National Weather Service new destructive, severe thunderstorm alert explained The alert is specifically for thunderstorms, such as last year's derecho, with winds over 80 miles per hour and/or baseball sized hail. Author: Jenna Webster Updated: 6:38 PM CDT July 23, 2021 DAVENPORT, Iowa — The National Weather Service announced it's expanding its alert warning categories to include destructive, severe thunderstorms, beginning Aug. 2. A destructive thunderstorm is defined as having winds over 80 miles per hour and/or hail at 2.75" in diameter (the size of a baseball). Last summer's derecho would be part of this category and people in the affected area would have gotten the alert.