Autumn Temperatures for Eastern Third of the Country; Super Typhoon Approaching Guam

Cooler and less humid conditions for the eastern third of the nation as a cold slides off the east coast. Combination of dry and breezy conditions may result in spread of wildfires across the lower Mississippi Valley. In addition, fire weather conditions linger into portions of Southern California. For Guam, a super typhoon is approaching. 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.