History of Severe thunderstorm warning in Timeline
A severe thunderstorm warning is a public alert issued by weather forecasting agencies when one or more severe thunderstorms are detected via Doppler radar, spotters, or reports from emergency management, law enforcement, or the public. This warning, unlike a watch, targets areas that are expected to be directly affected by the storm within an hour. Severe thunderstorms pose risks such as property damage and injury from large hail, strong winds, and flooding from heavy rainfall. The criteria for issuing such warnings vary between countries.
October 2007: Polygon Warning Introduction
In October 2007, the National Weather Service transitioned from issuing severe thunderstorm warnings on a per-county basis to using polygonal shapes. This change allowed more precise warnings based on the storm's projected path as determined by Doppler radar.
January 2010: Hail Criteria Upgrade
In January 2010, the National Weather Service upgraded the hail size criteria for severe thunderstorms from 0.75 inches to one inch in diameter. This change was prompted by public complacency due to frequent warnings and studies showing that hail did not cause significant damage until it reached one inch in diameter.
March 2012: Impact Based Warning System
In March 2012, the National Weather Service began implementing a multi-tier Impact Based Warning (IBW) system to notify the public and emergency management officials about severe weather phenomena. Initially rolled out at six offices in Kansas and Missouri, the system included event tags summarizing wind speeds, hail size, and potential tornadic development.
March 2013: IBW System Expansion
By March 2013, the National Weather Service expanded the Impact Based Warning (IBW) system to 33 additional Weather Forecast Offices in the Central Region. This expansion allowed more comprehensive coverage and better communication of severe weather risks.
2013: Introduction of Categorical Criteria
In 2013, the National Weather Service introduced categorical criteria for severe thunderstorm warnings to further explain the dangers of some thunderstorms. This included information on massive hailstones and damaging winds, especially those occurring in the Great Plains.
March 2014: Further IBW System Expansion
In March 2014, the National Weather Service further expanded the Impact Based Warning (IBW) system to include eight additional offices in the Eastern, Southern, and Western Regions. This continued the agency's efforts to improve severe weather warnings nationwide.
2016: Nationwide IBW Implementation
By 2016, the National Weather Service had fully implemented the Impact Based Warning (IBW) system across all its offices. This system provided detailed, location-specific warnings about severe weather threats, including hail and wind impacts.
August 16, 2019: Severe Thunderstorm Emergency
On August 16, 2019, the National Weather Service in Cheyenne, Wyoming, included the 'emergency' wording in a Severe Thunderstorm Warning for the first time. This was due to imminent three-inch diameter hail, as confirmed by storm spotters, threatening areas in Scotts Bluff, Banner, and Sioux Counties in Nebraska.
July 28, 2021: Categorical Damage Threat Indicators
Starting on July 28, 2021, the National Weather Service began using categorical damage threat indicators for severe thunderstorm warnings. These indicators, included at the bottom of warning texts, highlighted higher-end hail and wind events to better inform the public.