Severe Weather Awareness Week is underway in Alabama and we’ve certainly had to consider many safety measures recently. Be sure to check out the survey results from the National Weather Service in the previous post. Major damage occurred throughout portions of central Alabama as severe thunderstorms produced tornadoes yesterday. An EF-3 was classified to have hit Autauga County where 200 homes and 40 businesses were damaged. There were 50 injuries in that particular county, but the blessing is that there were no fatalities. Severe thunderstorms are what produce tornadoes and they need to be taken seriously. In fact, a majority of wind related damage in Chilton County yesterday was a result of thunderstorm straight-line winds. Today’s severe weather topic is on the power of thunderstorms and I wanted to share some thoughts. All thunderstorms produce lightning because lightning is what creates thunder and it’s important to note that thunder is natures way of saying seek shelter from an approaching thunderstorm. Even though we have criteria for a severe thunderstorm, I consider all storms severe with regard to the threat of cloud to ground lightning.
Thunderstorms go through various stages of development and there are different types of storms, such as multicell clusters, squall lines, or supercell storms. Thunderstorms are created by forced mechanical lift or by simple convective heating, such as warming of the earth’s surface. As this air rises, vertical air motions lead to the development of cumulus clouds. This is referred to as the “cumulus stage” of thunderstorm development. Secondly, as the air rising into cumulus clouds lifts into the middle layers of the atmosphere and as the air condenses, dry air aloft leads to evaporation. The evaporation process cools the air and this cooler air pools towards earths surface, resulting in a downdraft. These downdrafts can sometime be quite violent and you may have heard the term “downburst” before. This term is used to describe a severe downdraft of wind that sinks and spreads below the cloud base, along earth’s surface. Damage from downbursts is often evident by objects or trees toppled in a uniform direction. Winds can exceed 100 mph, which is as strong as a small tornado.
When an updraft and downdraft are present within a thunderstorm it is then in the “mature stage” of growth. This is when large hail, heavy rain, and lightning can be observed. If upper air winds are weak, thunderstorms may tend to weaken quickly as the downdraft may cut off the warm inflow of air or updraft. This is quite common with summer time single-cell thunderstorms. This stage is known as the dissipating stage however, the cool air reaching the surface can act like a mini cold front and ignite more thunderstorm development. The Supercell Thunderstorm is more tilted with height and can sustain itself for a longer portion of time. This tilt is due to faster winds aloft and that’s why you may hear us talk about upper level support with regard to thunderstorms or severe weather development. This presence of fast upper air winds can be evident by the large anvil that extends from the upper portion of some classic storms.
Severe thunderstorm watches are issued by the Storm Prediction Center and this does not mean there is a storm approaching, but this means that conditions are favorable for severe thunderstorms. A severe thunderstorm warning is issued when a storm has been sighted or detected and it is approaching your community. With the addition of polygon warnings, communities in the direct path of the storm will be warned by local media and the National Weather Service.
(Image Source: NOAA)
Wes Wyatt
WVUA Chief Meteorologist
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