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.
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.
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.
The following severe thunderstorm watch was issued for a large portion of the Northern Oklahoma and portions of Kansas and Missouri on April 25, 2017.
URGENT - IMMEDIATE BROADCAST REQUESTED Severe Thunderstorm Watch Number 159 NWS Storm Prediction Center Norman OK 555 PM CDT Tue Apr 25 2017
The NWS Storm Prediction Center has issued a
* Severe Thunderstorm Watch for portions of Southeast Kansas Western Missouri Central and eastern Oklahoma
* Effective this Tuesday afternoon and Wednesday morning from 555 PM until 200 AM CDT.
* Primary threats include... Scattered large hail likely with isolated very large hail events to 3 inches in diameter possible Isolated damaging wind gusts to 70 mph possible A tornado or two possible
SUMMARY...Strong to severe thunderstorms will continue to develop and intensify through the early/mid-evening hours, initially from central and northeast Oklahoma into far southeast Kansas. Large hail and damaging winds should be the most common hazard, although some tornado risk may exist as low-level moisture and low-shear both increase this evening.
The severe thunderstorm watch area is approximately along and 80 statute miles north and south of a line from 5 miles northwest of Oklahoma City OK to 50 miles north northeast of Joplin MO. For a complete depiction of the watch see the associated watch outline update (WOUS64 KWNS WOU9).
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.
AVIATION...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 500. Mean storm motion vector 23035.
URGENT - IMMEDIATE BROADCAST REQUESTED SEVERE THUNDERSTORM WATCH NUMBER 405 NWS STORM PREDICTION CENTER NORMAN OK 840 PM CDT MON MAY 30 2011 THE NWS STORM PREDICTION CENTER HAS ISSUED A SEVERE THUNDERSTORM WATCH FOR PORTIONS OF PARTS OF NORTHWEST IOWA PARTS OF CENTRAL KANSAS MUCH OF EASTERN NEBRASKA SMALL PART OF SOUTHEASTERN SOUTH DAKOTA EFFECTIVE THIS MONDAY NIGHT AND TUESDAY MORNING FROM 840 PM UNTIL 200 AM CDT. ...THIS IS A PARTICULARLY DANGEROUS SITUATION... WIDESPREAD DAMAGING WINDS WIND GUSTS TO 90 MPH LIKELY SCATTERED LARGE HAIL ISOLATED HAIL TO 1.5 INCHES IN DIAMETER POSSIBLE ISOLATED TORNADOES THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 75 STATUTE MILES EAST AND WEST OF A LINE FROM 35 MILES NORTHWEST OF SIOUX CITY IOWA TO 50 MILES SOUTH SOUTHEAST OF RUSSELL KANSAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU5). 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 402...WW 403... DISCUSSION...INTENSE SQUALL LINE HAS DEVELOPED N/S ACROSS CENTRAL NEB AND CONTINUES TO DEVELOP SWD AS IT MOVES EWD. VERY STRONG PRES RISE/FALL COUPLET IS DRIVING THE LINE EWD WITH THE FORCING FROM LARGE SCALE TROUGH ROTATING ACROSS THE NRN/CENTRAL PLAINS PROVIDING BOTH SHEAR AND UPWARD MOTION TO RESULT IN POTENTIAL FOR VERY DAMAGING WINDS. IN ADDITION BRIEF TORNADOES ARE STILL POSSIBLE BUT DAMAGING WINDS NOW ARE THE DOMINANT THREAT. AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 80 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 550. MEAN STORM MOTION VECTOR 23040. ...HALES
BULLETIN - IMMEDIATE BROADCAST REQUESTED SEVERE THUNDERSTORM WATCH OUTLINE UPDATE FOR WS 271 NWS STORM PREDICTION CENTER NORMAN OK 1135 AM EDT SUN JUN 2 2013 SEVERE THUNDERSTORM WATCH 271 IS IN EFFECT UNTIL 800 PM EDT FOR THE FOLLOWING LOCATIONS CTC003-005-030000- /O.NEW.KWNS.SV.A.0271.130602T1535Z-130603T0000Z/ CT . CONNECTICUT COUNTIES INCLUDED ARE HARTFORD LITCHFIELD MAC003-011-013-015-017-027-030000- /O.NEW.KWNS.SV.A.0271.130602T1535Z-130603T0000Z/ MA . MASSACHUSETTS COUNTIES INCLUDED ARE BERKSHIRE FRANKLIN HAMPDEN HAMPSHIRE MIDDLESEX WORCESTER MEC001-003-005-007-011-017-019-021-025-031-030000- /O.NEW.KWNS.SV.A.0271.130602T1535Z-130603T0000Z/ ME . MAINE COUNTIES INCLUDED ARE ANDROSCOGGIN AROOSTOOK CUMBERLAND FRANKLIN KENNEBEC OXFORD PENOBSCOT PISCATAQUIS SOMERSET YORK NHC001-003-005-007-009-011-013-015-017-019-030000- /O.NEW.KWNS.SV.A.0271.130602T1535Z-130603T0000Z/ NH . NEW HAMPSHIRE COUNTIES INCLUDED ARE BELKNAP CARROLL CHESHIRE COOS GRAFTON HILLSBOROUGH MERRIMACK ROCKINGHAM STRAFFORD SULLIVAN NYC001-019-021-027-031-035-039-057-083-091-093-095-111-113-115- 030000- /O.NEW.KWNS.SV.A.0271.130602T1535Z-130603T0000Z/ NY . NEW YORK COUNTIES INCLUDED ARE ALBANY CLINTON COLUMBIA DUTCHESS ESSEX FULTON GREENE MONTGOMERY RENSSELAER SARATOGA SCHENECTADY SCHOHARIE ULSTER WARREN WASHINGTON VTC001-003-005-007-009-011-013-015-017-019-021-023-025-027- 030000- /O.NEW.KWNS.SV.A.0271.130602T1535Z-130603T0000Z/ VT . VERMONT COUNTIES INCLUDED ARE ADDISON BENNINGTON CALEDONIA CHITTENDEN ESSEX FRANKLIN GRAND ISLE LAMOILLE ORANGE ORLEANS RUTLAND WASHINGTON WINDHAM WINDSOR ATTN...WFO...BOX...BTV...ALY...GYX...CAR...