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 typically within an hour. These storms are detected via radar, spotters, or reports and are characterized by large hail, high winds, and torrential rainfall, which can cause property damage and injury. Warnings are issued for areas in the direct path of these storms, differentiating them from watches which indicate potential for severe weather. Specific criteria for issuing a warning vary by country.

October 2007: NWS Changes Warning Issuance to Polygonal Shapes

Prior to October 2007, the National Weather Service issued severe thunderstorm warnings on a per-county basis. Afterwards, warnings for tornadoes and severe thunderstorms started to be outlined in polygonal shapes, based on the storm's projected path, and distributed to various agencies and websites.

January 2010: NWS Updates Severe Thunderstorm Criteria

In January 2010, the National Weather Service updated the criteria for a severe thunderstorm, increasing the hail size from 0.75 inches to 1 inch. This change was due to public complacency and studies showing hail needed to be larger to cause significant damage.

March 2012: NWS Implements Impact Based Warning System

In March 2012, the National Weather Service started implementing 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 at the bottom of the product text providing summarical estimates of straight-line wind speeds and hail size and an indication of possible tornadic development.

March 2013: Expansion of Impact Based Warning System

In March 2013, the categorical threat and damage indicator text of the Impact Based Warning system was expanded to 33 additional Central Region Weather Forecast Offices (WFOs).

2013: Categorical Criteria Introduced

Similar to the indicators used for tornado warnings since the 2013 implementation of its Impact Based Warning system, the categorical criteria—which will be applicable to all NWS Weather Forecast Offices—was introduced to further explain to the public of the dangers of some thunderstorms, especially those that occur in the Great Plains, that can produce massive hailstones.

March 2014: IBW indicators expanded to additional WFOs

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: Nationwide Implementation of Impact Based Warning System

In 2016, the Impact Based Warning (IBW) format was implemented across the entire National Weather Service agency.

August 16, 2019: Severe Thunderstorm Emergency Issued in Wyoming

On August 16, 2019, the National Weather Service in Cheyenne, Wyoming, incorporated the "emergency" wording into a Severe Thunderstorm Warning to denote that three-inch diameter hail was imminent in portions of Scotts Bluff, Banner and Sioux Counties in Nebraska.

July 28, 2021: NWS Incorporates 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.