You might hear about a storm system producing severe weather, but when it comes to thunderstorms, specific criteria must be satisfied for a storm to be deemed “severe”.
Here’s a quick rundown of each, though note only one of the three needs to be met:
1. Hail Must Be Quarter-Sized Or Larger
That’s about 1 inch diameter or bigger. As a general rule of thumb, this is the threshold at which hail can sometimes cause damage. In the case of quarter-sized hail, it can be damaging to shingles. Larger hail sizes can be more destructive.
Prior to 2010, this criteria was penny-sized hail, or 3/4 of an inch diameter. But, it was raised after research revealed hail smaller than quarters doesn’t usually cause significant damage.
2. Wind Gusts Must Be 58 MPH Or Greater
The National Weather Service issues a severe thunderstorm warning when radar detects that a storm might produce winds of 58 mph or higher. At least minor damage is possible when winds reach that threshold.
Many severe gusts in storms are never directly measured because weather observations are relatively sparse. However, the National Weather Service can use damage reports to verify where a thunderstorm produced severe wind gusts.
3. The Storm Poses A Tornado Threat
This is the most straightforward criteria.
A storm is automatically deemed severe if it’s producing a tornado, no matter what size hail or wind gusts (not associated with a tornado) it might contain. That said, the hail and wind criteria also are often satisfied in a storm that’s tornadic.
Not All Severe Thunderstorm Warnings Are The Same
One thing to note is that not all storms that prompt a severe thunderstorm warning have the same threat level.
The criteria for hail and wind mentioned earlier are the minimum thresholds. Warnings for a storm (or line of storms) with a threat of significantly stronger winds and/or large hail are 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.
Similar to this, not all tornado warnings are the same.