Additional Heavy Rain and Flooding Concerns in Central Texas; Tropical Storm Chantal to Impact the Coastal Carolinas

Heavy rainfall and flooding concerns remain for today across Central Texas and the Hill Country, with scattered thunderstorms expected this afternoon and evening. Tropical Storm Chantal has formed and will bring heavy rainfall and flooding concerns to portions of the coastal Carolinas. Life-threatening surf and rip currents are expected from the Southeast to Mid-Atlantic. Read More >

Severe Thunderstorms

What is a thunderstorm? This potentially complicated question can be answered in several ways, the most obvious being that it’s simply a “cloud that produces lightning.” A more descriptive, but equally simple definition is that a thunderstorm is “hot air going up.” A warm and humid (i.e., unstable) atmosphere is required for thunderstorm development. This is why spring and summer are the favored time of year for thunderstorms. If some atmospheric process forces the unstable air upward, the air will rise and cool until it saturates, causing a cloud to form. This rising air is called an “updraft.” As long as the updraft remains warmer than the surrounding environment, it will continue to rise, causing the cloud to grow vertically. On a typical day in the warm season, once a cloud grows to 20,000 to 30,000 feet, it will begin to produce heavy rain and lightning. The falling rain causes a “downdraft,” or sinking column of air to form. Therefore, a thunderstorm consists of two primary air currents: an updraft and a downdraft. The updraft takes the warm, humid air into the atmosphere; rain in the

Fig. 1. A “shelf cloud,” representing the leading edge of air spreading horizontally away from a thunderstorm downdraft. Wind gusts of 60 mph or more would likely be experienced along the leading edge and just to the rear of this cloud formation.

Fig. 2. Schematic of a mature thunderstorm, depicting the updraft (yellow arrows pointing toward top of the page) and the downdraft

downdraft brings cooler temperatures to the surface, thereby “stabilizing” the atmosphere.

A thunderstorm may eventually grow to a height of 50,000 feet or more before it stops developing. Generally speaking, the “taller” the storm, the more likely it is to produce violent weather.

Thunderstorms are characterized in two ways: general and severe. A Severe thunderstorm is one that produces large hail (i.e., one-inch diameter or larger), damaging wind gusts, and/or tornadoes.