Severe thunderstorm watch

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Map of average annual severe thunderstorm watches in the United States between 1999 and 2008.
Legend:

A severe thunderstorm watch (SAME code: SVA; sometimes referred to as "blue box" or "yellow box" by meteorologists and storm chasers) 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.[1]

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[2][3] from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth, Nova Scotia.[citation needed]

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).[4] 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[edit]

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

Example of Severe Thunderstorm Watch with PDS[edit]

  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

Example of a Watch Outline Update[edit]

  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...

References[edit]

  1. ^ "3.2.1.1 Geographical responsibilities". Retrieved 3 January 2013. 
  2. ^ "Severe Thunderstorm Watch". 25 February 2013. Retrieved 22 January 2014. 
  3. ^ "Severe Thunderstorm". 9 December 2013. Retrieved 22 January 2014. 
  4. ^ http://www.crh.noaa.gov/product_sites.php?site=ICT&product=WCN%7CWatch County Notification Messages

External links[edit]