A tornado watch means that the conditions have created a significant risk of a tornado occurring. In contrast, a tornado warning is used when the severe weather is actually occurring. If severe weather actually does occur, a tornado watch will be upgraded to a tornado warning or a severe thunderstorm warning.
Tornado watches in the United States 
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 are usually issued in the format of " __ miles (north and south; east and west; either side) of a line from __ miles (direction) of (City, State), to __ 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.) When displayed on a map, they appear as a red outline. In addition, a list of all counties included in its area of responsibility is issued by each NWS forecast office for each watch.
Examples of United States tornado watches 
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URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 227 NWS STORM PREDICTION CENTER NORMAN OK 615 PM CDT FRI MAY 4 2007 THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF WESTERN AND CENTRAL KANSAS WESTERN AND NORTHERN OKLAHOMA THE EASTERN TEXAS PANHANDLE AND NORTHWEST TEXAS EFFECTIVE THIS FRIDAY NIGHT AND SATURDAY MORNING FROM 615 PM UNTIL 200 AM CDT. TORNADOES...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 80 STATUTE MILES EAST AND WEST OF A LINE FROM 25 MILES WEST NORTHWEST OF RUSSELL KANSAS TO 75 MILES SOUTH SOUTHEAST OF CHILDRESS TEXAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU7). 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 226... DISCUSSION...WIDELY SCATTERED STORMS/SUPERCELLS POSSIBLE IN VERY UNSTABLE AIR MASS ALONG SLOWLY RETREATING DRY LINE THIS EVENING. STRENGTHENING ASCENT/WIND FIELD ASSOCIATED WITH APPROACHING FOUR CORNERS UPPER TROUGH MAY ENHANCE TORNADO POTENTIAL AFTER DARK AS LLJ INCREASES TO AROUND 50 KTS. 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 24030. ...CORFIDI 
PDS watches 
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 This is a particularly dangerous situation (PDS) can be added to the watch; this is occasionally issued.
Tornado watches in Canada 
In Canada, the criteria used 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.
See also 
- National Weather Service. "Tornado Watch". Glossary - NOAA's National Weather Service. Retrieved 2010-01-17.
- National Weather Service. "Severe thunderstorm". Glossary - NOAA's National Weather Service. Retrieved 2010-01-17.
- Corfidi, Steve (2007-05-04). "Tornado Watch 227". Storm Prediction Center. Retrieved 2010-01-17.
- Corfidi, Steve (2009-04-25). "Tornado Watch 186". Storm Prediction Center. Retrieved 2010-01-17.
- NOAA (2004-04-18). "Storm Prediction Center Frequently Asked Questions (FAQ)". Retrieved 2008-05-23.
- Environment Canada (2007-07-10). "Tornado watch". Prairie and Northern Region Weather Watch and Weather Warning Criteria. Retrieved 2010-01-19.