'Severe'Isn't A Word We Just Throw Around
While thunderstorms might be the first thing that comes to mind when thinking about severe weather,it's important to remember that severe weather can come in many forms.
Any type of weather that can pose a risk to life and property is considered severe and hazardous weather. Tornadoes,hurricanes,floods,lightning,hail,strong winds and even winter storms that produce dangerous sleet and freezing rain can all be classified as severe or hazardous.
The National Weather Service will issue different alerts based on the risk. For example,we often hear of a "hurricane watch"and "hurricane warning"when a hurricane is approaching the coast. We will hear of a "flash flood warning"or a "flood emergency"when heavy rainfall is occurring. Strong winds can lead to a "high wind warning"or "red flag warning"when they pose a risk to life and property – or could result in wildfires.
We commonly 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."
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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 in 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 three-quarters of an inch in diameter. But it was raised after research revealed hail smaller than quarters doesn't usually cause significant damage.
(15-min details:For even more granular weather data tracking in your area,view your 15-minute details forecast in our Premium Pro experience.)
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.
Similarly,not all tornado warnings are the same. The NWS will issue a tornado warning if there is imminent risk of a twister. This is when you should be in your safe place on the bottom level of a sturdy structure,away from windows.
However,there are times when a "tornado emergency"is issued. This is the highest threat level for a tornado and is issued when a powerful tornado has been confirmed in the threat area and catastrophic damage has occurred.
Make sure you and your family have a plan in place,and review those plans for when severe weather strikes.
Chris Dolce has been a senior meteorologist with www.weathernow24.com for over 10 years after beginning his career with Weather Now 24 in the early 2000s.
Jennifer Gray is a weather and climate writer for www.weathernow24.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.
