TheNational Weather Service (NWS), a component of NOAA, defines a "Severe Thunderstorm" based on specific physical criteria.19To warrant aSevere Thunderstorm Warning, a storm must be producing, or be capable of producing,winds of 58 miles per hour (50 knots) or higherand/orhail that is at least 1 inch (quarter-sized) in diameter.20These thresholds were established because they represent the point at which thunderstorms begin to pose a significant threat to life and property, specifically causing structural damage and injury from flying debris or large hail.
In 2021, the NWS updated its warning system to include "Damage Threat" tags to better convey the severity of the storm:21
Base (Standard):1-inch hail and/or 58 mph winds.22
Considerable:1.75-inch (golf ball) hail and/or 70 mph winds.23
Destructive:2.75-inch (baseball) hail and/or 80 mph winds (this tag triggers a Wireless Emergency Alert or WEA).24
For theCEDPprofessional, understanding these specific criteria is essential forIncident Recognition. A 58 mph wind is strong enough to down trees and power lines, which can lead to secondary emergencies such as road closures and power outages. 1-inch hail is large enough to damage roofs and shatter vehicle windshields. When an NWS warning is issued, it is a trigger for the emergency manager to activate theMass Notification System, ensure that "Shelter-in-Place" protocols are ready for outdoor workers, and prepare theLogistics Sectionfor potential post-storm damage assessments. By using standardized criteria, the NWS ensures that the public and disaster professionals are not desensitized by warnings for "routine" thunderstorms, but instead take immediate protective actions for storms that meet these scientifically defined thresholds for "severity."
