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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 tornadoes.
A watch does not mean that the severe weather is actually occurring, only that atmospheric conditions have created a significant risk for it. If severe weather actually does occur, a tornado warning or severe thunderstorm warning would then be issued. Note that a watch is not required for a warning to be issued; tornado warnings are occasionally issued when a tornado watch is not active (i.e. when a severe thunderstorm watch is active, or when no watches are in effect), if a severe thunderstorm develops and has a confirmed tornado or strong rotation.
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. Occasionally, a tornado watch may replace a severe thunderstorm watch (or a portion of one) should conditions that were originally forecast to be conducive for non-tornadic severe thunderstorms change to allow possible tornado formation.
In Canada, the criteria used to issue a tornado watch 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.
The term "red box" refers to the assigned coloring used for watch box outlines used in Storm Prediction Center and National Weather Service products; television stations typically assign other colors (usually green) to highlight tornado watches in severe weather advisory displays.
Watch Outline Updates are relayed (and at the initial watch issuance, issued) by the Storm Prediction Center, however it is the local National Weather Service Weather Forecast Offices that decide what counties (in their warning area) are included or excluded in the watch, via a conference call with the SPC. As a result, watch products will sometimes display counties inside the watch outline that are not included in the counties listed, and vice versa; however the local Weather Forecast Office will need to expand to add these counties into the watch. A Watch Status Message works in a similar fashion; the SPC designates which areas it thinks where a threat still exists (the most common designation for this is on the basis of the location of surface features such as cold fronts and drylines that would delineate where the threat of severe thunderstorms has ended and where it will remain a possibility), and the NWS offices decide what counties to remove from the watch (the local offices will almost always follow the SPC recommendation on the status messages).
If conditions are no longer favorable for tornadoes in the watch area (either because atmospheric conditions that earlier supported tornadic development have become less conducive for them or were not present compared to predictions in earlier forecasts to form tornadoes), the tornado watch may either be replaced by a severe thunderstorm watch or cancelled outright; if no thunderstorm activity occurs, this leads to a tornado watch "bust", which may also factor into the Storm Prediction Center's decision as to whether to cancel the watch.
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