A tornado watch (SAME code: TOA, sometimes referred to as a red box by meteorologists and storm chasers) is issued when conditions are right for a tornado to form.[1] Since any thunderstorm capable of producing a tornado is defined as severe, a tornado watch is also automatically a severe thunderstorm watch.[2] In most cases, the potential exists for large hail and/or damaging winds in addition to the tornadoes.
A tornado watch must not be confused with a tornado warning: a watch does not imply that the severe weather is actually occurring, only that conditions have created a significant risk for it. If severe weather actually does occur, a tornado warning or severe thunderstorm warning will be issued.[1]
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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.[1] The watch boxes are usually issued in the format of " __ miles (north and south; east and west; either side) of a line from __ miles (direction) of (City, State), to __ 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.)[3] When displayed on a map, they appear as a red outline. In addition, a list of all counties included in its area of responsibility is issued by each NWS forecast office for each watch.[1]
URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 227 NWS STORM PREDICTION CENTER NORMAN OK 615 PM CDT FRI MAY 4 2007 THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF WESTERN AND CENTRAL KANSAS WESTERN AND NORTHERN OKLAHOMA THE EASTERN TEXAS PANHANDLE AND NORTHWEST TEXAS EFFECTIVE THIS FRIDAY NIGHT AND SATURDAY MORNING FROM 615 PM UNTIL 200 AM CDT. TORNADOES...HAIL TO 3 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 80 STATUTE MILES EAST AND WEST OF A LINE FROM 25 MILES WEST NORTHWEST OF RUSSELL KANSAS TO 75 MILES SOUTH SOUTHEAST OF CHILDRESS TEXAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU7). 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 226... DISCUSSION...WIDELY SCATTERED STORMS/SUPERCELLS POSSIBLE IN VERY UNSTABLE AIR MASS ALONG SLOWLY RETREATING DRY LINE THIS EVENING. STRENGTHENING ASCENT/WIND FIELD ASSOCIATED WITH APPROACHING FOUR CORNERS UPPER TROUGH MAY ENHANCE TORNADO POTENTIAL AFTER DARK AS LLJ INCREASES TO AROUND 50 KTS. AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 3 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 550. MEAN STORM MOTION VECTOR 24030. ...CORFIDI [3]
In the event that a tornado watch is likely to lead to a major tornado outbreak along with possible destructive winds and hail, enhanced wording with the words This is a particularly dangerous situation (PDS) can be added to the watch; this is occasionally issued.[4]
In Canada, the criteria used are 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.[5]
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