Tornado watch

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See Severe weather terminology for a comprehensive article on related weather terms.
An example of a tornado watch for parts of Kansas and Nebraska, issued by the Storm Prediction Center on May 5, 2007.

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 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.

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 and Status Messages[edit]

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).[1]

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.

Example of a tornado watch[edit]

   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 

Example of a Watch Outline Update[edit]

   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... 

Example of a Watch Status Message[edit]

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

..SPC..05/06/07

ATTN...WFO...TOP...DDC...ICT...GLD...GID...

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