Severe thunderstorm watch
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A severe thunderstorm watch (SAME code: SVA; sometimes referred to as "yellow 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 such that there is a significant risk 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 (in which case the existing severe thunderstorm watch, or a portion of it, would be replaced). 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. 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.
[edit] Example of a severe thunderstorm watch
SEL9
URGENT - IMMEDIATE BROADCAST REQUESTED
SEVERE THUNDERSTORM WATCH NUMBER 589
NWS STORM PREDICTION CENTER NORMAN OK
400 PM CDT SUN AUG 12 2007
THE NWS STORM PREDICTION CENTER HAS ISSUED A
SEVERE THUNDERSTORM WATCH FOR PORTIONS OF
SOUTHWEST IOWA
EXTREME NORTHWEST AND NORTH CENTRAL MISSOURI
EFFECTIVE THIS SUNDAY AFTERNOON AND EVENING FROM 400 PM UNTIL
1100 PM CDT.
HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 80
MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.
THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 55
STATUTE MILES NORTH AND SOUTH OF A LINE FROM 20 MILES SOUTH OF
OMAHA NEBRASKA TO 35 MILES NORTH OF KIRKSVILLE MISSOURI. FOR A
COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE
UPDATE (WOUS64 KWNS WOU9).
REMEMBER...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 AND OCCASIONALLY
DO PRODUCE TORNADOES.
OTHER WATCH INFORMATION...CONTINUE...WW 588...
DISCUSSION...BOW ECHO MCS APPEARS TO BE ORGANIZING W OF OMAHA...AND
THIS CONVECTIVE COMPLEX WILL LIKELY PROGRESS ESEWD ALONG A
BAROCLINIC ZONE NEAR THE MO/IA BORDER. MLCAPE VALUES OF 4000-5000
J/KG AND MODEST DEEP LAYER SHEAR ALONG THE BOUNDARY WILL HELP
MAINTAIN STORM INTENSITY INTO THE EVENING. RELATIVELY WIDESPREAD
DAMAGING WINDS WILL BE POSSIBLE ALONG THE PATH OF THIS
SYSTEM...ALONG WITH LARGE HAIL.
AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT
TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 70
KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 600. MEAN STORM
MOTION VECTOR 29035.
...THOMPSON
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