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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. In most cases, the potential exists for large hail and/or damaging winds in addition to the tornadoes.
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 probably be issued. Note that a watch is not required for a warning to 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.
Watch Outline Updates are relayed (and at the initial watch issuance, issued) by the Storm Prediction Center, however it is the local NWS offices that decide what counties (in their warning area) are included or excluded in the watch, via a conference call with the SPC. This is why you will sometimes see counties inside the watch outline that are not included in the counties listed, and vice versa; however the NWS local office will need to expand to add these counties into the watch. A Watch Status Message works in a similar fashion; the SPC says in what areas they think a threat still exists, and the NWS offices decide what counties to take out of the watch (the local offices will almost always follow the SPC recommendation on the status messages).
URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 239 NWS STORM PREDICTION CENTER NORMAN OK 905 PM CDT SAT MAY 5 2007 THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF MUCH OF KANSAS EFFECTIVE THIS SATURDAY NIGHT AND SUNDAY MORNING FROM 905 PM UNTIL 400 AM CDT. ...THIS IS A PARTICULARLY DANGEROUS SITUATION... DESTRUCTIVE TORNADOES...LARGE 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 115 STATUTE MILES EAST AND WEST OF A LINE FROM 30 MILES NORTH OF CONCORDIA KANSAS TO 20 MILES SOUTH OF MEDICINE LODGE KANSAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU9). 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 232...WW 234...WW 235...WW 236...WW 237...WW 238... DISCUSSION...SUSTAINED SUPERCELLS EXPECTED TO PERSIST MOIST...HIGHLY-SHEARED ENVIRONMENT OVER THE CNTRL PLNS ON ERN SIDE OF 70+ KT SSWLY MID LEVEL SPEED MAX. WHILE ACTIVITY MAY CONGEAL INTO ONE OR MORE SEMI-LINEAR CLUSTERS...POTENTIAL WILL REMAIN FOR TORNADOES. OTHER STORMS MAY FORM LATER TONIGHT NEAR DRY LINE IN WRN PART OF WW...AND IN WARM ADVECTION ZONE OVER NE KS. 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 22040. ...CORFIDI
BULLETIN - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH OUTLINE UPDATE FOR WT 239 NWS STORM PREDICTION CENTER NORMAN OK 905 PM CDT SAT MAY 5 2007 TORNADO WATCH 239 IS IN EFFECT UNTIL 400 AM CDT FOR THE FOLLOWING LOCATIONS KSC003-007-009-013-015-017-019-025-027-029-031-033-035-041-045- 047-049-051-053-055-057-059-061-065-069-073-077-079-081-083-085- 087-089-095-097-105-111-113-115-117-119-123-127-131-135-137-139- 141-143-145-147-149-151-155-157-159-161-163-165-167-169-173-175- 177-183-185-191-195-197-201-060900- /O.NEW.KWNS.TO.A.0239.070506T0205Z-070506T0900Z/ KS . KANSAS COUNTIES INCLUDED ARE ANDERSON BARBER BARTON BROWN BUTLER CHASE CHAUTAUQUA CLARK CLAY CLOUD COFFEY COMANCHE COWLEY DICKINSON DOUGLAS EDWARDS ELK ELLIS ELLSWORTH FINNEY FORD FRANKLIN GEARY GRAHAM GRAY GREENWOOD HARPER HARVEY HASKELL HODGEMAN JACKSON JEFFERSON JEWELL KINGMAN KIOWA LINCOLN LYON MARION MARSHALL MCPHERSON MEADE MITCHELL MORRIS NEMAHA NESS NORTON OSAGE OSBORNE OTTAWA PAWNEE PHILLIPS POTTAWATOMIE PRATT RENO REPUBLIC RICE RILEY ROOKS RUSH RUSSELL SALINE SEDGWICK SEWARD SHAWNEE SMITH STAFFORD SUMNER TREGO WABAUNSEE WASHINGTON ATTN...WFO...TOP...DDC...ICT...GID...GLD...
STATUS REPORT #1 ON WW 239
VALID 060435Z - 060540Z
THE SEVERE WEATHER THREAT CONTINUES ACROSS THE ENTIRE WATCH AREA.
FOR ADDITIONAL INFORMATION SEE MESOSCALE DISCUSSION 717