Many people in Southeast Wisconsin were awoken early last Thursday morning (July 29th) by the sound of a Wireless Emergency Alert (WEA) on their phone. Tornado Warnings, issued by the National Weather Service (NWS), trigger these alerts. The purpose is to let people know a dangerous storm is nearing their location.
Now, some Severe Thunderstorm Warnings will also set off these alerts. Not all severe storms, however, will meet the criteria needed to send out a WEA. Only if a Severe Thunderstorm is considered "destructive" by the local NWS office, will a WEA be sent. That "destructive" rating is reserved only for the strongest severe storms. "Destructive" storm criteria are 80+ mph winds, or baseball size hail (2.75-inches in diameter) or larger. According to the National Weather Service, only around 10% of severe storms each year across the nation would fit this high-end criterion.
Starting August 2, Severe Thunderstorms deemed “destructive” will activate a Wireless Emergency Alert (WEA) on smartphones. Criteria for a destructive threat is at least 2.75 inch diameter hail and/or 80 mph thunderstorm winds.— National Weather Service (@NWS) July 27, 2021
Read more: https://t.co/Vu9HyjsoRw pic.twitter.com/XeRvkwgSY0
The base criteria for a Severe Thunderstorm Warning are one or more of the following: 58+ mph winds, one inch diameter hail, and/or the storm is producing a tornado. Storms that meet this description can certainly cause damage and still must be taken seriously, but they will NOT cause the WEA to active on your smartphone. Not all thunderstorms are created equal. That is why only the most powerful storms will trigger a WEA.
Other weather events that will trigger a WEA on your smartphone, in addition to tornadoes and "destructive" severe thunderstorms, are Flash Flood Warnings that are labeled considerable (rare) or catastrophic (very rare), and Snow Squall Warnings.
None of the Severe Thunderstorm Warnings that were issued last week in Southeast Wisconsin would have met the "destructive" thunderstorm warning criteria.
When might people receive an alert for a high-end "destructive" severe thunderstorm? Derechos are a good example of storms where winds can be incredibly strong, and a heads up through your smart phone could be lifesaving. The recent Iowa derecho from August of 2020 had estimated winds of 140 mph! Large hail events are another example where a WEA might be used to alert people in the storm's path.
If you did not receive a WEA on your smartphone, and you were within a tornado warned area during last week's storms, be sure to check that Wireless Emergency Alerts are turned ON in your phone settings.
In addition, a weather radio is a great tool that will alert you to any incoming severe weather, including tornadoes and severe thunderstorms (of any level).
For more on the updated threat tags for severe storm warnings, see the National Weather Service press release.
More information from the Federal Communications Commission on WEA can be found here.