The NWS Just Made an Important Change to Severe Thunderstorm Warnings

 Chris Dolce

Published: August 2, 2021

National Weather Service severe thunderstorm warnings for the highest-impact threats of destructive winds or large hail will now automatically activate emergency alerts on mobile devices.

A severe thunderstorm warning is issued by the NWS when a storm is capable of producing wind gusts of at least 58 mph and/or hail at least 1 inch in diameter – roughly the size of a quarter. Sometimes, these warnings indicate a tornado is also possible.

Severe thunderstorm warnings are commonly issued each year across the United States based on those criteria, especially east of the Rockies, where warm, humid air is often found during the spring and summer. You can see this on the map below, which shows the number of severe thunderstorm warnings each NWS office issued in 2020. In many cases, the number of warnings issued was over 100 per office.

Severe thunderstorm warnings issued by local NWS forecast offices in 2020.

While all severe thunderstorm warnings should prompt you to take precautions in order to protect life and/or property, a smaller number of them indicate there is a higher-end threat of damaging winds and/or large hail.

Beginning Monday, the NWS will tweak how it issues severe thunderstorm warnings so that those higher-impact warnings receive a greater level of attention.

Severe thunderstorm warnings will now be placed in one of three threat level categories, according to a release from NOAA.

The first warning category will be for a storm (or line of storms) that meets the minimum severe thunderstorm criteria. These warnings will be issued much like they are now.

Warnings for a storm (or line of storms) with a threat of significantly stronger winds and/or large hail will be placed in one of two higher-end categories. The text of these warnings will state the thunderstorm damage threat is either "considerable" or "destructive," based on the minimum wind and/or hail criteria listed below.

Warnings in the highest category - "destructive" - will automatically activate a Wireless Emergency Alert (WEA) on mobile devices within the warned area. This emergency alert will be accompanied by a loud screech that will wake you when a particularly dangerous severe weather threat is in your area.

Other NWS warnings already trigger a WEA, including tornado warnings and higher-end flash flood warnings.

An example of a "destructive" severe thunderstorm warning that would trigger a WEA is shown below. The potential for hail up to 3 inches in diameter - larger than baseballs - is what triggered the destructive wording in this case.

Sample severe thunderstorm warning in the destructive category.

According to the NWS, only 10% of all severe thunderstorms reach the "destructive" criteria each year across the country.

The billion-dollar Aug. 10, 2020, Midwest derecho is an example of when the thunderstorm damage threat from high winds would be classified as "destructive" in severe thunderstorm warnings. This severe thunderstorm warning from that day in Iowa called for wind gusts up to 90 mph, clearing the 80 mph minimum criteria for that highest category.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.