What Makes A Severe Thunderstorm Severe?

Later this week, one of the first signs of spring will be appearing, in the form of thunderstorms. It is likely that some of these storms will be severe, and there may even be a tornado or two. But not all thunderstorms are severe, and there doesn't necessarily have to be a severe thunderstorm watch issued, for a storm to be severe. There are a few specific criteria that a storm must meet for a severe thunderstorm warning to be issued. These criteria are:

1. Hail 1 inch in diameter or greater.

Hail can be measured using a ruler, or by comparison using standardized objects like coins. Marbles are NOT considered a good comparison because their size varies. Hail is commonly referred to as penny-size, nickel-size, dime-size, and quarter-size. For hail that is exceptionally large, comparison is made between golf balls, tennis balls, softballs, and in very rare occasions, basketballs. If hail golf ball size or larger is falling, it indicates that a storm is very well organized and likely has a rotating updraft. Any storm producing giant hail should be watched closely for signs of a possible tornado. Some NWS weather forecasting offices consider  .75"  hail as meeting severe criteria. But 1 inch is considered a good general rule of thumb. HAIL-12


2.  Wind speeds of 58mph or greater. Wind speeds may be estimated by using visual clues from the Beaufort Scale, or measured directly using an anemometer. windspeedchart  OR

3. A tornado is present on the ground. A funnel cloud is NOT a tornado, but may be a precursor to a tornado forming. Funnel clouds must be in contact with the ground to be considered a tornado.


If conditions are favorable for these criteria to occur, a severe thunderstorm watch, or tornado watch will be issued. If the current weather conditions meet the criteria listed above, (#1 AND #2, OR #3) a severe thunderstorm warning, or tornado warning will be issued. Please note that any severe thunderstorm has the potential for a tornado to develop regardless of a watch or warning being issued for it. Likewise, a severe thunderstorm may occur without a watch or warning being issued. Weather conditions can change rapidly, without notice, so always be weather aware during a period of active weather.