See Severe weather terminology for a comprehensive article on this term and related weather terms. A NOAA national weather forecast This article describes severe weather terminology used by the U.S. National Weather Service. ...

Thunderstorm out flow from storm core shows up as sheets of wind driven rain spreading from right to left in 1982 photo.

A severe thunderstorm watch (SAME code: SVA; sometimes referred to as "yellow box" by meteorologists and storm chasers) is issued when weather conditions are favorable for the development of severe thunderstorms. If the thunderstorms are forecasted to be so severe that they may produce tornadoes, then a tornado watch (which also automatically implies a severe thunderstorm watch) is issued. A severe thunderstorm watch can also be upgraded to a tornado watch as conditions warrant (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. Image File history File links Size of this preview: 800 × 520 pixelsFull resolution (1828 × 1188 pixel, file size: 1. ... Image File history File links Size of this preview: 800 × 520 pixelsFull resolution (1828 × 1188 pixel, file size: 1. ... See: In logic, sameness is synonymous with identity. ... Meteorology is the scientific study of the atmosphere that focuses on weather processes and forecasting. ... NSSL vehicles on Project Vortex. ... A severe thunderstorm is a thunderstorm with winds 58 mph or greater, 3/4 inch or larger hail, or tornadoes. ... A tornado in central Oklahoma. ... A tornado watch is issued when weather conditions are favorable for the development of severe thunderstorms that are capable of producing tornados. ... Severe thunderstorm with a clear slot near main updraft core. ...

A watch does not mean that the severe weather is actually occurring, only that conditions have created a significant risk for it. If severe weather actually does occur, a severe thunderstorm warning or tornado warning will be issued, and urgent action should be taken. It should be noted that while a severe thunderstorm watch does not imply in its name the risk for tornadoes, they can and occasionally do strike even when a tornado watch is not issued. Severe thunderstorm with a clear slot near main updraft core. ... Tornado at beginning of life. ...

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 (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.) In addition, a list of all counties included in its area of responsibility is now issued by each NWS forecast office for each watch. The Storm Prediction Center, located in Norman, Oklahoma, is part of the National Centers for Environmental Prediction (NCEP), operating under the control of the National Weather Service, which in turn is part of the National Oceanic and Atmospheric Administration (NOAA) of the U.S. government. ... The National Weather Service (NWS) is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration (NOAA) of the United States government. ... Fig. ... A county is generally a sub-unit of regional self-government within a sovereign jurisdiction. ...

In the event that a severe thunderstorm watch is likely to lead to very destructive winds or hail (usually 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); it is far more common with tornado watches. A derecho is a widespread and long-lived, violent convectively induced windstorm that is associated with a fast-moving band of severe thunderstorms usually taking the form of a bow echo. ... A particularly dangerous situation (PDS) is a type of enhanced wording used by the National Weather Services Storm Prediction Center in Norman, Oklahoma on certain weather watches. ...

A similar warning is issued by Environment Canada's Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth ,Nova Scotia. Environment Canada is the department of the government of Canada with responsibility for coordinating environmental policies and programs as well as preserving and enhancing the natural environment and conservation of wildlife. ... The Meteorological Service of Canada (MSC) is a Canadian government agency which provides meteorological information. ... Motto: By Sea, Land, and Air We Prosper Location of Vancouver within the Greater Vancouver Regional District in British Columbia, Canada Coordinates: , Country  Canada Province  British Columbia Region Lower Mainland Regional District Greater Vancouver Incorporated 1886 Government  - Mayor Sam Sullivan (NPA)  - City Council List of Councilors Suzanne Anton (NPA) Peter... Nickname: Motto: Industry - Integrity - Progress Location of Edmonton within census division number 11, Alberta, Canada. ... Nickname: Motto: Concordia Salus (well-being through harmony) Coordinates: , Country Canada Province Quebec Founded 1642 Established 1832 Government  - Mayor Gérald Tremblay Area [1][2][3]  - City 365. ... Ferry running between Halifax and Dartmouth, docked at Dartmouth Ferry Terminal. ... Motto: Munit Haec et Altera Vincit(Latin) One defends and the other conquers Capital Halifax Largest city Halifax Regional Municipality Official languages English Government - Lieutenant-Governor Mayann E. Francis - Premier Rodney MacDonald (PC) Federal representation in Canadian Parliament - House seats 11 - Senate seats 10 Confederation July 1, 1867 (1st) Area...

Example of a severe thunderstorm watch

 SEL2 URGENT - IMMEDIATE BROADCAST REQUESTED SEVERE THUNDERSTORM WATCH NUMBER 562 NWS STORM PREDICTION CENTER NORMAN OK 355 PM CDT SAT AUG 4 2007 THE NWS STORM PREDICTION CENTER HAS ISSUED A SEVERE THUNDERSTORM WATCH FOR PORTIONS OF WESTERN AND CENTRAL IOWA EASTERN NEBRASKA EXTREME SOUTHEAST SOUTH DAKOTA EFFECTIVE THIS SATURDAY AFTERNOON AND EVENING FROM 355 PM UNTIL 1100 PM CDT. HAIL TO 2.5 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 125 STATUTE MILES EAST AND WEST OF A LINE FROM 25 MILES NORTH NORTHWEST OF SIOUX CITY IOWA TO 10 MILES WEST OF LAMONI IOWA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU2). 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. DISCUSSION...AIR MASS IS VERY UNSTABLE AS MIXED PARCEL CAPE HAS REACHED 2000 J/KG. THUNDERSTORMS ARE EXPECTED TO DEVELOP NEAR SFC LOW OVER EXTREME SERN SD AND MOVE SEWD NEAR/ALONG BOUNDARY THAT EXTENDS INTO W CENTRAL IA. MAIN THREAT ACROSS MUCH OF THE AREA WILL BE DAMAGING WINDS AND HAIL. HOWEVER...DEEP LAYER SHEAR IS 50 KT ACROSS THE ERN QUARTER OF THE WATCH INDICATING AN ISOLATED TORNADO THREAT. AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 2.5 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 450. MEAN STORM MOTION VECTOR 30035. ...MCCARTHY 

External links

• Meteorological Service of Canada
• Current Watches