Weather Service unveils new severity ratings for thunderstorms

Not all thunderstorms are equal. Now the National Weather Service is adding new damage threat categories to severe thunderstorm warnings.

The two damage threat categories will depend on hail size and wind speeds.

A “considerable threat” is a storm with golf ball-sized hail and wind speeds of 70 miles per hour or greater, and a “destructive threat” warning is a storm with baseball-sized hail and 80 mile-per-hour winds or greater.

“The difference between 58-60 mph winds and 90 mph winds like we saw in the derecho last year, that makes a pretty large difference,” Illinois Climatologist Trent Ford explained.

The billion-dollar August 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.

The storm tracked across eastern Iowa into Illinois with winds of up to 140 mph. Widespread damage to buildings was reported, and millions of acres of crops were damaged or destroyed.

Winds estimated at 100 mph impacted Princeton, Illinois in Bureau County where a 150-foot communications tower collapsed and numerous power poles were snapped. Long-duration power outages occurred across the region.

If a destructive threat severe thunderstorm warning is issued, a new alert will be sent out.

“The storms that are classified as that by the National Weather Service will automatically trigger a Wireless Emergency Alert, or a WEA, and that is an alert that goes directly to smartphones,” Ford said.

This emergency alert will be accompanied by a loud screech that will alert you when a particularly dangerous severe weather threat is in your area. Other warnings already trigger a Wireless Emergency Alert to phones, including tornado warnings and higher-end flash flood warnings.

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