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A tornado warning (SAME code: TOR) is an alert issued by weather services to warn that severe thunderstorms with tornadoes may be imminent. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation. When this happens, the tornado sirens may sound in that area, letting people know that a tornado was seen or is forming nearby. Issuance of a tornado warning indicates that residents should take immediate safety precautions. It is a higher level of alert than a tornado watch, but it can be surpassed by an even higher alert known as a tornado emergency.
The first official tornado forecast (and tornado warning) was made by United States Air Force Capt. (later Col.) Robert C. Miller and Major Ernest Fawbush, on March 25, 1948. The USAF pioneered tornado forecasting and tornado warnings, mainly due to the Weather Bureau's strong discouragement/ban on the use of the word "tornado" in forecasts or statements, fearing that it would cause the public to panic if they predicted tornadoes. In 1950, the Weather Bureau revoked their ban on the word "tornado", thus allowing public tornado warnings. Chief of Bureau F.W. Reichelderfer officially lifted the ban on issuing tornado warnings in a Circular Letter issued on July 12, 1950 to all first order stations. "Weather Bureau employees should avoid statements that can be interpreted as a negation of the Bureau's willingness or ability to make tornado forecasts." 
Despite the U.S. Weather Bureau's lifting of their ban on tornado warnings, the Federal Communications Commission continued to ban television and radio from broadcasting tornado warnings on-air due to possibly inciting panic in the public. Broadcast media followed this ban until 1954, when meteorologist Harry Volkman broadcast the first televised tornado warning over WKY-TV (now KFOR-TV) in Oklahoma City, due to his belief that the banning of tornado warnings over broadcast media cost lives;
For many years until the early 1980s an intermediate type of tornado advisory known as a Tornado Alert was defined by the National Weather Service and issued by local offices thereof. A Tornado Alert indicated that tornado formation was imminent and in theory covered situations such as visible rotation in clouds and some other phenomena which are portents of funnel formation. The use of this advisory began to decline after 1974 but was still listed on public information materials issued by various media outlets, local NWS offices, &c. for another decade or so.
The kinds of situations which called for Tornado Alerts in the past now generally result in a Tornado Warning with clarifying verbiage specifying that the warning was issued because rotation was detected in one way or another, that a wall cloud has formed, and so on. The preferred response to both the Tornado Alerts and Warnings is to take shelter immediately, so distinguishing them could be seen as splitting hairs, especially since storm prediction methods have improved.
The Tornado Alert was finally eliminated because it was made largely obsolete by the advent of Doppler weather radar, which can detect rotational funnel cloud formations earlier than is typically possible by trained spotters and members of the public. With fewer false-positives, radar also helped reduce public confusion over storm types, strengths, and precise locations. The last Tornado Alert officially issued was discussed in earnest following the 3–4 April 1974 Super Outbreak.
A tornado warning is issued when any of the following conditions has occurred:
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.
The SKYWARN program, which teaches people how to spot tornadoes, funnel clouds, wall clouds, and other severe weather phenomena, is offered by the National Weather Service. Used in tandem with Doppler radar information, eyewitness reports can be very helpful for warning the public of an impending tornado, especially when used for ground truthing.
Other spotter groups such as the Amateur Radio Emergency Service, news media, local law enforcement agencies/emergency management organizations, cooperative observers, and the general public also relay information to the National Weather Service for ground truthing.
When a large, extremely violent tornado is about to impact a densely populated area, the Weather Service has the option of issuing a severe weather statement with unofficial, enhanced wording; this is called a tornado emergency. This category of weather statement is the highest and most urgent level relating to tornadoes, albeit unofficially. The first tornado emergency was declared on Monday, May 3, 1999, when an F5 tornado did major damage in the Oklahoma City area. In some cases, like the F3 that struck the Indianapolis, IN area on Friday, September 20, 2002, a Tornado Emergency has been declared in the initial issuance of the Tornado Warning.
The levels of severity increase as follows:
Tornado warnings can also be intensified by added wording mentioning that the storm is life-threatening, that it is an extremely dangerous situation, that a large, violent and/or destructive tornado is on the ground, etc.
The NOAA Weather Radio audio of a tornado warning issued for Greensburg, Kansas on May 4, 2007. Greensburg was struck by an EF-5 tornado while the warning was in effect.
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An example of a tornado warning issued in Oklahoma
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Below is an example of a tornado warning issued by the National Weather Service.
WFUS53 KICT 142213 TORICT KSC169-142245- /O.NEW.KICT.TO.W.0011.110414T2213Z-110414T2245Z/ BULLETIN - EAS ACTIVATION REQUESTED TORNADO WARNING NATIONAL WEATHER SERVICE WICHITA KS 513 PM CDT THU APR 14 2011 THE NATIONAL WEATHER SERVICE IN WICHITA HAS ISSUED A * TORNADO WARNING FOR... NORTHEASTERN SALINE COUNTY IN CENTRAL KANSAS... * UNTIL 545 PM CDT * AT 512 PM CDT...TRAINED WEATHER SPOTTERS REPORTED A FUNNEL CLOUD NEAR NEW CAMBRIA...OR 7 MILES EAST OF SALINA. A TORNADO MAY DEVELOP AT ANY TIME. DOPPLER RADAR SHOWED THIS DANGEROUS STORM MOVING NORTH AT 40 MPH. * LOCATIONS IMPACTED INCLUDE... NEW CAMBRIA. PRECAUTIONARY/PREPAREDNESS ACTIONS... TAKE COVER NOW. MOVE TO AN INTERIOR ROOM ON THE LOWEST FLOOR OF A STURDY BUILDING. AVOID WINDOWS. IF IN A MOBILE HOME...A VEHICLE OR OUTDOORS...MOVE TO THE CLOSEST SUBSTANTIAL SHELTER AND PROTECT YOURSELF FROM FLYING DEBRIS. && LAT...LON 3895 9736 3893 9736 3892 9737 3891 9736 3874 9736 3875 9761 3896 9761 3896 9737 TIME...MOT...LOC 2213Z 181DEG 34KT 3885 9746 HAIL 1.00IN $$ DUNTEN
Below is an example of an Environment Canada issued tornado warning for southeastern Saskatchewan.
344 WFCN13 CWWG 262334 TORNADO WARNING UPDATED BY ENVIRONMENT CANADA AT 5:34 PM CST TUESDAY 26 JUNE 2012. ---- TORNADO WARNING FOR: R.M. OF WHEATLANDS INCLUDING MORTLACH AND PARKBEG R.M. OF CARON INCLUDING CARONPORT AND CARON R.M. OF MOOSE JAW INCLUDING PASQUA AND BUSHELL PARK CITY OF MOOSE JAW. TORNADO WARNING ENDED FOR: R.M. OF RODGERS INCLUDING CODERRE AND COURVAL R.M. OF HILLSBOROUGH INCLUDING CRESTWYND AND OLD WIVES LAKE. ---- ==DISCUSSION== AT 5:30 PM CST, PUBLIC REPORTS A LARGE TORNADO CURRENTLY ON THE GROUND WEST OF MOOSE JAW. RADAR INDICATES THE SEVERE THUNDERSTORM ASSOCIATED WITH THIS TORNADO IS CURRENTLY JUST SOUTH OF MORTLACH AND IS SLOWLY TRACKING NORTHEASTWARDS TOWARDS THE CITY OF MOOSE JAW.