A tornado watch (SAME code: TOA) 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 (NWS) - issues watches for areas likely to produce tornadoes and severe thunderstorms. The watch boxes (or weather watches, WWs) are usually outlined 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 when atmospheric conditions are deemed sufficient to breed development of severe thunderstorms that are capable of producing significant tornadoes (either from supercells that develop strong, low-level rotation under typical tornadogenesis during the storm's maturation stage, or by thunderstorms that initiate mesocyclone maturation during their early development through a combination of sufficient wind shear and very high convective available potential energy (CAPE) values). 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 (colors used may vary, but very seldom if ever red, which is usually reserved for identifying tornado warnings; yellow is the most commonly used to denote them) 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.
Below is an example issued by the National Weather Service in Norman, Oklahoma.
SEL3 URGENT - IMMEDIATE BROADCAST REQUESTED Tornado Watch Number 233 NWS Storm Prediction Center Norman OK 215 PM CDT Fri May 24 2019 The NWS Storm Prediction Center has issued a * Tornado Watch for portions of eastern Kansas northwest Oklahoma * Effective this Friday afternoon and evening from 215 PM until 1000 PM CDT. * Primary threats include... A few tornadoes possible Scattered large hail and isolated very large hail events to 2 inches in diameter possible Scattered damaging wind gusts to 70 mph possible SUMMARY...Strong to severe storms will gradually increase from northern Oklahoma through eastern Kansas this afternoon and evening. Large hail, damaging wind gusts and a few tornadoes will be possible. The tornado watch area is approximately along and 75 statute miles north and south of a line from 60 miles northwest of Clinton OK to 55 miles southeast of Topeka KS. For a complete depiction of the watch see the associated watch outline update (WOUS64 KWNS WOU3). PRECAUTIONARY/PREPAREDNESS ACTIONS... 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 231...WW 232... AVIATION...Tornadoes and a few severe thunderstorms with hail surface and aloft to 2 inches. Extreme turbulence and surface wind gusts to 60 knots. A few cumulonimbi with maximum tops to 500. Mean storm motion vector 24035. ...Dial
Below is another example issued by the National Weather Service in Norman, Oklahoma.
BULLETIN - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH OUTLINE UPDATE FOR WT 68 NWS STORM PREDICTION CENTER NORMAN OK 825 AM EDT THU MAR 26 2009 TORNADO WATCH 68 IS IN EFFECT UNTIL 200 PM EDT FOR THE FOLLOWING LOCATIONS ALC031-045-061-067-069-261800- /O.NEW.KWNS.TO.A.0068.090326T1225Z-090326T1800Z/ AL . ALABAMA COUNTIES INCLUDED ARE COFFEE DALE GENEVA HENRY HOUSTON FLC005-013-045-059-063-131-133-261800- /O.NEW.KWNS.TO.A.0068.090326T1225Z-090326T1800Z/ FL . FLORIDA COUNTIES INCLUDED ARE BAY CALHOUN GULF HOLMES JACKSON WALTON WASHINGTON GAC035-037-053-061-063-077-079-099-113-145-149-151-171-197-199- 201-207-215-231-239-243-249-253-255-259-261-263-269-273-285-293- 307-261800- /O.NEW.KWNS.TO.A.0068.090326T1225Z-090326T1800Z/ GA . GEORGIA COUNTIES INCLUDED ARE BUTTS CALHOUN CHATTAHOOCHEE CLAY CLAYTON COWETA CRAWFORD EARLY FAYETTE HARRIS HEARD HENRY LAMAR MARION MERIWETHER MILLER MONROE MUSCOGEE PIKE QUITMAN RANDOLPH SCHLEY SEMINOLE SPALDING STEWART SUMTER TALBOT TAYLOR TERRELL TROUP UPSON WEBSTER GMZ750-261800- /O.NEW.KWNS.TO.A.0068.090326T1225Z-090326T1800Z/ CW . ADJACENT COASTAL WATERS INCLUDED ARE COASTAL WATERS FROM APALACHICOLA TO DESTIN FL OUT 20 NM ATTN...WFO...TAE...FFC...
STATUS REPORT #1 ON WW 68 VALID 261425Z - 261540Z SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 20 ESE PNS TO 30 N DHN TO 15 E ATL. ..SPC..03/26/09 ATTN...WFO...TAE...FFC...