A cut-off low (or cutoff low), sometimes referred to as the weatherman's woe, is defined as "a closed upper-level

low Low or LOW or lows, may refer to: People * Low (surname), listing people surnamed Low Places * Low, Quebec, Canada * Low, Utah, United States * Lo Wu station (MTR code LOW), Hong Kong; a rail station * Salzburg Airport (ICAO airport code: LO ...

which has become completely displaced (cut off) from basic westerly current, and moves independently of that current" 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 ...

. Cut-off lows form in mid-latitudes (usually in the

subtropics The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north and ...

or between 20° and 45°) and would remain nearly stationary for days.

Formation

A cut-off low is a

cold-core low A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure syst ...

where wind in the upper levels of the

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

is "cut off"" from the primary westerly winds of the

jet stream Jet streams are fast flowing, narrow, meandering thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are west ...

. They are formed when a

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 ...

in the upper-air flow pinches off and separates into a closed circulation. It is defined by concentric isotherms around the core of the low. Because they are a feature of the mid- to upper-troposphere, they may not be visible on a

surface weather analysis Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations. Weather maps are created by plotting or tra ...

. Because they are separated from the main westerly flow, cut-off lows can move slowly and erratically. In certain arrangements, known as a

block Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...

or a blocking pattern, they can remain in place for long periods of time. Whilst cut-off lows can form at any time of the year, they are more common in autumn, winter and spring in much of the areas affected, particularly in Australia and the

Mediterranean Basin In biogeography, the Mediterranean Basin (; also known as the Mediterranean Region or sometimes Mediterranea) is the region of lands around the Mediterranean Sea that have mostly a Mediterranean climate, with mild to cool, rainy winters and w ...

, when a mass of polar air is brought towards more southern regions (or northern, in the southern hemisphere) by the jet stream moving between 5 and 9 km

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

Characteristics

The diameter of a cut-off low can vary from a few hundred to a thousand kilometers. The air there is homogeneous and without a front line separating it from the surrounding masses, while having a decisive influence on the weather. It then most often leads to an

atmospheric blocking Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively "blocking" or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones.Glossary of Mete ...

circulation where we witness the formation of an upper level low. High cold drops, between 1,000 and 10,000 m, are regions of low stability, while low cold drops are regions of relatively stable air. Composed of very cold air of polar origin, it typically has a horizontal extent of 300 to 1000 km and is 5,000 to 10,000 meters above

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...

.Climatological Features of Cutoff Low Systems in the Northern Hemisphere
Journal of Climate by Raquel Nieto, Luis Gimeno, Laura de la Torre, Pedro Ribera, David Gallego, Ricardo García-Herrera, José Agustín García, Marcelino Nuñez, Angel Redaño, and Jerónimo Lorente. 15 August 2005. Retrieved 6 April 2022.

Movement

A cut-off low has a slow movement, typically over a confined region, where it produces heavy rainfall. They are volatile,

baroclinic In fluid dynamics, the baroclinity (often called baroclinicity) of a stratified fluid is a measure of how misaligned the gradient of pressure is from the gradient of density in a fluid. In meteorology a baroclinic flow is one in which the densi ...

systems that meander to the west with strong

convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen * "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that united the four Wei ...

and an ascending motion, especially when they are deepening. A cut-off low can persist from a few days to more than a week. It may be absorbed into the general circulation while another forms in the same place a few days later. The evolution and movement of a cold low, like any weather blockage, is therefore uncertain.

Effects

Cut off lows typically create unsettled weather and, in the warm season, they may produce a lot of

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. A cut-off low has a slow movement, typically over a confined region, where it produces heavy rainfall, and can result in severe flooding. For example, a cut-off low was responsible for the July 2021 floods in Europe

Regional impacts

In southeastern Australia, cut-off lows can be associated with

Australian east coast low Australian east coast lows (known locally as east coast lows, maritime lows and sometimes as east coast cyclones) are extratropical cyclones or low-pressure systems on the coast of southeastern Australia that may be caused by both mid-latitude an ...

s, which are

subtropical cyclone A subtropical cyclone is a weather system that has some characteristics of both tropical cyclone, tropical and an extratropical cyclone. As early as the 1950s, meteorologists were uncertain whether they should be characterized as Tropical cyclo ...

s or

extratropical cyclones Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of p ...

that originate from the east. In eastern Australia, a cut-off low can bring accumulating and widespread snowfall at low-level areas and as well as elevated regions in the

.Sydney shivers through coldest day since 1984 as snow blankets NSW
By Sarah McPhee and Daniella White from The Daily Telegraph. June 10, 2021 In Spain, cut-off lows are often referred to by the name "

cold drop A cold drop (from the Spanish "gota fría") is a term used in Spain that has commonly come to refer to any high impact rainfall events occurring in the autumn along the Spanish Mediterranean coast. In Europe, cold drops belong to the characterist ...

References