FRANKFORT, Ky. (KT) – Not all severe thunderstorms are created equal, so the National Weather Service will soon be making changes to the warnings they issue to more specifically describe their severity and to trigger special messages on smartphones for the strongest ones.
Up until now, a severe thunderstorm warning was issued when winds are forecast to reach at least 58 miles per hour and hail one inch in diameter or more is expected. That will not be changed, but effective August 2 there will be additional information contained in the warnings known as damage threats, for stronger storms.
The criteria for a considerable damage threat will be hail at least 1.75 inches in diameter, or golf ball-sized, and/or thunderstorm winds over 70 mph.
A destructive damage threat will include hail 2.75 inches in diameter, baseball-sized, or greater, and/or 80 mph thunderstorm winds. In addition, those warnings will automatically activate a Wireless Emergency Alert on the screens of smartphones that are located within the warned area.
The National Weather Service says, on average, only 10% of all severe thunderstorms reach the destructive category each year. Most of them are damaging wind events known as “Derechoes,” and some of the more intense thunderstorms, or “Supercell” storms, that can produce very large hail in their path.
The new destructive damage threat category is meant to convey that urgent action is needed, a life-threatening event is occurring, which may cause substantial damage to property.
The NWS notes that 13 of the 22 costliest weather disasters in 2020 were due to severe thunderstorms, and if the new destructive damage threat tag were included, would have triggered a Wireless Emergency Alert for many of them, including the costliest thunderstorm in U. S. history, a derecho that affected Iowa in August 2020, which had maximum estimated winds of 140 mph, and caused $11 billion in damage.
This is not the same as severe thunderstorm or tornado watches, which are issued when there is a possibility of the severe weather developing and can cover several states. The warnings are only issued when severe weather is occurring or is imminent and normally cover a smaller area.