Squall line

Last updated February 14, 2024

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 accompanied by abrupt and gusty wind shifts). Linear thunderstorm structures often contain heavy precipitation, hail, frequent lightning, strong straight-line winds, and occasionally tornadoes or waterspouts. Particularly strong straight-line winds can occur where the linear structure forms into the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes can grow to become derechos as they move swiftly across a large area. On the back edge of the rainband associated with mature squall lines, a wake low can be present, on very rare occasions associated with a heat burst.

Theory

Polar front theory was developed by Jacob Bjerknes, derived from a dense network of observation sites in Scandinavia during World War I. This theory proposed that the main inflow into a cyclone was concentrated along two lines of convergence, one ahead of the low and another trailing behind the low. The trailing convergence zone was referred to as the squall line or cold front. Areas of clouds and rainfall appeared to be focused along this convergence zone. The concept of frontal zones led to the concept of air masses. The nature of the three-dimensional structure of the cyclone was conceptualized after the development of the upper air network during the 1940s.^[1]

A squall line over 1,000 mi (1,600 km) long across the Gulf of Mexico and the Eastern U.S (radar coverage is from ground radars, so middle image does not cover the portion over the Gulf). The rightmost image is a couple of hours after the other two, showing the strongest portion of the line as it passes through Florida, Georgia, and South Carolina.

Life cycle

Organized areas of thunderstorms activity reinforce pre-existing frontal zones, and they can outrun cold fronts. This outrunning occurs within the westerlies in a pattern where the upper-level jet splits into two streams. The resultant mesoscale convective system (MCS) forms at the point of the upper level split in the wind pattern in the area of best low-level inflow.

The convection then moves east and toward the equator into the warm sector, parallel to low-level thickness lines. When the convection is strong linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise.^[2] This feature is commonly depicted in the warm season across the United States on surface analyses, as they lie within sharp surface troughs.

If squall lines form over arid regions, a dust storm known as a haboob may result from the high winds in their wake picking up dust from the desert floor.^[3] Well behind mature squall lines, a wake low can develop on the back edge of the rain shield,^[4] which can lead to a heat burst due to the warming up of the descending air mass which is no longer being rain-cooled.^[5]

Smaller cumulus or stratocumulus clouds, along with cirrus, and, sometimes, altocumulus or cirrocumulus, can be found ahead of the squall line. These clouds are the result of former cumulonimbus clouds having disintegrated, or an area of only minor instability ahead of the main squall line.

As supercells and multi-cell thunderstorms dissipate due to a weak shear force or poor lifting mechanisms, (e.g. considerable terrain or lack of daytime heating) the gust front associated with them may outrun the squall line itself and the synoptic scale area of low pressure may then infill, leading to a weakening of the cold front; essentially, the thunderstorm has exhausted its updrafts, becoming purely a downdraft dominated system. The areas of dissipating squall line thunderstorms may be regions of low CAPE, low humidity, insufficient wind shear, or poor synoptic dynamics (e.g. an upper-level low filling) leading to frontolysis.

From here, a general thinning of a squall line will occur: with winds decaying over time, outflow boundaries weakening updrafts substantially and clouds losing their thickness.

Characteristics

Updrafts

The leading area of a squall line is composed primarily of multiple updrafts, or singular regions of an updraft, rising from ground level to the highest extensions of the troposphere, condensing water and building a dark, ominous cloud to one with a noticeable overshooting top and anvil (thanks to synoptic scale winds). Because of the chaotic nature of updrafts and downdrafts, pressure perturbations are important. Precipitation cooled air from downdrafts usually outwardly just above the surface and lifts air into the updrafts unless gushing too far out and cutting off this inflow. Visually this process may take the form of a shelf cloud, often with a turbulent appearance.

Pressure perturbations

Pressure perturbations around thunderstorms are noteworthy. With buoyancy rapid within the lower and mid-levels of a mature thunderstorm, updraft and downdraft create distinct mesocenters of pressure. As thunderstorms organized in squall lines, the northern end of the squall line is commonly referred to as the cyclonic end, with the southern side rotating anticyclonically (in Northern hemisphere). Because of the Coriolis force, the northern end may evolve further, creating a "comma shaped" wake low, or may continue in a squall-like pattern. The updraft ahead of the line create a mesolow too while the downdraft just behind the line will produce a mesohigh.

Wind shear

Wind shear is an important aspect of a squall line. In low to medium shear environments, mature thunderstorms will contribute modest amounts of downdrafts, enough to help create a leading edge lifting mechanism – the gust front. In high shear environments created by opposing low level jet winds and synoptic winds, updrafts and consequential downdrafts can be much more intense (common in supercell mesocyclones). The cold air outflow leaves the trailing area of the squall line to the mid-level jet, which aids in downdraft processes.

Severe weather indicators

Severe squall lines typically bow out due to the formation of a stronger mesoscale high-pressure system (a mesohigh) within the convective area due to strong descending motion behind the squall line, and could come in the form of a downburst.^[6] The pressure difference between the mesoscale high and the lower pressures ahead of the squall line cause high winds, which are strongest where the line is most bowed out.

Another indication of the presence of severe weather along a squall line is its morphing into a line echo wave pattern (LEWP). A LEWP is a special configuration in a line of convective storms that indicates the presence of a low-pressure area and the possibility of damaging winds, large hail, and tornadoes. At each kink along the LEWP is a mesoscale low-pressure area, which could contain a tornado. In response to very strong outflow southwest of the mesoscale low, a portion of the line bulges outward forming a bow echo. Behind this bulge lies the mesoscale high-pressure area.^[7]

Depiction on maps

Squall lines are depicted on National Weather Service (NWS) surface analyses as an alternating pattern of two red dots and a dash labelled "SQLN" or "SQUALL LINE".^[8]

Variations

Derecho

A derecho (from Spanish: "derecho" meaning "straight")^[9] is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms usually taking the form of a bow echo. Derechos blow in the direction of movement of their associated storms, similar to a gust front, except that the wind is sustained and generally increases in strength behind the "gust" front. A warm weather phenomenon, derechos occur mostly in summer, between May and August in the Northern hemisphere. They can occur at any time of the year and occur as frequently at night as in the daylight hours.^[10]

The traditional criteria that distinguish a derecho from a severe thunderstorm are sustained winds of 58 miles per hour (93 km/h) during the storm as opposed to gusts, high or rapidly increasing forward speed, and geographic extent (typically 250 nautical miles (500 km; 300 mi) in length.)^[10] In addition, they have a distinctive appearance on radar (bow echo); several unique features, such as the rear inflow notch and bookend vortex, and usually manifest two or more downbursts. Although these storms most commonly occur in North America, derechos occur elsewhere in the world. Outside North America they may be called by different names. For example, in Bangladesh and adjacent portions of India, a type of storm known as a "Nor'wester" may be a progressive derecho.^[10]

Related Research Articles

<span class="mw-page-title-main">Thunderstorm</span> Type of weather with lightning and thunder

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 sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.

A supercell is a thunderstorm characterized by the presence of a mesocyclone; a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms, supercells are the overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local weather up to 32 kilometres (20 mi) away. They tend to last 2–4 hours.

A mesocyclone is a meso-gamma mesoscale region of rotation (vortex), typically around 2 to 6 mi in diameter, most often noticed on radar within thunderstorms. In the northern hemisphere it is usually located in the right rear flank of a supercell, or often on the eastern, or leading, flank of a high-precipitation variety of supercell. The area overlaid by a mesocyclone’s circulation may be several miles (km) wide, but substantially larger than any tornado that may develop within it, and it is within mesocyclones that intense tornadoes form.

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 increase of the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.

<span class="mw-page-title-main">Derecho</span> Widespread, long-lived, straight-line wind storm

A derecho 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.

<span class="mw-page-title-main">Cyclogenesis</span> The development or strengthening of cyclonic circulation in the atmosphere

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.

An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.

<span class="mw-page-title-main">Mesoscale convective system</span> Complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

A bow echo is the characteristic radar return from a mesoscale convective system that is shaped like an archer's bow. These systems can produce severe straight-line winds and occasionally tornadoes, causing major damage. They can also become derechos or form Line echo wave pattern (LEWP).

An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or gust fronts of thunderstorms; some of the most dramatic arcus formations mark the gust fronts of derecho-producing convective systems. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.

Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes and these vary in methods of formation. Despite ongoing scientific study and high-profile research projects such as VORTEX, tornadogenesis is a volatile process and the intricacies of many of the mechanisms of tornado formation are still poorly understood.

A multicellular thunderstorm cluster is a thunderstorm that is composed of multiple cells, each being at a different stage in the life cycle of a thunderstorm. It appears as several anvils clustered together. A cell is an updraft/downdraft couplet. These different cells will dissipate as new cells form and continue the life of the multicellular thunderstorm cluster with each cell taking a turn as the dominant cell in the group.

The rear-inflow jet is a component of bow echoes in a mesoscale convective system that aids in creating a stronger cold pool and downdraft. The jet forms as a response to a convective circulation having upshear tilt and horizontal pressure gradients. The cold pool that comes from the outflow of a storm forms an area of high pressure at the surface. In response to the surface high and warmer temperatures aloft due to convection, a mid-level mesolow forms behind the leading edge of the storm.

Outflow, in meteorology, is air that flows outwards from a storm system. It is associated with ridging, or anticyclonic flow. In the low levels of the troposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud on weather satellite imagery or a fine line on weather radar imagery. For observers on the ground, a thunderstorm outflow boundary often approaches in otherwise clear skies as a low, thick cloud that brings with it a gust front.

Convective storm detection is the meteorological observation, and short-term prediction, of deep moist convection (DMC). DMC describes atmospheric conditions producing single or clusters of large vertical extension clouds ranging from cumulus congestus to cumulonimbus, the latter producing thunderstorms associated with lightning and thunder. Those two types of clouds can produce severe weather at the surface and aloft.

Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are fed by inflow focused along their cold front and warm fronts. Tropical cyclones require a large inflow of warmth and moisture from warm oceans in order to develop significantly, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere. Once the flow of warm and moist air is cut off from thunderstorms and their associated tornadoes, normally by the thunderstorm's own rain-cooled outflow boundary, the storms begin to dissipate. Rear inflow jets behind squall lines act to erode the broad rain shield behind the squall line, and accelerate its forward motion.

A mesovortex is a small-scale rotational feature found in a convective storm, such as a quasi-linear convective system, a supercell, or the eyewall of a tropical cyclone. Mesovortices range in diameter from tens of miles to a mile or less and can be immensely intense.

A line echo wave pattern (LEWP) is a weather radar formation in which a single line of thunderstorms presenting multiple bow echoes forms south of a mesoscale low-pressure area with a rotating "head". LEWP often are associated with a multiple-bow serial derecho and often produce tornadoes, some of which can be strong. The existence of a LEWP on radar means that a serial derecho has developed or is likely to develop soon, much as a hook echo indicates the same for a tornado.

The following is a glossary of tornado terms. It includes scientific as well as selected informal terminology.

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

References

↑ University of Oklahoma (2004). "The Norwegian Cyclone Model" (PDF). Archived from the original (PDF) on February 25, 2009. Retrieved May 21, 2017.
↑ Office of the Federal Coordinator for Meteorology (2008). "Chapter 2: Definitions" (PDF). NOAA. pp. 2–1. Archived from the original (PDF) on 2009-05-06. Retrieved 2009-05-03.
↑ Western Region Climate Center (2002). H. Archived 2017-05-21 at the Wayback Machine Desert Research Institute. Retrieved on 2006-10-22.
↑ Wake Low. American Meteorological Society. 2009. ISBN 978-1-878220-34-9. Archived from the original on 2011-06-06. Retrieved 2019-09-26.{{cite book}}: |work= ignored (help)
↑ Heat burst. American Meteorological Society. 2009. ISBN 978-1-878220-34-9. Archived from the original on 2011-06-06.{{cite book}}: |work= ignored (help)
↑ Johnson, R. H.; P. J., Hamilton (July 1988). "The relationship of surface pressure features to the precipitation and airflow structure of an intense midlatitude squall line". Mon. Wea. Rev. 116 (7): 1444–1472. Bibcode:1988MWRv..116.1444J. doi: 10.1175/1520-0493(1988)116<1444:TROSPF>2.0.CO;2 .
↑ Line echo wave pattern. American Meteorological Society. 2009. ISBN 978-1-878220-34-9 . Retrieved 2009-05-03.{{cite book}}: |work= ignored (help)
↑ Weather Prediction Center. "WPC Product Legends". National Weather Service . Retrieved September 3, 2015.
↑ Merriam-Webster's Spanish/English Dictionary (2009). Derecho. Merriam-Webster, Incorporated. Retrieved on 2009-05-03.
1 2 3 F. Corfidi; Jeffry S. Evans; Robert H. Johns (Feb 1, 2015). "About Derechos". Storm Prediction Center of the National Weather Service . Retrieved March 5, 2015.

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[OU-1] University of Oklahoma (2004). "The Norwegian Cyclone Model" (PDF). Archived from the original (PDF) on February 25, 2009. Retrieved May 21, 2017.

[2] Office of the Federal Coordinator for Meteorology (2008). "Chapter 2: Definitions" (PDF). NOAA. pp. 2–1. Archived from the original (PDF) on 2009-05-06. Retrieved 2009-05-03.

[3] Western Region Climate Center (2002). H. Archived 2017-05-21 at the Wayback Machine Desert Research Institute. Retrieved on 2006-10-22.

[4] Wake Low. American Meteorological Society. 2009. ISBN 978-1-878220-34-9. Archived from the original on 2011-06-06. Retrieved 2019-09-26.{{cite book}}: |work= ignored (help)

[5] Heat burst. American Meteorological Society. 2009. ISBN 978-1-878220-34-9. Archived from the original on 2011-06-06.{{cite book}}: |work= ignored (help)

[Johnson-6] Johnson, R. H.; P. J., Hamilton (July 1988). "The relationship of surface pressure features to the precipitation and airflow structure of an intense midlatitude squall line". Mon. Wea. Rev. 116 (7): 1444–1472. Bibcode:1988MWRv..116.1444J. doi: 10.1175/1520-0493(1988)116<1444:TROSPF>2.0.CO;2 .

[7] Line echo wave pattern. American Meteorological Society. 2009. ISBN 978-1-878220-34-9 . Retrieved 2009-05-03.{{cite book}}: |work= ignored (help)

[8] Weather Prediction Center. "WPC Product Legends". National Weather Service . Retrieved September 3, 2015.

[9] Merriam-Webster's Spanish/English Dictionary (2009). Derecho. Merriam-Webster, Incorporated. Retrieved on 2009-05-03.

[noaa-10] 1 2 3 F. Corfidi; Jeffry S. Evans; Robert H. Johns (Feb 1, 2015). "About Derechos". Storm Prediction Center of the National Weather Service . Retrieved March 5, 2015.

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