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 the warned area, typically within an hour. These storms are detected via radar, spotter reports, or emergency management agencies. The primary threats associated with severe thunderstorms include large hail, damaging winds, and flash flooding due to heavy rainfall, all of which can cause property damage and injuries. The criteria for issuing a warning vary by country, but the overall purpose is to alert the public to take immediate precautions.

October 2007: Change in severe weather warnings issuance

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

January 2010: Severe thunderstorm definition changes

In January 2010, the National Weather Service updated the definition of a severe thunderstorm to include hail of one inch (2.5 cm) in diameter or larger. This change was prompted by public complacency and studies showing hail smaller than one inch did not cause significant damage.

March 2012: Implementation of Impact Based Warning system

In March 2012, the National Weather Service began implementing a multi-tier Impact Based Warning (IBW) system to better communicate the severity of severe weather. This included event tags for wind speeds, hail size, and potential tornadic development. The product text also included a summary of impacts to life and property.

March 2013: Expansion of Impact Based Warning system

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

2013: Impact Based Warning system for tornado warnings

Since the 2013 implementation of the Impact Based Warning system, the categorical criteria which will be applicable to all NWS Weather Forecast Offices was introduced to explain to the public the dangers of thunderstorms, especially those in the Great Plains, that can produce baseball-sized hailstones or larger and winds that can cause damage equivalent to hurricanes and tornadoes.

March 2014: Further expansion of Impact Based Warning system

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

2016: Nationwide implementation of Impact Based Warning system

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

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

On August 16, 2019, the National Weather Service in Cheyenne, Wyoming, incorporated the "emergency" wording into a Severe Thunderstorm Warning to denote the imminent threat of three-inch diameter hail in parts of Nebraska.

July 28, 2021: Incorporation of categorical damage threat indicators

Starting July 28, 2021, the National Weather Service incorporated categorical damage threat indicators for high-end hail and/or wind events into severe thunderstorm warning text products.