Non-Tornadic Severe Weather Phenomena
Recall that a downburst is defined as a strong downdraft with an outrush of damaging winds on or near the ground. Downbursts are subdivided based on their size. If the swath of damaging winds is 2.5 miles or greater in diameter, then it is termed a macroburst. If the swath is less than 2.5 miles, it is called a microburst. In general, microbursts are quick-hitting events and are extremely dangerous to aviation. Microbursts are sub-classified as dry or wet microbursts, depending on how much (or little) rain accompanies the microburst when it reaches the ground.
The formative stage of a microburst occurs as the downdraft begins its descent from the cloud base. The microburst accelerates downward, reaching the ground a short time later. The highest wind speeds can be expected shortly after the microburst impacts the ground. As the cold air of the microburst moves away from the center of the impact point, a "curl" will develop. Winds in this "curl" will accelerate even more, resulting in even greater danger to aircraft in the area. After several minutes, the microburst dissipates, but other microbursts may follow a short while later.
While spotting microbursts may not seem as dramatic as spotting tornadoes, it is important to the NWS, the public, and the aviation interests that microbursts be identified and reported. Listed below are some visual clues for identifying microbursts.
Patches of virga mark potential microburst formation areas. Virga is defined as precipitation which evaporates before reaching the ground. As the precipitation evaporates, it cools the air and starts a downdraft. If atmospheric conditions are right, the downdraft may accelerate and reach the ground as a microburst. Localized areas or rings of blowing dust raised from the ground usually mark the impact point of dry microbursts.
A small, intense, globular rain area, with an area of lighter rain in its wake, may mark a wet microburst. A rain foot, a marked outward distortion of the edge of a precipitation area, is also a visual indicator of a possible wet microburst . As the microburst reaches the ground and moves away from its impact point, a plume of dust may be raised from the ground. This plume is called a dust foot and also marks a possible microburst.
Recall that a flash flood is defined as a rapid rise in water usually associated with heavy rains from a thunderstorm. For many years, flash floods were the leading cause of death and injury among weather phenomena. Although casualty rates from flash floods are decreasing, many people still unnecessarily fall victim to flash floods.
The atmospheric conditions which cause flash floods have been found to be somewhat different from those which produce severe thunderstorms. The typical flash flood environment has abundant moisture through a great depth of the atmosphere. Low values of vertical wind shear are usually present. Flash flooding commonly occurs at night, rather than in the late afternoon or evening. Flash flooding is typically produced by either large, slow-moving storms or by "train effect" storms. The "train effect" occurs when several storms sequentially mature and drop their rainfall over the same area. This can occur when multicell cluster or squall line storms are present.
There are three types of flooding which may occur due to excessive rainfall over an area in a short period of time. The main difference lies in the terrain on which the rain falls. The first type is the classic "wall of water" which occurs in canyons and mountainous areas. In this type of flooding, rainwater rapidly runs off and is funneled into deep canyons and gorges, where it quickly rushes downstream. The second type, called "ponding," is common in relatively flat areas. The rainwater collects in drainage ditches and other low-water crossings and is particularly a problem in rural areas. The third type is "urban flooding." Extensive concrete and pavement in urban areas results in a large amount of rainwater runoffwhich collects in street intersections, underpasses, and dips in roads.
Last Modified: Mon June 29, 1998