Tornado Warning - Criteria

Criteria

A tornado warning is issued when any of the following conditions has occurred:

• a tornado is reported on the ground, or
• a funnel cloud is reported, or
• strong low-level rotation is indicated by weather radar, or
• a waterspout is headed for landfall.

A tornado warning means there is immediate danger for the warned and immediately surrounding area—if not from the relatively narrow tornado itself, from the severe thunderstorm producing (or likely to produce) it. All in the path of such a storm are urged to take cover immediately, as it is a life-threatening situation. A warning is different from a tornado watch (issued by a national guidance center, the Storm Prediction Center) which only indicates that conditions are favorable for the formation of tornadoes.

Generally (but not always), a tornado warning also indicates that the potential is there for severe straight-line winds and/or large hail from the thunderstorm. A severe thunderstorm warning can be upgraded suddenly to a tornado warning should conditions warrant.

In the United States, local offices of the National Weather Service issue warnings for tornadoes and severe thunderstorms based on the path of a storm, although entire counties are sometimes included, especially if they are small. Warnings were issued on a per-county basis before October 2007.

In Canada, similar criteria are used and warnings are issued by regional offices of the Meteorological Service of Canada of Environment Canada in Vancouver, Edmonton, Winnipeg, Toronto, Montreal and Halifax. (In the province of Ontario, Emergency Management Ontario recently began issuing red alerts for areas of the province that are already under an Environment Canada issued tornado warning. These red alerts sometimes override the tornado warning if local government or media are participating in the program.)

Tornado warnings are generated via AWIPS then disseminated through various communication routes accessed by the media and various agencies, on the internet, to NOAA satellites, and on NOAA Weather Radio. Tornado sirens are also usually activated for the affected areas if present.

Advances in technology, both in identifying conditions and in distributing warnings effectively, have been credited with reducing the death toll from tornadoes. The average warning times have increased substantially to about 15 minutes (in some cases, to more than an hour's warning of impending tornadoes). The U.S. tornado death rate has declined from 1.8 deaths per million people per year in 1925 to only 0.11 per million in 2000. Much of this change is credited to improvements in the tornado warning system.