Severe Weather

By John Wilson, Extension Educator

Last week, March 21-25, was Nebraska Severe Weather Awareness Week. I think it is interesting that we have a whole week designated as Severe Weather Awareness Week, but only one day in November designated as Winter Weather Awareness Day.

Since we are just entering the severe weather season, I thought this would be a good time to review terminology used with severe weather and the appropriate actions required with each. In general, watches indicate conditions are favorable for the development of certain weather conditions. Usually these cover a large area and don’t require immediate action, but let people know they should keep advised of developing weather conditions.

On the other hand, warnings indicate that the weather condition is occurring, is imminent, or has been indicated by radar or confirmed by a trained weather spotter. In the case or a warning, you should take immediate action to protect yourself and others.

A severe thunderstorm watch means that the potential exists for the development of thunderstorms which may produce large hail or damaging winds. When a watch is issued, you can go about your normal activities, but keep an eye to the sky and an ear to a weather radio or your local radio and television stations for further updates and possible warnings.

A severe thunderstorm warning, on the other hand, means that a severe thunderstorm is occurring or is imminent, based on doppler radar information. You should move indoors to a place of safety. The term severe refers to hail that is quarter size, 1.0 inch in diameter, or larger and/or wind gusts to 58 mph or more. If golf ball size hail, about 1.6 inches in diameter, 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 monitored closely for signs of a possible tornado.

Although lightning can be deadly it is not a criterion for what the National Weather Service defines as severe since any ordinary thunderstorm can produce lots of lightning. Also, excessive rainfall may lead to flash flooding, but heavy rain is not a criterion for the term severe. Severe strictly refers to hail at least one inch in diameter or wind gusts of at least 58 mph.

A tornado watch, like a severe thunderstorm watch, means that conditions are favorable for severe thunderstorms to form, but it also means that a few storms may be capable of producing a tornado. A tornado warning is the ultimate in severe warnings, it means that a tornado is either occurring or imminent based on radar. You should take cover immediately.

A flash flood watch is issued when conditions are favorable for flash flooding in flood-prone areas, usually when grounds are already saturated from recent rains, or when upcoming heavy rains will have the potential to cause a flash flood.

A flash flood warning is issued when a flash flood is imminent or occurring in the warned area. A flash flood is a sudden, violent flood after a heavy rain, especially when runoff is channeled through narrow valleys or ditches. Rainfall intensity and duration, topography, soil conditions, and ground cover contribute to flash flooding.

For more information on weather watches and warnings, visit the National Weather Service website at www.weather.gov or the National Oceanic and Atmospheric Administration, or NOAA, website at www.noaa.gov.

John Wilson