A tornado watch (code: WT; sometimes referred to as a "red box" by meteorologists and storm chasers) is issued when weather conditions are favorable for the development of severe thunderstorms that are capable of producing tornadoes. A tornado watch therefore automatically implies that it is also a severe thunderstorm watch.
A watch does not mean that the severe weather is actually
occurring, only that conditions have created a significant risk for it. If severe weather actually does occur, a severe thunderstorm warning or tornado warning will be issued, and urgent action should be taken.
In the United States, the Storm Prediction Center (a national guidance center of the National Weather Service) issues watches for areas likely to produce tornadoes and severe thunderstorms. The watch boxes (or weather watches, WWs) are usually issued in the format of x miles north and south, or east and west, or either side of a line from y miles direction of city, state, to z miles another direction of another city, state. For example: "50 miles either side of a line from 10 miles northeast of Columbia, South Carolina to 15 miles south-southwest of Montgomery, Alabama". ("Either side" means perpendicular to the center line.) In addition, a list of all counties included in its area of responsibility is now issued by each NWS forecast office for each watch.
In the event that a tornado watch is likely to lead to a major tornado outbreak along with possible destructive winds and hail, a particularly dangerous situation (PDS) can be added to the watch.
A watch must not be confused with a tornado warning.
Example of a tornado watch:
SEL0
URGENT - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 730
NWS STORM PREDICTION CENTER NORMAN OK
250 PM CDT THU AUG 18 2005
THE NWS STORM PREDICTION CENTER HAS ISSUED A
TORNADO WATCH FOR PORTIONS OF
EXTREME NORTHEAST IOWA
EXTREME SOUTHEAST MINNESOTA
CENTRAL AND SOUTHWEST WISCONSIN
EFFECTIVE THIS THURSDAY AFTERNOON AND EVENING FROM 250 PM UNTIL
900 PM CDT.
TORNADOES...HAIL TO 1.5 INCHES IN DIAMETER...THUNDERSTORM WIND
GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
AREAS.
THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 60 STATUTE MILES
EAST AND WEST OF A LINE FROM 50 MILES NORTH OF CAMP DOUGLAS
WISCONSIN TO 50 MILES SOUTH OF LONEROCK WISCONSIN. FOR A COMPLETE
DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE
(WOUS64 KWNS WOU0).
REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA.
PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING
WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE
WARNINGS.
OTHER WATCH INFORMATION...CONTINUE...WW 729...
DISCUSSION...SCATTERED THUNDERSTORMS ARE EXPECTED TO CONTINUE TO
INCREASE THIS AFTERNOON NEAR SURFACE LOW/WARM FRONT FROM EXTREME
SOUTHEAST MN INTO WI. OTHER STORMS MAY DEVELOP SOUTHWARD INTO
EXTREME NORTHEAST IA. VERTICAL SHEAR PROFILES WILL PROMOTE
SUPERCELL POTENTIAL NEAR WARM FRONT...WITH THE RISK OF ISOLATED
TORNADOES. DAMAGING WINDS ARE LIKELY TO BE THE MAIN THREAT FARTHER
SOUTH.
AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
SURFACE AND ALOFT TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE
WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
500. MEAN STORM MOTION VECTOR 26030.
...HART
Current Watches
In Canada, the criteria used is the same and watches are issued by regional offices of the Meteorological Service of Canada of Environment Canada in Vancouver, Edmonton, Toronto, Montreal and Halifax, on a county or regional basis.
Tornado warning
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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 should 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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Typical Tornado Warning in the United States
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BULLETIN - EAS ACTIVATION REQUESTED
TORNADO WARNING
NATIONAL WEATHER SERVICE NASHVILLE TN
ISSUED BY NATIONAL WEATHER SERVICE MORRISTOWN TN
325 PM CDT FRI APR 7 2006
THE NATIONAL WEATHER SERVICE IN NASHVILLE HAS ISSUED A
TORNADO WARNING FOR...
BEDFORD COUNTY IN MIDDLE TENNESSEE
MARSHALL COUNTY IN MIDDLE TENNESSEE
MAURY COUNTY IN MIDDLE TENNESSEE
THIS INCLUDES THE CITY OF COLUMBIA
RUTHERFORD COUNTY IN MIDDLE TENNESSEE
WILLIAMSON COUNTY IN MIDDLE TENNESSEE
UNTIL 415 PM CDT
AT 321 PM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR WAS TRACKING
A LARGE AND EXTREMELY DANGEROUS TORNADO NEAR COLUMBIA...MOVING EAST
AT 40 MPH. THIS IS AN EXTREMELY DANGEROUS SITUATION. TAKE COVER
NOW!!
THE TORNADO IS EXPECTED TO BE NEAR...
SPRING HILL BY 330 PM CDT
IN ADDITION TO THE TORNADO...THIS STORM IS CAPABLE OF PRODUCING HAIL
UP TO THE SIZE OF SOFTBALLS AND DESTRUCTIVE OUTFLOW WINDS IN EXCESS
OF 50 MPH.
THE SAFEST PLACE TO BE DURING A TORNADO IS IN A BASEMENT OR A STORM
SHELTER. GET UNDER A WORKBENCH OR OTHER PIECE OF STURDY FURNITURE. IF
NO BASEMENT IS AVAILABLE...SEEK SHELTER ON THE LOWEST FLOOR OF THE
STRUCTURE IN AN INTERIOR HALLWAY...ROOM OR CLOSET. USE BLANKETS OR
PILLOWS TO COVER YOUR BODY AND ALWAYS STAY AWAY FROM WINDOWS.
IF IN MOBILE HOMES OR VEHICLES...EVACUATE THEM AND GET INSIDE A
SUBSTANTIAL SHELTER. IF NO SHELTER IS AVAILABLE...LIE FLAT IN THE
NEAREST DITCH OR OTHER LOW SPOT AND COVER YOUR HEAD.