Alerts for destructive thunderstorms to appear on your phone

source：washingtonpost.com

Before a violent thunderstorm complex unleashed winds up to nearly 140 mph in Iowa last August, many residents had no idea it was coming. It was the most costly thunderstorm disaster in U.S. history, but severe thunderstorm warnings issued by the National Weather Service did not sound on smartphones like they do for tornadoes and severe flash floods.

Starting Monday, the Weather Service is implementing changes to their warning alerts to differentiate the threats posed by typical severe thunderstorms and those that are particularly dangerous, such as the Iowa complex last year. With winds rivaling those in a major hurricane, meteorologist classified it as a “derecho,” which is an extreme, fast-moving wind storm.

A thunderstorm is considered “severe,” according to the National Weather Service, if it contains one or more of the following: 1) hail at least one-inch in diameter, or roughly the size of a quarter; 2) damaging winds of at least 58 mph; 3) a tornado.

But there is a big difference between a severe thunderstorm with 58 mph vs. 90 mph winds, or between a storm with quarter-size vs. softball-size hail.

The new change will introduce two key categories, “considerable” and “destructive,” into thunderstorm warning text, creating higher tiers for the more intense storms.

Warnings will get the “considerable” tag when a storm could produce hail of at least 1.75 inches in diameter (golf ball-size) and/or 70 mph winds. One threat level higher, and storms will be tagged as “destructive” with hail at least 2.75 inches in diameter (baseball size) and/or 80 mph winds.