Tornado warning
A tornado warning is issued when:
a tornado is reported on the ground,
a waterspout is headed toward landfall,
a funnel cloud is reported in the sky, or
depending on the circumstances:
a thunderstorm with a threshold strong, tight rotation signature is indicated by doppler radar, or
a rotating wall cloud is reported (in context of all other available information).
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.
In the United States, local offices of the National Weather Service issue warnings for tornadoes and severe thunderstorms on a per-county basis, narrowing down to parts of counties in many cases, and usually with a narrower pathcast of where the tornado(s) is expected to track within the area is mentioned in the warning message.
In Canada, similar criterias are used and warnings are issued by regional offices of the Meteorological Service of Canada of Environment Canada in Vancouver, Edmonton, Toronto, Montreal and Halifax.
Tornado warnings are generated via computer then disseminated through various communication routes accessed by the media and various agencies, on the internet, to NOAA satellites, and on NOAA Weather Radio.
The first tornado warning was issued by the meteoroligical staff of Tinker Air Force Base in 1947 and was also coincidentally the first successful tornado forecast.
Advances in technology, both in indentifying 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; and in some cases to more than a one 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.
A warning must not be confused with 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.
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