Tornadoes sometimes form with mesovortices within squall lines (QLCS, quasi-linear convective systems), most often in middle latitudes regions. Mesocyclonic tornadoes may also form with embedded supercells within squall lines.
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Intense bow echoes responsible for widespread, extensive wind damage are called derechos, and move quickly over large territories. A wake low or a mesoscale low-pressure area forms behind the rain shield (a high pressure system under the rain canopy) of a mature squall line and is sometimes associated with a heat burst. Squall lines often cause severe straight-line wind damage, and most non-tornadic wind damage is caused from squall lines. Although the primary danger from squall lines is straight-line winds, some squall lines also contain weak tornadoes.
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If outflow boundaries or squall lines form over arid regions, a haboob may result. Squall lines are depicted on NWS surface analyses as an alternating pattern of two red dots and a dash labelled SQLN or SQUALL LINE, while outflow boundaries are depicted as troughs with a label of OUTFLOW BOUNDARY.
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Meteorology Online: Squall. Retrieved on 2006-11-19. Significant squall lines with multiple bow echoes are known as derechos.Robert H. Johns and Jeffry S. Evans.
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In the winter, squall lines can occur albeit less frequently – bringing heavy snow and/or thunder and lightning – usually over inland lakes (i.e. Great Lakes region).
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Severe thunderstorms can occur from any type of storm cell. However, multicell, supercell, and squall lines represent the most common forms of thunderstorms that produce severe weather.
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At sea, wind speeds typically reach level 8 on the Beaufort scale while waves become drastically higher when the bar reaches overhead and squall lines begin to arrive.
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Classic derechos occur with squall lines that contain bow- or spearhead-shaped features as seen by weather radar that are known as bow echoes or spearhead echoes. Squall lines typically "bow out" due to the formation of a mesoscale high pressure system which forms within the stratiform rain area behind the initial convective line. This high pressure area is formed due to strong descending air currents behind the squall line, and could come in the form of a downburst.Parke, Peter S. and Norvan J. Larson (2005).
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16 November Narrow Frontal Rain band Floods and severe weather. Retrieved on 2008-12-26. narrow, convective rainbands known as squall lines generally in the cyclone's warm sector, ahead of strong cold fronts associated with extratropical cyclones.
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How a squall line is depicted by the NWS on weather maps Squall lines are depicted on National Weather Service surface analyses as an alternating pattern of two red dots and a dash labelled "SQLN" or "SQUALL LINE".
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He finished his Ph.D. at Colorado State University (CSU) in Ft. Collins in 1992. At CSU he participated in more field work, including researching squall lines in Australia and his dissertation was titled Observational and Theoretical Study of Squall Line Evolution.
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These occur during the onset of rainy season when tumultuous winds accompanied by sheets of rain as well as spectacular thunder and lightning batter the coast. The tornadoes, however, were welcomed by settlers in the region since it dissipates extreme heat and humidity during the last days of the dry season. These tornadoes are often embedded in the African squall lines and produced by the severe thunderstorms that they bring. There are experts that attribute the formation of the tornado to the large hail, supported by wind shears in the northern part of the squall lines that veer and increase in height.
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Three-Dimensional Kinematics of Rainbands in Midlatitude Cyclones. Retrieved on 2008-12-24 and tend to be wide and stratiform in nature. Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes.Glossary of Meteorology (2009).
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Retrieved on 2008-03-02. Squall lines account for 30% of the large thunderstorm complexes which move through the region.Brian A. Klimowski and Mark R. Hjelmfelt (2000-08-11). Climatology and Structure of High Wind-Producing Mesoscale Convective Systems Over the Northern High Plains.
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Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform precipitation also occurs. Graupel and hail indicate convection. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts.
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Here also, wind velocities and magnitudes have a generally mild and low nature often varying from . Nevertheless, high velocities and magnitudes sometimes occur especially during the passage of squall lines associated with large rainy seasons or regular thunderstorms. These winds usually blow from a southwest to west direction.
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Frontal mechanisms, referred to then as "squall lines", were not yet understood. Surface observations were collected only twice daily at stations around the country, and by the time these data were collected and hand-drawn maps created, the information lagged actual weather conditions by hours.Brown, 2002, pp. 13, 19, 68.
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In addition, squall lines spawned multiple tornadoes, damaging evacuation shelters in Pascagoula and Gautier. Beach erosion occurred along the coastline, resulting in some property damage on beach houses. Around Biloxi, coastal casinos and the shipyards experienced little from the storm. Inland, high winds and flooding caused extensive damage to homes.
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Areas of precipitation help determine the frontal type and location. Mesoscale systems and boundaries such as tropical cyclones, outflow boundaries and squall lines are also analyzed on surface weather analyses. Isobars are commonly used to place surface boundaries from the horse latitudes poleward, while streamline analyses are used in the tropics.Bureau of Meteorology.
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A macroburst, normally associated with squall lines, is a strong downburst larger than . A wet microburst consists of precipitation and an atmosphere saturated in the low-levels. A dry microburst emanates from high-based thunderstorms with virga falling from their base. All types are formed by precipitation-cooled air rushing to the surface.
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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.
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The air masses separated by a front usually differ in temperature and humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage.
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At least three low-pressure systems formed and moved northward over the Great Plains. Adequate moisture and wind shear on the warm side of the system allowed for repeated rounds of severe and tornadic supercells and squall lines over a six day period. The outbreak ended as an surface anti-cyclone pushed through the region.
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By the evening hours, several squall lines developed across the region. Unstable air over much of Missouri allowed for these storms to maintain themselves as they moved slowly eastward. Around 7:00 p.m. CDT (0000 UTC), a rapidly moving vorticity maximum over Iowa resulted in a significant increase in thunderstorms over the area, some of which were tornadic.
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A group of thunderstorms over Brazil photographed by the Space Shuttle Challenger. This is the most common type of thunderstorm development. Mature thunderstorms are found near the center of the cluster, while dissipating thunderstorms exist on their downwind side. Multicell storms form as clusters of storms but may then evolve into one or more squall lines.
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Another strong tornado, rated EF3, resulted in houses destroyed near Eminence, Kentucky. Concentrated tornado activity also occurred in parts of Missouri and Illinois with 22 tornadoes in the region, some as strong as EF2, related to a series of squall lines with many embedded tornadoes. None of those tornadoes resulted in any fatalities. Overall damages from this system amounted to $12,800,000.
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Approaching weather fronts are often visible from the ground, but are not always as well defined as this. A weather front is a boundary separating bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage.
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Rainbands within tropical cyclones are curved in orientation. Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity. Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes.
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WSR-88D Distance Learning Operations Course, slide 30 Rotation associated with quasi-linear convective systems (QLCSs) or squall lines can trip the TVS but do so less reliably as the couplets typically are more transient, are shallower, smaller, and weaker. This rotation may be considered a mesovortex rather than a mesocyclone but these do produce tornadoes and damaging straight-line winds.
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"Barat" is a term for a northwest squall in Manado Bay in Sulawesi. "Sumatra" squall is a term used in Singapore and Peninsular Malaysia for squall lines that form over the island of Sumatra and move east across the Straits of Malacca. Gusts can reach up to 28 m/s (100 km/h).National Environment Agency Guide To Singapore's Weather, archived by archive.
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Most commonly, independent squalls occur along front lines, and may contain heavy precipitation, hail, frequent lightning, dangerous straight line winds, and possibly funnel clouds, tornadoes and waterspouts.Weatherquestions.com. What is a Squall Line? Retrieved on 2006-11-19. Squall lines require significant low-level warmth and humidity, a nearby frontal zone, and vertical wind shear from an angle behind the frontal boundary.
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Activity shifted into central Florida on March 30 as a frontal boundary stalled out across the state. During the two-day period, ten tornadoes touched down and a series of squall lines produced widespread wind damage on March 31. Significant damage took place in several communities and damage exceeded $5 million. Seven people were injured when one of the tornadoes hit a local festival.
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Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes which develop within the summer season are known as derechos, and they move quite fast through large sections of territory. On the back edge of the rainband associated with mature squall lines, a wake low can be present, sometimes associated with a heat burst.
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Cumulonimbus (from Latin cumulus, "heaped" and nimbus, "rainstorm") is a dense, towering vertical cloud, forming from water vapor carried by powerful upward air currents. If observed during a storm, these clouds may be referred to as thunderheads. Cumulonimbus can form alone, in clusters, or along cold front squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes and hailstones.
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By plotting the freezing line, isotherms can be useful in determination of precipitation type. Mesoscale boundaries such as tropical cyclones, outflow boundaries and squall lines also are analyzed on surface weather analyses. Isobaric analysis is performed on these maps, which involves the construction of lines of equal mean sea level pressure. The innermost closed lines indicate the positions of relative maxima and minima in the pressure field.
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Squall lines associated with Dean brought near-hurricane-force wind gusts and brief, heavy rainfall. Winds caused little effect other than isolated power outages, especially on the Outer Banks. Aside from the minor damage caused by Dean, there was also a positive side to the storm. Throughout coastal waters off North Carolina, king mackerel appeared in near-record numbers, improving the local fishing industry.
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Three cargo ships and seven boats ran aground and two other boats sank. In Antigua and Barbuda, winds from Omar were recorded at , with gusts to . Intense squall lines brought torrential rains, falling at rates of at times, peaking at per hour from 0900 UTC to 1200 UTC on October 16\. The maximum recorded rainfall was however, up to was estimated to have fallen in the mountains.
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By plotting the freezing line, isotherms can be useful in determination of precipitation type. Mesoscale convective systems such as tropical cyclones, outflow boundaries and squall lines also are analyzed on surface weather analyses. Isobaric analysis is performed on these maps, which involves the construction of lines of equal mean sea level pressure. The innermost closed lines indicate the positions of relative maxima and minima in the pressure field.
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In addition, squall lines spawned multiple tornadoes, damaging evacuation shelters in Pascagoula and Gautier. One location observed hurricane-force winds, with a sustained wind speed of at Keesler Air Force Base. In Harrison County, winds downed power lines, signs, and traffic lights, while damaging some roofs. Impact was similar but more severe in Jackson County, with more considerable damage to the roofs of some buildings, while a few mobile homes were damaged or overturned.
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H. Desert Research Institute. Retrieved on 2006-10-22. Well behind mature squall lines, a wake low can develop on the back edge of the rain shield, which can lead to a heat burst due to the warming up of the descending air mass which is no longer being rain-cooled. Smaller cumulus or stratocumulus clouds, along with cirrus, and, sometimes, altocumulus or cirrocumulus, can be found ahead of the squall line.
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Some derechos develop a radar signature resembling that of a hurricane in the low levels. They may have a central eye free of precipitation, with a minimum central pressure and surrounding bands of strong convection, but are really associated with an MCS developing multiple squall lines, and are not tropical in nature. These storms have a warm core, like other mesoscale convective systems. One such derecho occurred across the Midwestern U.S. on 21 July 2003.
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Where treefall gaps are usually formed by falling trees or branches due to old age or disease, large-scale blow-downs are caused by squall lines, which generate heavy rain and strong winds.Negrón‐Juárez, R. I., Chambers, J. Q., Guimaraes, G., Zeng, H., Raupp, C. F. M., Marra, D. M., Ribeiro, G. H. P. M., et al.(2010). Widespread Amazon forest tree mortality from a single cross‐basin squall line event. Geophysical Research Letters.
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Animated satellite cloud imagery is a better tool for their early detection and tracking. The low-level convergence caused by the cut-off low can trigger squall lines and rough seas, and the low-level spiral cloud bands caused by the upper level circulation are parallel to the low- level wind direction. This has also been witnessed with upper level lows which occur at higher latitudes. For example, in areas where small-scale snow bands develop within the cold sector of extratropical cyclones.
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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.
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Downbursts are created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. Dry downbursts are associated with thunderstorms with very little rain, while wet downbursts are created by thunderstorms with high amounts of rainfall. Microbursts and macrobursts are downbursts at very small and larger scales, respectively. Another variety, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking.
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The majority of deaths associated with Hilda in Louisiana were a result of tornadoes spawned by the hurricane in its outer peripheral rainbands and squall lines, which began tracking across the region on October 3. Six tornadoes and two waterspouts occurred in Louisiana due to the hurricane. Despite tracking for only near Larose, Louisiana, a violent F4 tornado killed 22 people and injured 165 others, destroying 35 homes in the process. Automobiles in the path of the twister were also damaged.
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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. 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 duststorm known as a haboob may result from the high winds in their wake picking up dust from the desert floor.Western Region Climate Center (2002).
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It then spreads outward in all directions. "Dry downbursts" are associated with thunderstorms with very little precipitation, while wet downbursts are generated by thunderstorms with large amounts of rainfall. Microbursts are very small downbursts with winds that extend up to 2.5 miles (4 km) from their source, while macrobursts are large-scale downbursts with winds that extend in excess of 2.5 miles (4 km). The heat burst is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking.
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A meso-beta scale vortex Mesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers. Examples of mesoscale weather systems are sea breezes, squall lines, and mesoscale convective complexes. Vertical velocity often equals or exceeds horizontal velocities in mesoscale meteorological systems due to nonhydrostatic processes such as buoyant acceleration of a rising thermal or acceleration through a narrow mountain pass.
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Picture of cold front (left part of the image) moving over the Czech Republic A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines.
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Tree damage from the EF1 tornado in Tennessee In Missouri, high winds caused widespread damage and knocked out power to 7,629 residences. Several homes and businesses lost their windows due to wind gusts produced within squall lines. In Huntsville, Alabama, a car carrying four people lost control on rain-slicked roads and crashed into a van, killing one of the occupants and injuring the other three. Another fatality occurred in Kentucky when a utility worker was knocked over by high winds and fell to his death while trying to restore power.
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Weather fronts mark the boundary between two masses of air of different temperature, humidity, and densities, and are associated with the most prominent meteorological phenomena. Strong cold fronts typically feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Such fronts form west of the circulation center and generally move from west to east; warm fronts form east of the cyclone center and are usually preceded by stratiform precipitation and fog. Warm fronts move poleward ahead of the cyclone path.
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Areas under and immediately adjacent to the downburst are the areas which receive the highest winds and rainfall, if any is present. Also, because the rain-cooled air is descending from the middle troposphere, a significant drop in temperatures is noticed. Due to interaction with the ground, the downburst quickly loses strength as it fans out and forms the distinctive "curl shape" that is commonly seen at the periphery of the microburst (see image). Downbursts usually last only a few minutes and then dissipate, except in the case of squall lines and derecho events.
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In winter, Lake effect snowfall occurs. In spring and summer, lake breezes can penetrate inland, creating narrow boundaries more inland causing cloudy conditions, severe thunderstorms, and convective rainfall events. This is known as the "lake breeze front" or "lake breeze thunderstorms" phenomenon, in which intense, sharply defined squall lines develop quickly on summer afternoons amplified by localized wind patterns between the Great Lakes. This is seen by the tendency for thunderstorms from the west to weaken/dissipate as they approach Toronto Pearson Airport, located in the southeastern part of the region.
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If squall lines form over arid regions, a duststorm known as a haboob can result from the high winds picking up dust in their wake from the desert floor. If outflow boundaries move into areas of the atmosphere which are stable in the low levels, such through the cold sector of extratropical cyclones or a nocturnal boundary layer, they can create a phenomenon known as an undular bore, which shows up on satellite and radar imagery as a series of transverse waves in the cloud field oriented perpendicular to the low-level winds.
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Tornadoes, which form within stronger thunderstorms, grow until they reach their mature stage. This is when the rear flank downdraft of the thunderstorm, fed by rain-cooled air, begins to wrap around the tornado, cutting off the inflow of warm air which previously fed the tornado. Inflow can originate from mid-levels of the atmosphere too. When thunderstorms are able to organize into squall lines, a feature known as a rear inflow jet develops to the south of the mid-level circulation associated with its northern bookend vortex.
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These methods modify the covariance matrix used in the computations and, consequently, the posterior ensemble is no longer made only of linear combinations of the prior ensemble. For nonlinear problems, EnKF can create posterior ensemble with non-physical states. This can be alleviated by regularization, such as penalization of states with large spatial gradients. For problems with coherent features, such as hurricanes, thunderstorms, firelines, squall lines, and rain fronts, there is a need to adjust the numerical model state by deforming the state in space (its grid) as well as by correcting the state amplitudes additively.
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Mesoscale meteorology is the study of atmospheric phenomena that has horizontal scales ranging from 1 km to 1000 km and a vertical scale that starts at the Earth's surface and includes the atmospheric boundary layer, troposphere, tropopause, and the lower section of the stratosphere. Mesoscale timescales last from less than a day to weeks. The events typically of interest are thunderstorms, squall lines, fronts, precipitation bands in tropical and extratropical cyclones, and topographically generated weather systems such as mountain waves and sea and land breezes.Online Glossary of Meteorology, American Meteorological Society , 2nd Ed., 2000, Allen Press.
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In North America these storms most prominently form in the semi-arid Great Plains during the spring and summer months. Moving east and southeast, they often collide with moist air masses from the Gulf of Mexico, leading to the formation of HP supercells in areas just to the west of Interstate 35 before dissipating (or coalescing into squall lines) at variable distances farther east. LP supercells have been observed as far east as Illinois and Indiana, however. LP supercells can occur as far north as Montana, North Dakota, and even in the Prairie Provinces of Alberta, Saskatchewan, and Manitoba in Canada.
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Another signature associated with this situation is the Enhanced-V feature where the cold cloud tops forming at the overshooting top fan out in a V shape as cloud matter is blown downwind at that level. Both features can be seen on visible satellite imagery, during daytime, by the shadows they cast on surrounding clouds. In multicellular storms and squall lines, the mid-level jet stream is often intersecting the line and its dry air introduced into the cloud is negatively unstable. This results in drying of the cloudy air in the region where the jet plunge groundward.
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Summer heat episodes are usually broken by cooler, drier periods if only for brief periods. Thunderstorms are also a regular occurrence and can pop up quickly, especially west and north of the city in areas more prone to the "lake breeze front" or "lake breeze thunderstorms" phenomenon, in which intense, sharply defined squall lines develop quickly on summer afternoons amplified by localized wind patterns between the Great Lakes.Lake Breeze Weather These storms sometimes, but not always move into the city causing localized flooding resulting from downpours with high rainfall amounts, intense lightning but less often severe winds knocking down trees and powerlines.
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Brief funnels also are observed with some rear flank downdrafts (RFDs) (within inflow or outflow areas, and especially within inflow-outflow interchange areas as RFDs interact with mesocyclones or flanking line updrafts) and streamwise vorticity currents (SVCs) feeding into mesocyclones. Although not considered a separate kind of funnel cloud, some funnel clouds form with mesovortices associated with squall lines, which also can become tornadoes but are often not visible as funnel clouds or tornadoes because they usually occur within obscuring precipitation. Other "fair weather" funnel clouds include horseshoe clouds which are a very transient phenomena associated with extremely weak vortices.
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Jeffrey W. Frame is an American atmospheric scientist known for observational and modeling studies of severe convective storms and for teaching meteorology. He was a scientist for VORTEX2 and other field research programs. Frame is from Michigan and graduated summa cum laude from the University of Michigan (UM) with a B.S. in Atmospheric, Oceanic, and Space Sciences in 2001. Moving on to the Pennsylvania State University (PSU) for graduate studies, he earned a M.S. in 2003 with the thesis The Interaction of Simulated Squall Lines with Complex Terrain and a Ph.D. in 2008 with the dissertation The Dynamical Impact of Anvil Shading on Simulated Supercell Thunderstorms.
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The outflow boundary indicated by the presence of this shelf cloud preceded a derecho in Minnesota For thunderstorms, outflow tends to indicate the development of a system. Large quantities of outflow at the upper levels of a thunderstorm indicate its development. Too much outflow in the lower levels of a thunderstorm, however, can choke off the low-level inflow which fuels it. Squall lines typically bow out the most, or bend the most convex outward, at the leading edge of low level outflow due to the formation of a mesoscale high-pressure area which forms within the stratiform rain area behind the initial line.
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Thunderstorms, squall lines, drylines, sea and land breezes, mountain breeze and valley breezes, mountain waves, mesolows and mesohighs, wake lows, mesoscale convective vortices (MCVs), tropical cyclone and extratropical cyclone rainbands, macrobursts, gust fronts and outflow boundaries, heat bursts, urban heat islands, and other mesoscale phenomena can cause weather conditions in a localized area to be significantly different from that dictated by the ambient large-scale conditions. As such, meteorologists need to understand these phenomena in order to improve forecast skill. Observations are critical to understanding the processes by which these phenomena form, evolve, and dissipate. The long-term observing networks (ASOS, AWOS, Coop), however, are too sparse and report too infrequently for mesoscale research.
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Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low pressure areas are present. Some bow echoes in the summer are called derechos, and move quite fast through large sections of territory. On the back edge of the rain shield associated with mature squall lines, a wake low can form, which is a mesoscale low pressure area that forms behind the mesoscale high pressure system normally present under the rain canopy, which are sometimes associated with a heat burst. This kind of storm is also known as "Wind of the Stony Lake" (Traditional Chinese:石湖風 – shi2 hu2 feng1, Simplified Chinese: 石湖风) in southern China.
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MCC moving through New England: August 2, 2006 0600 UTC 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 they generally form near weather fronts. Most mesoscale convective systems develop overnight and continue their lifespan through the next day. They tend to form when the surface temperature varies by more than between day and night.
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The development supercells and later squall lines was anticipated, and was also expected to impact the Mid- Atlantic states. At around midday on November 15, the SPC issued a slight risk for severe weather for the 16th in regions concentrated primarily around Iowa and Missouri, forecasting the possibility of storms particularly during the evening to overnight hours of November 16. The system associated with those storms was to track eastward and strengthen, and as a result the SPC also issued a Day 3 slight risk earlier on November 15 for a large swath of the Eastern United States in effect for November 17\. The slight risk included a 30% chance of severe thunderstorm activity was forecast for an area concentrated on Indiana and Ohio.
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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. 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, or 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.
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Mesoscale convective systems are thunderstorm regions which may be round or linear in shape, on the order of or more across in one direction but smaller than extratropical cyclones, and include systems such as tropical cyclones, squall lines, and Mesoscale Convective Complexes (MCCs), among others. MCS is a more generalized term which includes systems that do not satisfy the stricter size, shape, or duration criteria of an MCC. They tend to form near weather fronts and move into areas of 1000-500 mb thickness diffluence, which are areas where the low to mid level temperature gradient broadens, which generally steers the thunderstorm clusters into the warm sector of extratropical cyclones, or equatorward of warm fronts. They can also form along any convergent zones within the tropics.
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Both tornadoes were rated as EF2. The same storm system and its associated cold front produced a bow echo that traveled through Michigan, most of Southern Ontario and parts of Southern Quebec where a second squall line formed in eastern Ontario. With the squall lines, one F0 tornado was confirmed by Environment Canada investigators in Ottawa, Ontario tracking about through the Britannia, Lincoln Heights and Carlington Heights communities where it tore off the roofs of two apartment complexes, downed numerous trees, power lines and signs. Widespread heavy damage was reported elsewhere including to planes at the Rockcliffe Airport as well as trees, power lines, street signs and several other buildings including houses and a school elsewhere across the city and in Gatineau, Quebec due to winds locally at .
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Wake lows form due to adiabatic warming in the wake of mature squall lines at the back edge of their rain shields, where evaporative cooling is unable to offset warming due to atmospheric subsidence, or downward motion. They can be caused by gravity waves which duct through boundary layers which are deep and cold to the north of a weather front. As with mesoscale high-pressure areas behind a squall line, when new thunderstorm development stops along the squall line, the wake low will weaken as well. Clearing conditions will accompany the wake low, due to the descending warm air mass associated with the feature. Within the United States, these systems have been observed to form in the Mississippi river valley, Southeast,National Weather Service Office Huntsville, Alabama (2009).
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Rasmussen became a research meteorologist at the National Severe Storms Laboratory (NSSL) and then the Cooperative Institute for Mesoscale Meteorological Studies (CIMMS). After the study of squall lines his interest returned to supercells from the microphysical aspects of cloud particles to mesoscale environments modulating storm behavior. He was the field commander (FC) of Project VORTEX in 1994-1995 where he worked with lead forecaster Charles A. Doswell III, participated in SUB-VORTEX and VORTEX-99, STEPS, IHOP, and served on the steering committee and was a lead principal investigator (PI) for VORTEX2 in 2009-2010VORTEX2 as well as project manager for VORTEX-SE in 2016-2017. Since his college days Rasmussen was a major contributor to Storm Track magazine although by the mid-1990s his previously intense interest in storm chasing was waning.
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Severe weather, in form of strong straight-line winds can be expected in areas where the squall line itself is in the shape of a bow echo, within the portion of the line which bows out the most. Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low pressure areas are present. Some bow echoes that develop within the summer season are known as derechos, and they move quite fast through large sections of territory. On the back edge of the rain shield associated with mature squall lines, a wake low can form, which is a mesoscale low pressure area that forms behind the mesoscale high pressure system normally present under the rain canopy, which are sometimes associated with a heat burst.
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A weather map consisting of a station model plot of Oklahoma Mesonet data overlaid with WSR-88D weather radar data depicting possible horizontal convective rolls as a potential contributing factor in the incipient 3 May 1999 tornado outbreak In meteorology (and climatology), a mesonet, portmanteau of mesoscale network, is a network of (typically) automated weather and environmental monitoring stations designed to observe mesoscale meteorological phenomena. Dry lines, squall lines, and sea breezes are examples of phenomena that can be observed by mesonets. Due to the space and time scales associated with mesoscale phenomena, weather stations comprising a mesonet will be spaced closer together and report more frequently than synoptic scale observing networks, such as ASOS. The term mesonet refers to the collective group of these weather stations, and are typically owned and operated by a common entity.
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By the late evening, most of the discrete storms that existed earlier in the day had merged into squall lines and bow echoes, with any remaining mesocyclones embedded within these bands rather than individual storms. Forced along the extent of a cold front, the primary squall line raced eastward across Ohio at approximately 60 mph (95 km/h), reaching Pennsylvania by 0200 UTC. As a result of decreased atmospheric instability, the threat for tornadoes and hail had greatly diminished by this time, while the probability for damaging winds remained elevated to high; as a result, watches issued east of Ohio were severe thunderstorm watches rather than tornado watches. The final tornado associated with the outbreak was an EF0 that dissipated at 0340 UTC in Lincoln County, Tennessee (south of the area impacted by the main outbreak of tornadoes); based on Central Standard Time, this would have still occurred on November 17.
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Hodograph plot of wind vectors at various heights in the troposphere, which is used to diagnose vertical wind shear Wind shear, sometimes referred to as wind gradient, is a difference in wind speed and direction over a relatively short distance in the Earth's atmosphere. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across weather fronts and near the coast, and vertical shear typically near the surface, though also at higher levels in the atmosphere near upper level jets and frontal zones aloft. Wind shear itself is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near microbursts and downbursts caused by thunderstorms, weather fronts, areas of locally higher low level winds referred to as low level jets, near mountains, radiation inversions that occur because of clear skies and calm winds, buildings, wind turbines, and sailboats.
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Squall lines, or solid bands of strong thunderstorms, can form ahead of cold fronts and lee troughs due to the presence of significant atmospheric moisture and strong upper level divergence, leading to hail and high winds. When significant directional wind shear exists in the atmosphere ahead of a cold front in the presence of a strong upper-level jet stream, tornado formation is possible. Although tornadoes can form anywhere on Earth, the greatest number occur in the Great Plains in the United States, because downsloped winds off the north–south oriented Rocky Mountains, which can form a dryline, aid their development at any strength. Explosive development of extratropical cyclones can be sudden. The storm known in Great Britain and Ireland as the "Great Storm of 1987" deepened to with a highest recorded wind of , resulting in the loss of 19 lives, 15 million trees, widespread damage to homes and an estimated economic cost of £1.2 billion (US$2.3 billion).
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