History of Severe thunderstorm warning in Timeline

A severe thunderstorm warning is issued when severe thunderstorms are imminent or occurring, posing a threat of large hail, high winds, and flooding. These warnings are triggered by Doppler radar detection, spotter reports, or information from emergency services or the public. Unlike a watch, a warning indicates an immediate threat to areas in the storm's path, typically within an hour. The specific criteria for issuing a warning differ by country, but the purpose remains the same: to alert the public to potentially dangerous weather conditions.

October 2007: Change in warning issuance

Prior to October 2007, the National Weather Service issued severe thunderstorm warnings on a per-county basis.

January 2010: Hail size criteria upgrade

In January 2010, the National Weather Service upgraded the hail size criteria for a thunderstorm to be considered severe to one inch (2.5 cm) in diameter. This change was due to public complacency from frequent warnings and studies indicating hail did not cause significant damage until it reached one inch.

March 2012: Implementation of Impact Based Warning system

In March 2012, the National Weather Service implemented a multi-tier Impact Based Warning (IBW) system. This system included event tags in severe thunderstorm warnings, providing summaries of wind speeds, hail size, and potential for tornadic development, as well as impacts to life and property.

March 2013: Expansion of IBW indicators

In March 2013, the categorical threat and damage indicator text of the Impact Based Warning system, initially implemented at six NWS offices in Kansas and Missouri, was expanded to 33 additional Central Region WFOs.

2013: Impact Based Warning system implementation

In 2013, the Impact Based Warning system was implemented. This was similar to the indicators used for tornado warnings, the categorical criteria was introduced to further explain to the public of the dangers of some thunderstorms that can produce massive hailstones of baseball size or larger and winds that can cause damage equivalent to hurricanes and tornadoes.

March 2014: Further expansion of IBW indicators

In March 2014, eight additional National Weather Service offices operating within the Eastern, Southern, and Western Region divisions began using the Impact Based Warning indicators.

2016: Agency-wide implementation of IBW format

In 2016, the entire National Weather Service agency began using the Impact Based Warning format for severe thunderstorm warnings.

August 16, 2019: Severe Thunderstorm Emergency declared

On August 16, 2019, at 1:31 a.m. MDT, the National Weather Service in Cheyenne, Wyoming, incorporated the "emergency" wording into a Severe Thunderstorm Warning. This "severe thunderstorm emergency" statement was included to denote that three-inch diameter hail was imminent in portions of Scotts Bluff, Banner, and Sioux Counties in Nebraska.

July 28, 2021: Incorporation of categorical damage threat indicators

Since July 28, 2021, the NWS has incorporated categorical damage threat indicators for higher-end hail and/or wind events at the bottom of text products for severe thunderstorm warnings, including update statements to existing warnings issued as Severe Weather Statements.