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Severe Thunderstorm Criteria

Source:, Featuring Scott Dennstaedt

Subscriber question:

"The Aeronautical Information Manual (AIM) suggests that pilots avoid severe thunderstorms by 20 miles. How do you know while enroute if a thunderstorm is severe?" - Gail C.

weather image


"There's no question that all thunderstorms should be avoided by pilots, severe or not. But as the AIM suggests, severe thunderstorms demand a wide berth. Visually, it may be difficult or fundamentally impossible to ascertain severity while enroute given that many thunderstorms tend to have a similar appearance when viewed from a distance. However, the National Weather Service (NWS) does identify thunderstorms that are severe when they meet certain criteria.

The NWS identifies a thunderstorm as severe whenever it is producing or is expected to produce a tornado, wind gusts in excess of 58 miles per hour (50 knots) or hail that is at least 1 inch (quarter-size) in diameter. Notice that the criteria for severity do not include lightning or heavy precipitation. When a thunderstorm meets one or more of these minimum criteria, the local NWS forecast office will issue a severe thunderstorm or tornado warning accordingly.

Before you depart, you can see any current warnings on the local NWS forecast office's website. They are depicted as red (tornado) or yellow (severe thunderstorm) polygons on the NEXRAD radar display ( Normally, severe thunderstorm or tornado warnings are not broadcast to pilots.

Along with other criteria, convective SIGMETs are issued for thunderstorms identified as severe. Therefore, if the local weather forecast office issues a severe thunderstorm or tornado warning, the Aviation Weather Center will subsequently issue a convective SIGMET, be it a line, area or single cell. The text of the convective SIGMET will identify the thunderstorms as severe. A convective SIGMET is broadcast over HIWAS and is also available through flight watch (EFAS) or FSS. You may also be able to get the latest convective SIGMETs assuming you have an active datalink weather display in the cockpit."

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