History of Severe thunderstorm warning in Timeline

A severe thunderstorm warning is issued by weather agencies when severe thunderstorms are imminent or occurring, posing a direct threat to areas in their path, usually within an hour. These storms can bring large hail, high winds, and flooding, leading to property damage and injuries. Warnings are triggered by Doppler radar detection, spotter reports, or emergency agency/public confirmation. The specific criteria for issuing a warning differ by country, but the purpose remains the same: to alert the public to immediate danger.

October 2007: Change in warning issuance method

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

January 2010: Hail size upgrade for severe thunderstorm classification

In January 2010, the National Weather Service (NWS) upgraded the hail size criteria for a thunderstorm to be considered severe from 0.75 inches to one inch in diameter. This change was made due to public complacency from frequent warnings and studies showing that 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 provided event tags at the bottom of severe thunderstorm warnings and Severe Weather Statements, offering estimates of straight-line wind speeds, hail size, and potential tornadic development. It also summarized impacts to life and property caused by hail and winds.

March 2013: Expansion of Impact Based Warning indicators

In March 2013, the categorical threat and damage indicator text of the Impact Based Warning (IBW) system was expanded to 33 additional Central Region Weather Forecast Offices (WFOs). These indicators were applicable to all agency's Weather Forecast Offices, primarily those operating within the agency's Central and Southern Region divisions.

2013: Introduction of Categorical Criteria for Tornado Warnings

In 2013, categorical criteria, similar to those used for tornado warnings with the Impact Based Warning system, were introduced to further explain the dangers of certain thunderstorms, especially in the Great Plains. These thunderstorms can produce massive hailstones and winds causing damage equivalent to hurricanes and tornadoes. Warnings with the "destructive" tag were disseminated via Wireless Emergency Alerts.

March 2014: Further expansion of Impact Based Warning indicators

In March 2014, eight additional Weather Forecast Offices (WFOs) operating within the Eastern, Southern, and Western Region divisions began using the Impact Based Warning (IBW) indicators.

2016: Agency-wide adoption of Impact Based Warning format

In 2016, the entire National Weather Service agency began using the Impact Based Warning (IBW) format.

August 16, 2019: Use of "emergency" wording in a Severe Thunderstorm Warning

On August 16, 2019, the National Weather Service in Cheyenne, Wyoming, used the "emergency" wording in a Severe Thunderstorm Warning for the first time. This "severe thunderstorm emergency" was issued due to imminent three-inch diameter hail in parts of Nebraska.

July 28, 2021: Incorporation of Categorical Damage Threat Indicators

Since July 28, 2021, the National Weather Service 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 issued as Severe Weather Statements. The implementation may have occurred as late as August 2 in certain County Warning Areas.