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

Thunderstorm out flow from storm core shows up as sheets of wind driven rain spreading from right to left in 1982 photo.

A severe thunderstorm watch (code: WS; sometimes referred to as "blue box" by meteorologists and storm chasers) is issued when weather conditions are favorable for the development of severe thunderstorms. If the thunderstorms are forecasted to be so severe that they may produce tornadoes, then a tornado watch (which also automatically implies a severe thunderstorm watch) is issued. A severe thunderstorm watch can also be upgraded to a tornado watch as conditions warrant. A watch must not be confused with a severe thunderstorm 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 severe thunderstorm warning or tornado warning will be issued, and urgent action should be taken. It should be noted that while a severe thunderstorm watch does not imply in its name the risk for tornadoes, they can and occasionally do strike even when a tornado watch is not 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 severe thunderstorm watch is likely to lead to very destructive winds or hail (usually a major derecho event), enhanced wording with the words particularly dangerous situation (PDS) can be added to the watch. This is rare with severe thunderstorm watches (since the tornado threat has to remain low enough to only warrant a severe thunderstorm watch); it is far more common with tornado watches.

A similar warning is issued by Environment Canada's Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth ,Nova Scotia.

  SEL9
  
  URGENT - IMMEDIATE BROADCAST REQUESTED
  SEVERE THUNDERSTORM WATCH NUMBER 559
  NWS STORM PREDICTION CENTER NORMAN OK
  1140 PM CDT WED JUN 29 2005
  
  THE NWS STORM PREDICTION CENTER HAS ISSUED A
  SEVERE THUNDERSTORM WATCH FOR PORTIONS OF 
  
         CENTRAL WISCONSIN
  
  EFFECTIVE THIS WEDNESDAY NIGHT AND THURSDAY MORNING FROM 1140 PM
  UNTIL 500 AM CDT.
  
  HAIL TO 1.5 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
  MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.
  
  THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 40
  STATUTE MILES EAST AND WEST OF A LINE FROM 100 MILES NORTH
  NORTHWEST OF WAUSAU WISCONSIN TO 50 MILES SOUTH SOUTHEAST OF
  LONEROCK WISCONSIN.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE
  ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU9).
  
  A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
  FAVORABLE FOR 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. SEVERE THUNDERSTORMS CAN PRODUCE TORNADOES WITH NO ADVANCE
  WARNING. MOVE IMMEDIATELY TO A STORM SHELTER IF A TORNADO IS SIGHTED.
  
  OTHER WATCH INFORMATION...CONTINUE...WW 556...WW 557...WW 558...
  
  DISCUSSION...SEVERAL BOWING SEGMENTS HAVE DEVELOPED IN SW AND W
  CENTRAL WI...WITH MOTION OF THESE STORMS TO THE ENE AT 45-55 KT. 
  THOUGH INSTABILITY DIMINISHES FARTHER E INTO CENTRAL/ERN WI...THE
  STRONG WIND FIELDS AND ORGANIZED NATURE OF THE CONVECTION SUGGEST
  THAT THE THREAT FOR DAMAGING WINDS AND ISOLATED LARGE HAIL MAY
  PERSIST THE NEXT FEW HOURS INTO CENTRAL WI.
  
  AVIATION...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 450. MEAN STORM
  MOTION VECTOR 25035.
  
  
  ...THOMPSON

• Meteorological Service of Canada
• Current Watches