History of Severe thunderstorm warning in Timeline

A severe thunderstorm warning is issued by weather agencies when severe thunderstorms are imminent or occurring, posing immediate threats like large hail, high winds, and flooding. Unlike a watch, a warning indicates that a direct impact is expected, typically within an hour, and is based on radar detection, spotter reports, or emergency agency information. These warnings are designed to protect people and property from potential damage and injury caused by the storm's intensity. Criteria for issuing a warning vary depending on the specific country's standards.

October 2007: Change in Severe Weather Warning Issuance

Prior to October 2007, the National Weather Service issued severe weather warnings on a per-county basis. In October 2007, NWS forecast offices outlined warnings for tornadoes and severe thunderstorms in polygonal shapes for map-based weather hazard products.

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, due to public complacency and studies showing that hail did not cause significant damage until it reached one inch. This change occurred in January 2010.

March 2012: Implementation of Impact Based Warning (IBW) system

In March 2012, the National Weather Service implemented a multi-tier Impact Based Warning (IBW) system to notify the public and emergency management officials of the severity of specific severe weather phenomena. Severe thunderstorm warnings included event tags summarizing straight-line wind speeds, hail size, and possible tornadic development. The product text also provided a summary of impacts to life and property. The implementation of the IBW system happened in March 2012.

March 2013: Expansion of categorical threat and damage indicator text

In March 2013, the categorical threat and damage indicator text was expanded to 33 additional Central Region WFOs. The initial implementation started at six NWS offices in Kansas and Missouri. This expansion happened in March 2013.

2013: Categorical criteria for tornado warnings

In 2013, the Impact Based Warning system was implemented. Categorical criteria was introduced to further explain to the public the dangers of some thunderstorms. Hailstones of baseball size (2.75 inches [7.0 cm]) or larger may fall fast enough to severely injure if not kill a person by repeated blunt trauma, and winds that can cause damage equivalent to hurricanes and tornadoes. When deemed necessary, initial warnings and Severe Weather Statement updates containing the "destructive" tag will be disseminated into the Wireless Emergency Alerts system as well. The implementation occurred in 2013.

March 2014: IBW indicators introduced to additional offices

In March 2014, eight additional offices operating within the Eastern, Southern and Western Region divisions began using the Impact Based Warning (IBW) indicators. The categorical threat and damage indicator text expanded into different regions, beginning in March 2014.

2016: Nationwide implementation of Impact Based Warning format

In 2016, the entire National Weather Service agency began using the Impact Based Warning format. This brought the warnings format to all Weather Forecast Offices.

August 16, 2019: Severe Thunderstorm Warning includes "emergency" wording

On August 16, 2019, the National Weather Service in Cheyenne, Wyoming, incorporated the "emergency" wording into a Severe Thunderstorm Warning. This denoted that three-inch diameter hail was imminent in portions of Nebraska.

July 28, 2021: Incorporation of categorical damage threat indicators

Starting July 28, 2021, the NWS began incorporating 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). The date of this update was July 28, 2021.