A Severe Thunderstorm Watch outlines an area where hail 3/4 inch diameter or larger hail and damaging thunderstorm winds are expected to occur during a three to six hour period. Typical watches cover about 25,000 square miles, or about half the size of Iowa.

A Severe Thunderstorm Warning means severe thunderstorms that contain 3/4 hail and damaging winds in excess of 58 mph are occuring or are imminent. Seek shelter immediately if a severe thunderstorm warning is issued for your area.

There are many forms of hazardous, or severe weather. The primary mission of the National Weather Service is to provide forecasts and warnings for the protection of life and property. Forecasts can range from the climate and seasonal outlooks for agricultural interests provided by the Climate Prediction Center (www.cpc.ncep.noaa.gov), to the daily county forecasts for the public issued by local NWS offices. The NWS issues warnings for heavy snow, freezing rain, high winds, flash flooding, river flooding, thunderstorms, tornadoes, tropical storms, and hurricanes. In a broad sense, these are all defined as severe weather as any of them can and do pose a threat to life and property.

How does the National Weather Service (NWS) define a severe thunderstorm?

The term severe thunderstorm refers to a thunderstorm producing hail that is dime size, 0.75 inches in diameter or larger, and/or wind gusts to 58 mph or greater, and/or a tornado. Although lightning can be deadly it is not a criterion for what the NWS defines as severe thunderstorm since any ordinary thunderstorm can produce a lot of lightning. Also, excessive rainfall may lead to flash flooding, but heavy rain is not a criterion for a severe thunderstorm.

What is organized severe weather and why is it more important than non-organized severe weather?

To some degree, all thunderstorms are organized. We refer to "organized" severe storms as those which persist for several hours and we are able to forecast consistently. The most long-lived, intense tornadoes and largest hail are usually produced by supercell thunderstorms, while the most serious convective wind storms are produced by bow echoes. The most serious severe storms tend to form in areas where the vertical wind shear is relatively strong and has particular characteristics, while shorter-lived storms are most common when vertical wind shear is weak.

What environmental parameters are most often observed with organized severe weather?

 
In general, severe thunderstorms form in areas where moisture, instability, and lift are present. Additionally, long-lived severe thunderstorms are often associated with strong vertical wind shear (e.g., winds that change direction and increase in speed with height). Unfortunately, thunderstorm initiation is not well-understood, and weather observations are too widely spaced to detect all important features.