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See Severe weather terminology for a comprehensive article on related weather terms.

Severe thunderstorm with a clear slot near main updraft core. Typically a tornado, if present, will form in this area.

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 implies that it is also a severe thunderstorm watch. A tornado watch must not be confused with a tornado warning.

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 tornado warning or severe thunderstorm warning will be issued.

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, enhanced wording with the words particularly dangerous situation (PDS) can be added to the watch; this is occasionally issued.

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.

  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

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