VIII. NON-TORNADIC SEVERE WEATHER PHENOMENA

Downbursts

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.

Figure 25 shows the life cycle of a microburst. The formative stage of a microburst occurs as the downdraft begins its descent from the cloud base (figure 26). 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 (figure 27). As the cold air of the microburst moves away from the center of the impact point, a "curl" will develop (figure 28). 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.

Figure 26: Formative stage of a wet microburts. The downdraft (in the developing heavy rain area) is accelerating toward the ground. Photo - Bill Bunting

Figure 27: Impact stage of a wet microburst. This marks the most dangerous stage in the microburst's life. Photo - Bill Bunting.

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. This is shown in figure 28. A rain foot, a marked outward distortion of the edge of a precipitation area, is also a visual indicator of a possible wet microburst (figure 29). 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 (figure 30).

Figure 28: Dissipating stage of a wet microburst. The curl is still evident on the edges of the microburst's impact area. Photo - Bill Bunting.

Figure 29: A rain foot, an outward deflection of the rain shaft, may also suggest a wet microburst. View is to the west. Photo - Charles Doswell III.

Figure 30: Similar to the rain foot, the dust foot indicates the presence of a microburst. Photo - Alan Moller.

Flash Floods

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 m a large amount of rainwater runoff which collects in street intersections, underpasses, and dips in roads.

As mentioned in chapter II, it is difficult to set spotting and reporting guidelines regarding flash flooding. Local differences in geography, soil type and character, and urbanization result in widely varying amounts of runoff for a given amount of rain. Consult your local NWS office for guidelines regarding flash flooding in your area. Of course, keep the safety rules outlined in chapter III in mind anytime flash flooding is a possibility.
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