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Shelf cloud signs of a tornado8/26/2023 ![]() This shelf cloud preceded a derecho in MinnesotaĪt ground level, shelf clouds and roll clouds can be seen at the leading edge of outflow boundaries. In the extreme case, a derecho can cover a huge area more than 200 miles (320 km) wide and over 1,000 miles (1,600 km) long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds, but the generative process is somewhat different from that of most downbursts. All types are formed by precipitation-cooled air rushing to the surface. A dry microburst emanates from high-based thunderstorms with virga falling from their base. A wet microburst consists of precipitation and an atmosphere saturated in the low-levels. ![]() This feature is commonly depicted in the warm season across the United States on surface analyses, as they lie within sharp surface troughs.Ī macroburst, normally associated with squall lines, is a strong downburst larger than 4 kilometres (2.5 mi). When the convection is strong and 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 which is normally just ahead of its radar signature. The convection then moves east and toward the equator into the warm sector, parallel to low-level thickness lines. ![]() 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. This outrunning occurs within the westerlies in a pattern where the upper-level jet splits into two streams. Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. Microburst soundings show the presence of mid-level dry air, which enhances evaporative cooling. They are normally associated with individual thunderstorms. The term was defined as affecting an area 4 kilometres (2.5 mi) in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. Definition Ī microburst is a very localized column of sinking air known as a downburst, producing damaging divergent and straight-line winds at the surface that are similar to but distinguishable from tornadoes which generally have convergent damage. When an outflow boundary moves into a more stable low level environment, such as into a region of cooler air or over regions of cooler water temperatures out at sea, it can lead to the development of an undular bore. Wet microbursts occur in atmospheres where the low levels are saturated, while dry microbursts occur in drier atmospheres from high-based thunderstorms. Microbursts have a diameter of influence less than 4 kilometres (2.5 mi), while macrobursts occur over a diameter greater than 4 kilometres (2.5 mi). Strong versions of these features known as downbursts can be generated in environments of vertical wind shear and mid-level dry air. If a thunderstorm runs into an outflow boundary, the low-level wind shear from the boundary can cause thunderstorms to exhibit rotation at the base of the storm, at times causing tornadic activity. Outflow boundaries create low-level wind shear which can be hazardous during aircraft takeoffs and landings. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary ( cold front, dry line, another outflow boundary, etc.). 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. Outflow boundary on radar with radial velocity and frontal boundary drawn in.Īn 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.
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