary significantly over different county warning areas based on climate variability. This product will be deprecated sometime in 2021, and be superseded by the Extreme Cold Warning product, which will include dual criteria for extreme wind chill and actual temperature values, and have its use expanded to WFOs elsewhere in the conterminious United States.
*:Particularly dangerous situation wind chill warning (NPW) – Extremely low wind chills of or lower creating an enhanced risk of frostbite, hypothermia and death are imminent or occurring.
*Wind chill advisory
A wind chill advisory is a hazardous weather statement issued by Weather Forecast Offices (WFO) of the National Weather Service (NWS) in the United States to alert the public that wind chills are forecast to reach values low enough that it poses a ...
(NPW) – Dangerous wind chills making it feel very cold are imminent or occurring. The apparent temperature and wind speed criteria vary significantly over different county warning areas based on climate variability.
* Wind chill watch (NPW) – Extreme wind chills that are capable of causing life-threatening medical conditions associated with accelerated heat loss from exposed skin are possible within the next 12 to 48 hours; the apparent temperature and wind speed criteria vary significantly over different county warning areas.
Aviation
The following advisories are issued by the National Weather Service Aviation Weather Center
The Aviation Weather Center (AWC) provides weather information and forecasts for air flights over United States territory and at certain altitudes for global traffic. It works with customers, such as commercial airlines, and international partners ...
(outside of Alaska) or Alaska Aviation Weather Unit. Atmospheric ash plume advisories/warnings are also issued by the United States Geological Survey
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...
( Aviation Color Codes).
*Airport weather warning (AWW) – Addresses weather phenomena (including but not limited to surface wind gusts around or above , freezing rain, heavy snow or thunderstorms producing cloud-to-ground lightning within of the airport and/or hail) capable of adversely affecting ground operations at regional and international airports. AWWs are intended for the use of airport managers, fixed-based operators, airline ground personnel, and other personnel responsible for the safety of ground operations.
*Center weather advisory (CWA) – Advisories issued when conditions just below severe (SIGMET) criteria. CWAs are issued for thunderstorms, turbulence, icing, and ceiling and visibility limits (IFR
In aviation, instrument flight rules (IFR) is one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other is visual flight rules (VFR).
The U.S. Federal Aviation Administration's (FAA) ''Instrument Fly ...
).
*SIGMET
SIGMET, or Significant Meteorological Information (AIM 7-1-6), is a severe weather advisory that contains meteorological information concerning the safety of all aircraft. Compared to AIRMETs, SIGMETs cover more severe weather.
Types
There a ...
(significant meteorological information; SIG) – Advises of weather that is potentially hazardous to all aircraft and is affecting or is forecast to affect at least .[
**Convective SIGMET (SIG) – Implies severe or greater ]turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
, severe icing, and low level wind shear
Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal ...
. Issued for severe surface weather (including surface winds greater than or equal to , hail at the surface greater than or equal to in diameter, or tornadoes); embedded thunderstorms; line of thunderstorms; thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at least .[
**Non-convective SIGMET (SIG) – These SIGMETs may be issued for: severe icing; severe or extreme turbulence; ]dust storm
A dust storm, also called a sandstorm, is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Fine particles are transp ...
s and/or sand storm
A dust storm, also called a sandstorm, is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Fine particles are transport ...
s lowering visibilities to less than ; or volcanic ash. SIGMET advisories are issued for six hours during hurricanes and four hours for other weather-related events.[
* Volcanic ash advisories (VAA) – Advisory issued for all ash plumes detected by satellite imagery, including the location of the ]volcano
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates are ...
, location/description of ash plume, forecast (at 6, 12 and 18 hours), and a graphic of the ash plume location/forecast. VAAs are issued by Volcanic Ash Advisory Center
A Volcanic Ash Advisory Center (VAAC) is a group of experts responsible for coordinating and disseminating information on atmospheric volcanic ash clouds that may endanger aviation. As at 2019, there are nine Volcanic Ash Advisory Centers located a ...
s in Anchorage (Alaska) and Washington (Contiguous United States
The contiguous United States (officially the conterminous United States) consists of the 48 adjoining U.S. states and the Federal District of the United States of America. The term excludes the only two non-contiguous states, Alaska and Hawaii ...
, Caribbean
The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean Se ...
, Central America
Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...
, most of the North Pacific
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
, and South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
north of 10°S).
VAAs are standardized worldwide by the International Civil Aviation Organization
The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international sc ...
.
Tropical weather
*Tropical storm warning
Tropical cyclone warnings and watches are alerts issued by national weather forecasting bodies to coastal areas threatened by the imminent approach of a tropical cyclone of tropical storm or hurricane intensity. They are notices to the local popul ...
(TRW) – Tropical storm conditions (gale- and storm-force sustained winds of ) are expected within the specified coastal or inland area within 36 hours (24 hours for Guam
Guam (; ch, Guåhan ) is an organized, unincorporated territory of the United States in the Micronesia subregion of the western Pacific Ocean. It is the westernmost point and territory of the United States (reckoned from the geographic cent ...
, as locally defined by the Barrigada
Barrigada ( ch, Barigåda) is a village in the United States territory of Guam. A largely residential municipality, its main village is located south of the Antonio B. Won Pat International Airport near the intersections of Routes 8, 10, and 16. ...
-based WFO) in advance of the forecast onset of tropical-storm-force winds. These winds may be accompanied by storm surge, and coastal and/or river flooding. Tropical Storm Warnings may also be issued in association with a subtropical or post-tropical cyclone.
*Tropical storm watch (TRA) – Tropical storm conditions (gale- and storm-force sustained winds of ) are possible within the specified coastal or inland area within 48 hours in advance of the forecast onset of tropical-storm-force winds. These winds may be accompanied by storm surge, and coastal and/or river flooding. Tropical Storm Watches may also be issued in association with a subtropical or post-tropical cyclone.
*Hurricane warning (HUW) – Hurricane conditions (sustained winds of or greater) are expected within the specified coastal or inland area within 36 hours in advance of the forecast onset of tropical-storm-force winds. A Hurricane Warning can remain in effect if dangerously high storm surge, or a combination of dangerous storm surge and waves continue, even when winds fall below hurricane criteria. (This product is issued as a Typhoon Warning by the NWS Forecast Office in Barrigada, Guam
Barrigada ( ch, Barigåda) is a village in the United States territory of Guam. A largely residential municipality, its main village is located south of the Antonio B. Won Pat International Airport near the intersections of Routes 8, 10, and 16 ...
, which uses a 24-hour maximum lead time before the onset of tropical-storm-force winds.)
*Hurricane watch (HUA) – Hurricane conditions (sustained winds of or greater) are possible within the specified coastal or inland area within 48 hours in advance of the forecast onset of tropical-storm-force winds. (This product is issued as a Typhoon Watch by the NWS Forecast Office in Barrigada, Guam and its partnering tropical weather forecasting agency, the Central Pacific Hurricane Center
The Central Pacific Hurricane Center (CPHC) of the United States National Weather Service is the official body responsible for tracking and issuing tropical cyclone warnings, watches, advisories, discussions, and statements for the Central Pacifi ...
.)
* Hurricane local statement (HLS) – Issued by NWS Weather Forecast Offices for hurricane-threatened zones within the office's Warning Area, providing detailed weather conditions, evacuation decisions made by local law enforcement and emergency management agencies, and other necessary safety precautions for residents in the cyclone's path.
*Extreme wind warning
An extreme wind warning ( SAME code EWW) is an alert issued by the National Weather Service for areas that will experience sustained surface winds 100 knots (115 mph, 185 km/h, 51 m/s) or greater within one hour, due to a lan ...
(EWW) – An extreme wind event producing sustained surface winds of or greater, associated with the eyewall of a major hurricane (Category 3 or higher on the Saffir-Simpson Scale), non-convective winds, downslope winds or a derecho is occurring or expected to occur in the specified coastal or inland area within one hour. Extreme Wind Warnings—which should be issued no prior than two hours before the onset of extreme winds—are not issued by WFOs located within the Pacific basin.
Other hazards
*Air quality alert (AQA) – Significant atmospheric pollution (through inhalable particulates or ozone) is expected to accumulate in a given area, and may be hazardous to people suffering from respiratory conditions. Criteria developed in conjunction with the local or state EPA and the product issued at their request.
* Air stagnation advisory (NPW) – Atmospheric conditions stable enough to cause air pollutants to accumulate in a given area. Criteria are developed in conjunction with the local or state EPA and the product is issued at their request.
* Ashfall advisory (NPW) – Ash rain resulting in deposition of less than of ashfall accumulation at the surface is occurring or likely. Ashfall from the large airborne ash plume may originate directly from a volcanic eruption
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...
, from the re-suspension (by wind) of a significant amount of relic ash, or by large fires. Ashfall accumulations may affect the health of people suffering from respiratory illnesses.
*Ashfall warning (NPW) – A volcano undergoing a major eruption is creating the likelihood of significant impact on the public (with higher risk to the health of people with respiratory illnesses) and maritime activity due to ashfall accumulation forecast to be greater than or equal to , significant debris, lava, or lahar flows.
*Blowing dust advisory (NPW) – Considerable blowing sand or dust are expected to reduce visibilities to between or less, but greater than over a widespread or localized area, caused by strong sustained winds of or greater.
*Blowing dust warning (NPW) – Life-threatening dust will reduce visibilities to near zero over a widespread or localized area, caused by strong sustained winds of or greater.
* Civil emergency message (CEM) – A message intended to warn the public of an in-progress or imminent significant threat(s) to public safety and/or property. Although the hazard that warrants such a message to be issued is less specific than that meriting a Civil Danger Warning, it is given higher priority than the Local Area Emergency.
*Dense fog advisory (NPW) – Widespread or localized fog reducing visibilities of or less (up to for marine-based advisories, depending on locally defined criteria) is occurring or is forecast within the next six to 12 hours.
**Freezing fog advisory (NPW) – Widespread dense fog reducing visibility to less than that occurs in a sub-freezing environment, producing very light glazed ice accumulations on exposed surfaces.
*Dense smoke advisory (NPW) – Widespread or localized smoke reducing visibilities to or less (up to for marine-based advisories, depending on locally defined criteria) is occurring; minimum visibility criteria may be defined by the accordant NWS Regional Headquarters or local Weather Forecast Office.
*Dust storm warning
A dust storm warning (SAME code: DSW) is issued by the National Weather Service in the United States when blowing dust is expected to frequently reduce visibility to or less, generally with winds of or more.
Beginning November 1, 2018, the Natio ...
(DSW) – Widespread or localized blowing dust reducing visibilities to or less, usually caused by sustained winds of or greater, is occurring or imminent.
*Gale warning
A gale warning is an alert issued by national weather forecasting agencies around the world in an event that maritime locations currently or imminently experiencing winds of gale force on the Beaufort scale. Gale warnings (and gale watches) a ...
(MWS) – Sustained surface winds or frequent gusts of is either forecast within 24 to 48 hours or is occurring in marine areas. The wind speeds must not be directly associated with a tropical cyclone.
*Gale watch A gale watch is issued by the National Weather Service of the United States when there is an increased risk for a gale-force wind event, meaning sustained surface winds, or frequent gusts, of , but the occurrence, location, and/or timing of the even ...
(MWS) – A gale-force wind event affecting marine areas producing sustained surface winds or frequent gusts of is forecast within the next few days, but its occurrence, location and/or specific timing remains uncertain.
*High wind warning
A gale warning is an alert issued by national weather forecasting agencies around the world in an event that maritime locations currently or imminently experiencing winds of gale force on the Beaufort scale. Gale warnings (and gale watches) al ...
(HWW) – Strong sustained winds of for one hour or longer, or wind gusts of or greater for any duration that are not associated with thunderstorms are occurring or will occur within six to 12 hours. Wind speeds may pose a hazard to pedestrian and vehicular travel (including toppling or creating flying debris from unsecured outdoor objects, and the risk of pushing cars outside their lane of travel and overturning high-profile vehicles), and cause disruption of electrical utilities and wired telecommunications services (including the downing of electrical lines). Warning criteria may vary by county warning area based on local climatological strong wind frequency. A ''tropical storm warning'' implies a ''high wind warning''.
*High wind watch (HWA) – Strong sustained winds of or greater for one hour or more, or wind gusts of for any duration are expected within 12 to 48 hours. Wind speeds may pose a hazard to pedestrian and vehicular travel (including toppling or creating flying debris from unsecured outdoor objects, and the risk of pushing cars outside their lane of travel and overturning high-profile vehicles), and cause disruption of electrical utilities and wired telecommunications services (including the downing of electrical lines). Watch criteria may vary depending on the county warning area based on local climatological strong wind frequency.
*Extreme wind warning (EWW) – Strong straight-line sustained winds of or above, or wind gusts over . Hurricane-like wind damage is expected, and severe structural damage is likely. Those in an extreme wind warning are asked to take cover immediately as if they were in a hurricane or tornado warning. A ''hurricane warning'' implies an ''extreme wind warning''.
*Hurricane force wind warning
A hurricane force wind warning is a warning issued by the National Weather Service of the United States when sustained winds or frequent gusts of 64 knots (118 km/h, 74 mph) or greater are either being observed or are predicted to occur. The winds ...
(NPW) – Sustained winds or frequent gusts of or greater are being observed or are predicted to occur. The wind speeds must not be directly associated with a tropical cyclone.
*Hurricane force wind watch (NPW) – Sustained surface winds or frequent gusts of or greater are forecast. The expected occurrence, location and/or timing of the wind event may be uncertain but is forecast to take place within the next few days. The wind speeds must not be directly associated with a tropical cyclone.
*Low water advisory (MWS) – Water levels are significantly below average levels over major lakes, coastal marine zones, and tidal marine areas, waterway, or river inlet within or adjacent to a marine zone, which could create a hazard to navigation. Issued primarily for areas within the Northern Region Headquarters near the Great Lakes
The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...
region.
*Special weather statement
A Special Weather Statement ( SAME event code SPS) is a form of weather advisory. Special Weather Statements are issued by the National Weather Service of the United States (the NWS) and the Meteorological Service of Canada (the MSC). There are no ...
(SPS) – An advisory issued when a hazard is approaching advisory criteria. In most cases, special weather statements typically highlight upcoming significant weather events (such as substantial temperature changes, dense fog and winter weather events). Short-fused convective statements (which superseded the significant weather advisory product used by most NWS offices in July 2021) are issued for non-severe thunderstorms near or below warning criteria.
*:Particularly dangerous situation special weather statement (SPS) – An enhanced advisory issued for hazards that do not have a specific code of their own, and pose an exceptionally high risk of damage and loss of life.
*Storm warning
At sea, a storm warning is a warning issued by the National Weather Service of the United States when winds between 48 knots (89 km/h, 55 mph) and 63 knots (117 km/h, 73 mph) are occurring or predicted to occur soon. The wi ...
(MWS) – Sustained surface winds or frequent gusts of is either predicted within 36 hours or occurring. Expected wind speeds are likely to cause significant navigational impairment and/or damage to commercial and recreational boating within the warned area. The wind speeds must not be directly associated with a tropical cyclone.
*Storm watch (MWS) – Sustained surface winds or frequent gusts of are forecast to occur, but its occurrence, location and/or timing remains uncertain. Expected wind speeds may cause significant navigational impairment and/or damage to commercial and recreational boating. The wind speeds must not be directly associated with a tropical cyclone.
*Wind advisory A wind advisory is generally issued by the National Weather Service of the United States when there are sustained thunderstorm winds of and/or gusts of over land. Winds over the said cap will trigger High wind warning, high wind alerts rather tha ...
(NPW) – Strong sustained winds of for one hour or longer and/or wind gusts of for any duration are expected within the next 12 to 24 hours. Wind speeds may pose a hazard to pedestrian and vehicular travel, topple and/or create flying debris from unsecured outdoor objects, and potentially disrupt public utilities and down electrical lines. Advisory criteria may vary by county warning area depending on local climatology; winds of the standard criteria magnitude occurring over an area that frequently experiences such wind speeds will not necessarily trigger a wind advisory.
Non-meteorological hazards and administrative messages
The National Weather Service also relays messages for non-weather related hazardous events in text products and NOAA Weather Radio broadcasts:
*911 telephone outage emergency (TOE) – Issued when 911
911 or 9/11 may refer to:
Dates
* AD 911
* 911 BC
* September 11
** 9/11, the September 11 attacks of 2001
** 11 de Septiembre, Chilean coup d'état in 1973 that outed the democratically elected Salvador Allende
* November 9
Numbers
* 91 ...
telephone service experiences a network outage over a geographic area or telephone exchange. Alternative phone numbers in which to reach 911 or dispatch emergency personnel will be provided to the public.
*Administrative message (ADR) – A non-emergency message providing updated information about an event that is either in progress, has expired or concluded early, pre-event preparation or mitigation activities, post-event recovery operations, or other Emergency Alert System-related administrative matters.
*Avalanche watch (AVA) – Indicates conditions are forecast to favor natural or human-induced avalanche
An avalanche is a rapid flow of snow down a slope, such as a hill or mountain.
Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and earth ...
s that could affect roadways, structures, or backcountry activities.
*Avalanche warning (AVW) – Avalanche activity—either natural or human-induced—is ongoing or imminent, and likely to affect roadways, structures or backcountry activities and threaten the lives of people within the avalanche's path. Safety precautions will be recommended or ordered as dictated by state law or local ordinance.
*Blue alert (BLU) – Issued by state and local authorities to warn the public of information relating to a law enforcement officer who is missing, seriously injured or was killed in the line of duty, or if there is an imminent, credible threat to an officer. Information on the suspect and safety instructions for if the suspect has been spotted within a specific community may be included. The NWS added the ability to convey these messages to its WFOs on a phased basis beginning in late 2019.
* Child abduction emergency (CAE) – An emergency message, usually issued in accordance with an Amber alert
An Amber Alert (alternatively styled AMBER alert) or a child abduction emergency alert ( SAME code: CAE) is a message distributed by a child abduction alert system to ask the public for help in finding abducted children. The system originated in ...
, informing the public and law enforcement about a missing child believed to have been abducted. Local or state law enforcement agencies investigating the abduction will provide descriptions of the missing child, suspect or vehicle used by the suspect, and ask the public to notify the requesting agency if they have information on the child's or suspect's whereabouts.
* Civil danger warning (CDW) – Assigned a higher priority than a Local Area Emergency, this indicates a hazardous event (such as the contamination of local water supply or a military or terrorist attack that is imminent or ongoing) presenting a danger to a significant civilian population, requiring specific protective action (such as evacuation or sheltering in place).
*Earthquake warning (EQW) – Indicates earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
activity is ongoing or imminent.
*Evacuation immediate (EVI) – Warns the public that immediate evacuation for a long-duration emergency event (such as for an approaching tropical cyclone, a wildfire or the release of flammable or explosive gas) has been recommended or ordered by state and/or local law enforcement.
*Hazardous materials warning (HMW) – Warns the public of the release of non-radioactive hazardous material (such as a flammable gas, toxic chemical, or biological agent) that may necessitate recommendations of evacuation (for an explosion, fire or oil spill hazard) or sheltering in place (for a toxic fume hazard).
* Law enforcement warning (LEW) – Issued by authorized law enforcement
Law enforcement is the activity of some members of government who act in an organized manner to enforce the law by discovering, deterring, rehabilitating, or punishing people who violate the rules and norms governing that society. The term en ...
agencies to warn the public of a bomb explosion, riot, or other criminal event (e.g., a jailbreak) that may result in the blockade roads, waterways, or facilities, the evacuation or denial of access to affected areas, and the arrest of violators or suspicious persons.
* Local area emergency (LAE) – An emergency message that defines an event of insignificant threat to public safety and/or property, which could escalate, contribute to more serious events, or disrupt critical public safety services (such as a disruption in water, electric or natural gas service, snowfall-induced road closures, or a potential terrorism threat).
*Nuclear power plant warning (NUW) – Warns of a hazardous materials event at a nuclear power plant
A nuclear power plant (NPP) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a electric generator, generato ...
classified as a Site Area Emergency (a hazardous event confined to the plant site) or General Emergency (an event affecting a less than radius around the plant) under Nuclear Regulatory Commission (NRC) guidelines. Authorized officials may recommend evacuation or medical treatment of exposed persons in nearby areas.
*Radiological hazard warning (RHW) – Indicates the loss, discovery, theft or release of a radiological hazard.
*Shelter in place warning (SPW) – Indicates an event (such as the release of hazardous materials containing toxic fumes or radioactivity harmful to residents in the warning area) in which the public is recommended to shelter in place and take necessary safety precautions.
*Tsunami watch (TSA) – A tsunami
A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...
is forecast to impact the specified area.
*Tsunami warning (TSW) – A tsunami with the potential of causing widespread coastal flooding accompanied by powerful currents and inundation and/or significant damage to adjacent property is imminent, expected or occurring and may continue for several hours after the wave's initial arrival, issued by the National Tsunami Warning Center
The National Tsunami Warning Center (NTWC) forms part of an international tsunami warning system (TWS). It serves as the operations center for all coastal regions of Canada and the United States, except Hawaii, the Caribbean, and the Gulf of Mexic ...
.
*Tsunami advisory – A tsunami with the potential to generate strong currents or waves dangerous to those in or very near the water is imminent, expected, or occurring. The threat may continue for several hours after initial arrival, but significant inundation of land and adjacent property is not expected.
* Volcano warning (VOW) – Indicates volcanic
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates a ...
activity that is ongoing or imminent.
Wind and tropical cyclones
Wind alerting is classified into groups of two Beaufort numbers, beginning at 6–7 for the lowest class of wind advisories. The last group includes three Beaufort numbers, 14–16. The actual alerts can be categorized into three classes: maritime wind warnings, land wind warnings, and tropical cyclone warnings. Advisory-force and gale-force winds will not trigger a separate wind advisory or warning if a Blizzard warning
A blizzard warning ( SAME code: BZW) is a hazardous weather statement issued by Weather Forecast Offices (WFO) of the National Weather Service (NWS) in the United States, which indicates heavy snowfall accompanied by sustained winds or frequent gu ...
is already in effect. However, as seen with Hurricane Sandy
Hurricane Sandy (unofficially referred to as ''Superstorm Sandy'') was an extremely destructive and strong Atlantic hurricane, as well as the largest Atlantic hurricane on record as measured by diameter, with tropical-storm-force winds spann ...
, if widespread high wind warnings are in effect prior to the issuance of a blizzard warning, the high wind warnings may be continued.
Wind alert terms and signals
*
†
‡
Hazardous weather risks
The various weather conditions described above have different levels of risk. The National Weather Service uses a multi-tier system of weather statements to notify the public of threatening weather conditions. These statements are used in conjunction with specific weather phenomena to convey different levels of risk. In order of increasing risk, these statements are:
*Outlook – A Hazardous Weather Outlook is issued daily to indicate that a hazardous weather or hydrologic event may occur in the next seven days. Each segment of the product will contain sections outlining hazardous weather in the short term (through Day 1) and long term (for Days 2-7), and spotter information. The outlook will include information about potential severe thunderstorms, heavy rain or flooding, winter weather, extremes of heat or cold, etc., that may develop over the next seven days with an emphasis on the first 24 hours of the forecast. It is intended to provide information to those who need considerable lead time to prepare for the event (such as emergency management
Emergency management or disaster management is the managerial function charged with creating the framework within which communities reduce vulnerability to hazards and cope with disasters. Emergency management, despite its name, does not actuall ...
agencies, Skywarn spotters and media outlets). If issued for a coastal area, the outlook will contain two segments: one for the marine zones and adjacent land-based (i.e., coastal) zones, and the other for the remainder of the land-based zones.
*Advisory – An advisory is issued when a hazardous weather or hydrologic event is occurring, imminent, or likely. Advisories are for "less serious" conditions than warnings that may cause significant inconvenience, and if caution is not exercised could lead to situations that may threaten life or property. The National Weather Service may activate weather spotters in areas affected by advisories to help them better track and analyze the event.
*Emergency – An Emergency is issued when an event that by itself cannot pose a threat to life or property, but may indirectly cause other events to happen that may pose a threat to life or property. An example of this would be a power outage
A power outage (also called a powercut, a power out, a power failure, a power blackout, a power loss, or a blackout) is the loss of the electrical power network supply to an end user.
There are many causes of power failures in an electricit ...
, which although not directly posing a hazard, may threaten public safety and critical services. The only existing exceptions to this are the tornado emergency
A tornado emergency is an enhanced version of a tornado warning, which is used by the National Weather Service (NWS) in the United States during imminent, significant tornado occurrences in highly populated areas. Although it is not a new warning ...
and flash flood emergency
A flash flood warning ( SAME code: FFW) is an hazardous weather statement issued by national weather forecasting agencies throughout the world to alert the public that a flash flood is imminent or occurring in the warned area. A flash flood is a ...
, which are issued to get the attention of the public to a major tornado or flash flood.
*Watch – A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location, or timing is still uncertain. It is intended to provide enough lead time so those who need to set their plans in motion can do so. A watch means that hazardous weather is possible. People should have a plan of action in case a storm threatens and they should listen for later information and possible warnings especially when planning travel or outdoor activities. The National Weather Service may activate weather spotters in areas affected by watches to help them better track and analyze the event.
*Warning – A warning is issued when a hazardous weather or hydrologic event is occurring, imminent, or likely. A warning means weather conditions pose a threat to life or property. People in the path of the storm need to take protective action. The National Weather Service may activate weather spotters in areas affected by warnings to help them better track and analyze the event.
*Statement – A statement is either issued as a follow-up message to a warning, watch, or emergency, that may update, extend, or cancel the message it is following up or a notification of significant weather for which no type of advisory, watch, or warning exists.
Convective outlook categories
The Storm Prediction Center (SPC) issues Day 1, Day 2, and Day 3 Convective Outlooks depicting forecast areas of general (non-severe) and severe thunderstorm threats across the contiguous United States, along with a text narrative discussion consisting of a plain-language summary of the threat type(s) and timing focused on areas of highest risk, and a technical discussion written in scientific language that usually includes a synoptic overview of convective patterns as well as, if necessary, a geographically specific narrative of meteorological reasoning and justification for the type of coverage and intensity applicable to the severe thunderstorm threat.
The categorical forecast in the Day 1-3 Convective Outlooks—which estimates a severe weather event occurring within of a point and derives the attendant risk areas from probability forecasts of tornadoes, damaging winds, and large hail on Days 1 and 2, and a combined severe weather risk on Day 3—specifies the level of overall severe thunderstorm risk via numbers, descriptive labeling, and colors as follows: (The Day 4-8 Convective Outlook assesses the percentile probability of severe thunderstorm activity during that period at the 15% and 30% likelihood.)
Warning impact statements
Many of the National Weather Service's Weather Forecast Offices—primarily those located within the Central and Southern Region Headquarters—use a multi-tier impact-based warning (IBW) system of impact statements to notify the public and emergency management officials of the severity of specific severe weather phenomena. The impact statement system—initially used only for tornado and severe thunderstorm warnings—was first employed by the WFOs in Wichita and Topeka, Kansas
Topeka ( ; Kansa language, Kansa: ; iow, Dópikˀe, script=Latn or ) is the Capital (political), capital city of the U.S. state of Kansas and the County seat, seat of Shawnee County, Kansas, Shawnee County. It is along the Kansas River in the ...
, and Springfield, St. Louis
St. Louis () is the second-largest city in Missouri, United States. It sits near the confluence of the Mississippi and the Missouri Rivers. In 2020, the city proper had a population of 301,578, while the bi-state metropolitan area, which e ...
and Kansas City/Pleasant Hill, Missouri
Pleasant Hill is a city in Cass and Jackson counties, Missouri, United States. The population was 8,113 at the 2010 census. It is part of the Kansas City metropolitan area.
Pleasant Hill is home for the National Weather Service Kansas City/Pleas ...
beginning with the 2012 Spring severe weather season, eventually expanded to include 33 additional National Weather Service Weather Forecast Offices within the Central Region Headquarters in 2013, and then to eight additional offices within the Eastern, Southern and Western Regions in the spring of 2014.[ Since July 28, 2021 (or as late as August 2 in certain County Warning Areas), the NWS has incorporated categorical “CONSIDERABLE” and “DESTRUCTIVE" damage threat indicators (similar to those incorporated into tornado warning products since the implementation of the Impact Based Warning system) at the bottom of the product text of certain severe thunderstorm warnings and related Severe Weather Statements to indicate higher-end hail and/or wind events caused by the parent storm cell.]
Under this system, the warning product will include text denoting the specific hazard (i.e., 60 mph wind gusts and quarter size hail) and applicable sourcing (either via indication from Doppler weather radar, or visual confirmation from storm spotters or other emergency management officials) and the level of impact to life and/or property. In order of increasing risk by warning type, these statements—which may be modified at the discretion of the regional forecast office—are:
Media distribution
Hazardous weather forecasts and alerts are provided to the public using the NOAA Weather Radio All Hazards
NOAA Weather Radio NWR; also known as NOAA Weather Radio All Hazards is an automated 24-hour network of VHF FM weather radio stations in the United States (U.S.) that broadcast weather information directly from a nearby National Weather Servi ...
system and through news media such as television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
, radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
and internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
sources. Many local television station
A television station is a set of equipment managed by a business, organisation or other entity, such as an amateur television (ATV) operator, that transmits video content and audio content via radio waves directly from a transmitter on the earth ...
s have overlay graphics which will either show a map or a list of the affected areas. The most common NWS weather alerts to be broadcast over NOAA Weather Radio using SAME technology are described in the following table:
Related weather scales as defined by the NWS
The NWS uses several scales in describing weather events or conditions. Several common scales are described below.
Hail diameter sizes
The size of individual hailstones that reach surface level is determined by speed of the updraft which create the individual ice crystals at atmospheric levels. Larger hailstones are capable of producing damage to property, and particularly with very large hailstones, resulting in serious injury or death due to blunt-force trauma induced by the impact of the hailstones. Hailstone size is typically correspondent to the size of an object for comparative purposes.
* Begins hail sizes within the severe hail criterion.
† Begins hail sizes within the Storm Prediction Center
The Storm Prediction Center (SPC) is a US government agency
A government or state agency, sometimes an appointed commission, is a permanent or semi-permanent organization in the machinery of government that is responsible for the oversight a ...
's significant severe criterion.
Beaufort wind scale
The Beaufort scale is an empirical measure that correlates wind speed to observed conditions at sea or on land.
†:Beaufort levels above 12 are non-standard in the United States. Instead, the Saffir–Simpson hurricane scale (Category 1, Category 2, etc.) is used.
Enhanced Fujita tornado intensity scale
The Enhanced Fujita scale
The Enhanced Fujita scale (abbreviated as EF-Scale) rates tornado intensity based on the severity of the damage they cause. It is used in some countries, including the United States, Canada, China, and Mongolia.
The Enhanced Fujita scale repl ...
, an updated version of the original Fujita scale
The Fujita scale (F-Scale; ), or Fujita–Pearson scale (FPP scale), is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation. The official Fujita scale category is determ ...
that was developed by Ted Fujita
was a Japanese-American meteorologist whose research primarily focused on severe weather. His research at the University of Chicago on severe thunderstorms, tornadoes, hurricanes, and typhoons revolutionized the knowledge of each. Although he is ...
with Allen Pearson, assigns a numerical rating from EF0 to EF5 to rate the damage intensity of tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
es. EF0 and EF1 tornadoes are considered "weak" tornadoes, EF2 and EF3 are classified as "strong" tornadoes, with winds of at least major hurricane force, where EF4 and EF5 are categorized as "violent" tornadoes, with winds corresponding to category 5 hurricane winds and rising to match or exceed the strongest tropical cyclones on record. The EF scale is based on tornado damage (primarily to buildings), which makes it difficult to rate tornadoes that strike in sparsely populated areas, where few man-made structures are found. The Enhanced Fujita scale went into effect on February 1, 2007.
Saffir–Simpson hurricane category scale
The Saffir–Simpson hurricane wind scale, assigns a numerical classification of hurricanes into five categories distinguished by the intensities of their sustained winds. The scale spans from Category 1 (winds of at least ) to Category 5 (exceeding ). Unlike the Enhanced Fujita Scale, which assigns ratings for tornadoes after damage has been incurred and thoroughly assessed, categories on the Saffir-Simpson scale are assigned to most active cyclones that reach the minimum hurricane threshold, even before landfall.
See also
* Emergency Alert System
The Emergency Alert System (EAS) is a national warning system in the United States designed to allow authorized officials to broadcast emergency alerts and warning messages to the public via cable, satellite, or broadcast television, and bot ...
* Glossary of climate change
* Glossary of environmental science
* Glossary of meteorology
* List of climate change topics
* List of severe weather phenomena
Severe weather phenomena are weather conditions that are hazardous to human life and property.
Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, a ...
* Local storm report
* National Weather Service
The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
* National Weather Service bulletin for New Orleans region
The National Weather Service bulletin for the New Orleans region of 10:11 a.m., August 28, 2005, was a particularly dire warning issued by the local National Weather Service, Weather Forecast Office in Slidell, Louisiana, warning of the devas ...
, generally considered the most seriously worded NWS forecast ever issued, done so the day before Hurricane Katrina
Hurricane Katrina was a destructive Category 5 Atlantic hurricane that caused over 1,800 fatalities and $125 billion in damage in late August 2005, especially in the city of New Orleans and the surrounding areas. It was at the time the cost ...
.
* NOAA Weather Radio
NOAA Weather Radio NWR; also known as NOAA Weather Radio All Hazards is an automated 24-hour network of VHF FM weather radio stations in the United States (U.S.) that broadcast weather information directly from a nearby National Weather Servi ...
* Severe weather terminology (Canada)
This article describes severe weather terminology used by the Meteorological Service of Canada, a branch within Environment and Climate Change Canada. The article primarily describes various weather warnings, and their criteria. Related weather s ...
* Severe weather terminology (Japan)
This article describes the Japan Meteorological Agency (JMA) severe weather terminology. The JMA defines precise meanings for nearly all its weather terms as the . This article describes JMA terminology and related JMA weather scales. Some terms m ...
* Specific Area Message Encoding
Specific Area Message Encoding (SAME) is a protocol used for framing and classification of broadcast emergency warning messages. It was developed by the United States National Weather Service for use on its NOAA Weather Radio (NWR) network, and w ...
References
External links
NWS Directives System
National Weather Service glossary of terms
National Weather Service, Chicago, Illinois
NWS National Hurricane Center
{{DEFAULTSORT:Severe Weather Terminology (United States)
Severe weather and convection
Weather warnings and advisories