Weather Safety: What Weather Is The Most Likely To Create A Tornado?

This article discusses what weather is most likely to produce tornadoes, when they occur most often, and some terms used when forecasting severe weather.

Ahh, spring, when the thoughts turn lightly to musings on "" tornadoes. These violent windstorms are a fact of life for those living in the Midwest and Southeast, where they are most common. Most people who live in these areas seem to have a sixth sense for the conditions that usually presage a tornado outbreak and will start turning on their weather radios and keeping an eye on the television.

The classic "recipe" for a tornado is when cool, dry air meets warm, moist air. This creates an unstable air mass, and is prone to producing thunderstorms. However, not every thunderstorm, even a severe one, will produce a tornado. Most tornadoes come from supercells, which are large thunderstorms having a defined circulation, or "mesocyclone." These storms produce tornadoes more frequently because of that rotating air. Even with current technology, however, the best meteorologists cannot predict with 100 percent accuracy which supercells will produce tornadoes and which will not. It is also impossible to predict how strong a given tornado might be, although an experienced severe weather meteorologist might venture an educated guess, based on what he or she sees on radar.

Tornadoes occur most often in the spring, and in the Southeast, in the fall, because these are the seasons when the weather varies the most in short periods of time. Chilly spring days are often followed by very warm weather, and the variance in temperatures ripens the air for mesocyclone formation. When the November, 1989 tornado hit Huntsville, Ala., the temperatures were in the 80s that afternoon. By the time the cold front that had spawned the storms had passed through that evening, snow was falling in the area, and temperatures had plummeted into the low 30s. This is not necessarily unusual in the Southeast in the fall. This kind of unstable weather is ideal for tornadic development.

When the National Weather Service deems a tornado outbreak likely, the Storms Prediction Center arm of the NWS issues a tornado watch for the area. The SPC will also usually issue a severe weather risk advisory, stating whether a given area has a mild, moderate or high risk of severe weather for a 24-hour period. This is based on their analysis of temperature and wind patterns that, in their experience, are likely to produce a severe weather outbreak. Rarely, the SPC will outlook a situation as a "particularly dangerous situation." This means the atmospheric conditions are extremely unstable and are liable to produce large, destructive tornadoes.

A tornado watch covers a large area and is issued several hours in advance of the approaching severe weather. It means that conditions are, or will become, favorable for tornado formation. The severe weather usually strikes an area close to the end of the watch period. The SPC issues the watch early so residents in the affected area will have time to prepare for bad weather.

A severe thunderstorm warning often precedes a tornado warning. These are issued by the local National Weather Service offices and mean that a large thunderstorm is moving through a specific county. A severe thunderstorm has winds in excess of 55 mph and often produce hail and intense cloud-to-ground lightning. These storms can, and sometimes do, produce tornadoes with little or no warning, meaning residents should keep an eye on the storm.

When a tornado watch is issued for an area, the county's Emergency Management Agency will usually activate its spotter network. These are people who have been trained to visually recognize tornado formation. They are usually Ham radio operators and can get the information to EMA authorities.

If a spotter calls in a funnel cloud sighting, or a tornado vortex signature is indicated on radar, the NWS office will issue a tornado warning for the county. The warning will include information about the tornado's speed and direction of movement. This will prompt warning sirens to sound and weather radios to alert.

Tornadoes are capricious storms, and while most move from southwest to northeast, this is not always the case. Residents in an affected county should carefully watch and listen to weather reports for their specific area, since tornadoes usually do localized damage. Most do not stay on the ground long "" rarely for more than a few minutes at a time.

A wealth of information about tornadoes can be found on the Storms Prediction Center Web site, along with some spectacular photos of funnel clouds.

When the air turns warm and moist, and there is cool, dry air behind it, a wise person will be alert to the possibility of severe weather.

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