The tornado outbreak of March 6–7, 2017 was a widespread severe weather and tornado outbreak that affected portions of the Midwestern United States in the overnight hours of March 6–7. Occurring just days after a deadly and more significant event across similar areas just a week prior, this particular outbreak led to 63 tornadoes within a 9-hour period as a

quasi-linear convective system 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 ...

and discrete supercell thunderstorms traversed the region. The most notable aspect of the outbreak was an

EF3 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 damaged or destroyed hundreds of structures within Oak Grove, Missouri, injuring 12 people but causing no fatalities. An EF1 tornado touched down near Bricelyn, the earliest known tornado on record in the state of Minnesota. Outside of tornadic activity, hundreds of damaging wind reports and a multitude of severe hail reports were documented.

Meteorological synopsis

On March 2, the Storm Prediction Center (SPC) highlighted the forecast evolution of an upper-level

trough Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), the l ...

and the potential for severe thunderstorms across portions of the southern Plains and the lower Mississippi River Valley valid for day 5/March 6. However, limited moisture return and low forecaster confidence prevented the delineation of a severe weather threat area at that time. The organization again omitted a threat area for the same period the next day, citing unpredictability in the duration and severity of strong thunderstorms. On March 4, the SPC outlined a Marginal risk of severe weather from northeastern Texas northward into extreme portions of southern Minnesota and southwestern Wisconsin. The next morning, a Slight risk was introduced across portions of northeastern Oklahoma, southeastern Kansas, much of Missouri, and northwestern Arkansas. Accounting for increased confidence in supercellular thunderstorm development ahead of a

(QLCS), a small Enhanced risk of severe weather was indicated across portions of northwestern Arkansas and southern Missouri during the afternoon hours of March 5. This risk area was expanded the day of the event, including a 10% hatched tornado probability area across southeastern Kansas and south-central Missouri where the SPC noted, "large hail and tornadoes, some of which may be significant, will be the primary risks with any supercells that can maintain discrete mode immediately ahead of a probable QLCS." 2017-03-06 SPC outlook verification

On the heels of a significant tornado outbreak that affected much of the same areas the week before, this event came to fruition as a synoptic upper-level trough tilted northwest to southeast from the Alaska Panhandle into the

Four Corners The Four Corners is a region of the Southwestern United States consisting of the southwestern corner of Colorado, southeastern corner of Utah, northeastern corner of Arizona, and northwestern corner of New Mexico. The Four Corners area ...

region. A strong shortwave trough on the base of the synoptic feature was expected to phase with a second shortwave over Utah, collectively progressing across the northern and central Plains throughout March 6. The northern half of the trough, meanwhile, was expected to evolve into a closed low in the middle levels of the atmosphere as it lifted into The Dakotas and then on up to southern Canada. At the surface, a rapidly-deepening area of low pressure (expected to fall to near by 00:00 UTC on March 7) was noted across South Dakota, with a cold front extending southward into New Mexico and a warm front extending eastward into the Great Lakes region. A dry line extended from central South Dakota down to northern Mexico. Both the cold front and the dry line were expected to track eastward throughout the day, with the former feature expected to overtake portions of the latter in eastern Kansas by the evening hours. Despite widespread low-level clouds in the warm sector, limiting the amount of atmospheric instability, mid-level Convective Available Potential Energy (CAPE) values were expected to rise to around 2000 J/kg across the western Ozarks and up to 500 J/kg as far north as north-central Minnesota. Partially modified Gulf moisture, with dewpoints in the 60s F across the Enhanced risk and 50s F farther north, was expected to combine with steep mid-level

lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...

s of 7–7.5 C/km to destabilize the environment. Although a capping inversion across the risk area limited convection throughout the day and despite the main forcing passing north of the highest risk area, two regimes were expected to evolve: a quasi-linear convective system along the cold front and the formation of discrete supercells ahead of said line. With strong effective bulk wind shear up to , effective storm relative helicity of 250–400 m2/s2, and even higher 0–1 km storm relative helicity values of 500–600 m2/s2, the environment became primed for a widespread severe weather and tornado outbreak. The first tornado watch of the day was issued at approximately 17:55 UTC across portions of Iowa, and several other watches were also issued across the

Midwest The Midwestern United States, also referred to as the Midwest or the American Midwest, is one of four Census Bureau Region, census regions of the United States Census Bureau (also known as "Region 2"). It occupies the northern central part of ...

before the final watch issued for portions across the Mid-South expired at 16:00 UTC on March 7. In total, the SPC logged 35 filtered reports of tornadoes, 352 filtered reports of damaging winds, and 98 filtered reports of severe hail. A vast majority of tornadoes that occurred during this outbreak were spawned by semi-discrete supercell structures and embedded mesovortices within a larger

, as development of true discrete supercell thunderstorms throughout the event was somewhat limited. However, a few isolated supercell thunderstorms did develop over northern Arkansas and southern Missouri during the late night hours of March 6 into the early morning hours of March 7. One of these isolated supercells produced a long-track EF2 tornado that struck

Parthenon, Arkansas Parthenon is an unincorporated community in Newton County, Arkansas, United States. Parthenon is located on Arkansas Highway 327, southwest of Jasper. Parthenon has a post office A post office is a public facility and a retailer that pro ...

and caused heavy damage. The most significant event of the outbreak was an

that moved directly through Oak Grove, Missouri, damaging or destroying hundreds of structures and injuring 12 people. No fatalities occurred as a result of this outbreak, but 19 people were injured.

Confirmed tornadoes

March 6 event

March 7 event

Footnotes

References

External links

Outbreak summaries from regional National Weather Service offices:
Topeka, Kansas, WFOSpringfield, Missouri, WFOKansas City, Missouri, WFODes Moines, Iowa, WFOTwin Cities, Minnesota, WFOLa Crosse, Wisconsin, WFOQuad Cities, IA/IL, WFOSaint Louis, Missouri, WFOCentral Illinois, WFO
{{DEFAULTSORT:Tornado outbreak of March 6-7, 2017 2016–17 North American winter 2017 in Kansas 2017 in Missouri 2017 in Illinois 2017 in Iowa 2017 in Minnesota March 2017 events in the United States Tornadoes of 2017