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A Severe Thunderstorm Watch (SAME code: SVA) is issued when weather conditions are favorable for the development of severe thunderstorms. If thunderstorms are expected to be of sufficient strength such that there is a significant risk that they may produce tornadoes, then a Tornado Watch (which also automatically implies that severe thunderstorms are possible) is issued. A Severe Thunderstorm Watch can also be upgraded to a Tornado Watch if conditions originally forecasted for limited to no tornadic development change to allow possible tornado formation (in which case the existing severe thunderstorm watch, or a portion of it, would be replaced). A watch must not be confused with a Severe Thunderstorm Warning.
Contents
- Watch Outline Updates and Status Messages
- Example of a severe thunderstorm watch
- Example of Severe Thunderstorm Watch with PDS
- Example of a Watch Outline Update
- References
A watch does not necessarily mean that severe weather is actually occurring, only that atmospheric conditions have created a significant risk for severe weather to occur. If severe weather actually does occur, a severe thunderstorm warning or tornado warning will be issued, and urgent action should be taken immediately. While a severe thunderstorm watch does not imply in its name the risk for tornadoes, the risk for tornadoes in such a scenario is not necessarily zero as severe thunderstorms can and occasionally do spawn tornadoes even after only a severe thunderstorm watch has been issued.
In the United States, the Storm Prediction Center (a national guidance center of the National Weather Service (NWS)) issues watches for areas of the lower 48 states that are likely to produce tornadoes and/or severe thunderstorms. The local NWS forecast offices in Hawaii and Alaska issue their own Severe Thunderstorm and Tornado Watches.
The terms "blue box" and "yellow box" refer to the assigned coloring used for watch box outlines used in Storm Prediction Center and National Weather Service products as well as by television broadcast media (in the case of the latter, blue is typically assigned to highlight severe thunderstorm watches whereas yellow is often assigned to severe thunderstorm warnings).
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.) When displayed on a map, they are usually shown as either a blue or yellow outline, depending on the source. In addition, a list of all counties included in its area of responsibility is now issued by each local NWS forecast office for each watch.
In the event that a Severe Thunderstorm Watch is likely to lead to very destructive winds or hail (usually from a major derecho event), enhanced wording with the words Particularly Dangerous Situation (PDS) can be added to the watch. This is rare with Severe Thunderstorm Watches, since the tornado threat has to remain low enough to only warrant a Severe Thunderstorm Watch and a higher tornado threat would warrant a normal Tornado Watch despite the extreme wind or hail threat. It is far more common with Tornado Watches.
Similar watches and warnings are issued by Environment Canada's Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth, Nova Scotia.
Watch Outline Updates and Status Messages
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, the severe thunderstorm watch may be cancelled outright; if no thunderstorm activity occurs or if thunderstorms that develop remain under severe criteria, this leads to a severe thunderstorm watch "bust", which may also factor into the Storm Prediction Center's decision as to whether to cancel the watch.
Example of a severe thunderstorm watch
The following severe thunderstorm watch was issued for a large portion of the path of the June 2012 North American derecho.
URGENT - IMMEDIATE BROADCAST REQUESTED SEVERE THUNDERSTORM WATCH NUMBER 436 NWS STORM PREDICTION CENTER NORMAN OK 305 PM EDT FRI JUN 29 2012 THE NWS STORM PREDICTION CENTER HAS ISSUED A SEVERE THUNDERSTORM WATCH FOR PORTIONS OF EXTREME SOUTHEAST INDIANA EXTREME NORTHERN KENTUCKY MOST OF CENTRAL AND SOUTHERN OHIO CENTRAL AND NORTHWEST WEST VIRGINIA EFFECTIVE THIS FRIDAY AFTERNOON AND EVENING FROM 305 PM UNTIL 1000 PM EDT. HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 80 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS. THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 80 STATUTE MILES NORTH AND SOUTH OF A LINE FROM 35 MILES WEST NORTHWEST OF DAYTON OHIO TO 45 MILES EAST OF PARKERSBURG WEST VIRGINIA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU6). REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE FAVORABLE FOR 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. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY DO PRODUCE TORNADOES. OTHER WATCH INFORMATION...CONTINUE...WW 435... DISCUSSION...A DEVELOPING MCS/BOW ECHO ACROSS INDIANA IS EXPECTED TO MOVE ESEWD THROUGH THE AFTERNOON/EVENING AT ROUGHLY 50 KT. THE STORM ENVIRONMENT DOWNSTREAM FEATURES STRONG INSTABILITY AND SUFFICIENT DEEP-LAYER FLOW/SHEAR TO MAINTAIN AN ORGANIZED MCS...AND THERE WILL BE THE POTENTIAL FOR WIDESPREAD DAMAGING WINDS /SOME SIGNIFICANT/ WITH THIS BOWING SYSTEM THROUGH THIS EVENING. AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 650. MEAN STORM MOTION VECTOR 29050. ...THOMPSON