Severe thunderstorm watch
Encyclopedia
A severe thunderstorm watch (SAME
code: SVA; sometimes referred to as "blue box" or "yellow box" by meteorologists and storm chaser
s) is issued when weather conditions are favorable for the development of severe thunderstorms. If the thunderstorms are forecast to be such that there is a significant risk that they may produce tornado
es, 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
.
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. While a severe thunderstorm watch does not imply in its name the risk for tornadoes, the risk for tornadoes in such is not zero and they can and occasionally do strike even after only a severe thunderstorm watch is issued.
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.) 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 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 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 similar warning is issued by Environment Canada
's Meteorological Service of Canada
from their offices in Vancouver
, Edmonton
, Toronto
, Montreal
and Dartmouth
, Nova Scotia
.
SEVERE THUNDERSTORM WATCH NUMBER 589
NWS STORM PREDICTION CENTER NORMAN OK
400 PM CDT SUN AUG 12 2007
THE NWS STORM PREDICTION CENTER HAS ISSUED A
SEVERE THUNDERSTORM WATCH FOR PORTIONS OF
SOUTHWEST IOWA
EXTREME NORTHWEST AND NORTH CENTRAL MISSOURI
EFFECTIVE THIS SUNDAY AFTERNOON AND EVENING FROM 400 PM UNTIL
1100 PM CDT.
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 55
STATUTE MILES NORTH AND SOUTH OF A LINE FROM 20 MILES SOUTH OF
OMAHA NEBRASKA TO 35 MILES NORTH OF KIRKSVILLE MISSOURI. 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.
OTHER WATCH INFORMATION...CONTINUE...WW 588...
DISCUSSION...BOW ECHO MCS APPEARS TO BE ORGANIZING W OF OMAHA...AND
THIS CONVECTIVE COMPLEX WILL LIKELY PROGRESS ESEWD ALONG A
BAROCLINIC ZONE NEAR THE MO/IA BORDER. MLCAPE VALUES OF 4000-5000
J/KG AND MODEST DEEP LAYER SHEAR ALONG THE BOUNDARY WILL HELP
MAINTAIN STORM INTENSITY INTO THE EVENING. RELATIVELY WIDESPREAD
DAMAGING WINDS WILL BE POSSIBLE ALONG THE PATH OF THIS
SYSTEM...ALONG WITH LARGE HAIL.
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 600. MEAN STORM
MOTION VECTOR 29035.
...THOMPSON
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
Specific Area Message Encoding
Specific Area Message Encoding or SAME is the protocol used to encode the Emergency Alert System and NOAA Weather Radio's SAME Public Warning System in the U.S...
code: SVA; sometimes referred to as "blue box" or "yellow box" by meteorologists and storm chaser
Storm chaser
Storm chaser can refer to:*Storm chasing, the pursuit of any severe weather condition*Storm Chaser, registered US Service Mark for Warren Faidley confirmed by the US Trademark Office as the first, full-time, professional storm chasing journalist...
s) is issued when weather conditions are favorable for the development of severe thunderstorms. If the thunderstorms are forecast to be such that there is a significant risk that they may produce tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es, then a tornado watch
Tornado watch
A tornado watch is issued when conditions are right for a tornado to form. Since any thunderstorm capable of producing a tornado is defined as severe, a tornado watch is also automatically a severe thunderstorm 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
Severe thunderstorm warning
A severe thunderstorm warning is issued when trained storm spotters or a Doppler weather radar indicate a strong thunderstorm is producing dangerously large hail or high winds, capable of causing significant damage. In the United States, it does not account for lightning or flooding...
.
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
Severe thunderstorm warning
A severe thunderstorm warning is issued when trained storm spotters or a Doppler weather radar indicate a strong thunderstorm is producing dangerously large hail or high winds, capable of causing significant damage. In the United States, it does not account for lightning or flooding...
or tornado warning
Tornado warning
A tornado warning is an alert issued by government weather services to warn that severe thunderstorms with tornadoes may be imminent. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation...
will be issued, and urgent action should be taken. While a severe thunderstorm watch does not imply in its name the risk for tornadoes, the risk for tornadoes in such is not zero and they can and occasionally do strike even after only a severe thunderstorm watch is issued.
In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, the Storm Prediction Center
Storm Prediction Center
The Storm Prediction Center , located in Norman, Oklahoma, is tasked with forecasting the risk of severe thunderstorms and tornadoes in the contiguous United States. The agency issues convective outlooks, mesoscale discussions, and watches as a part of this process...
(a national guidance center of the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
) 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
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
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 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 a major derecho
Derecho
A derecho is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo...
event), enhanced wording with the words particularly dangerous situation
Particularly dangerous situation
A particularly dangerous situation is a type of enhanced wording first used by the National Weather Service's Storm Prediction Center in Norman, Oklahoma on certain severe weather watches...
(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 similar warning is issued by Environment Canada
Environment Canada
Environment Canada , legally incorporated as the Department of the Environment under the Department of the Environment Act Environment Canada (EC) (French: Environnement Canada), legally incorporated as the Department of the Environment under the Department of the Environment Act Environment...
's Meteorological Service of Canada
Meteorological Service of Canada
The Meteorological Service of Canada , also known as "The Canadian Weather Service", is a division of Environment Canada, which primarily provides public meteorological information and weather forecasts and warnings of severe weather and other environmental hazards...
from their offices in Vancouver
Vancouver
Vancouver is a coastal seaport city on the mainland of British Columbia, Canada. It is the hub of Greater Vancouver, which, with over 2.3 million residents, is the third most populous metropolitan area in the country,...
, Edmonton
Edmonton
Edmonton is the capital of the Canadian province of Alberta and is the province's second-largest city. Edmonton is located on the North Saskatchewan River and is the centre of the Edmonton Capital Region, which is surrounded by the central region of the province.The city and its census...
, Toronto
Toronto
Toronto is the provincial capital of Ontario and the largest city in Canada. It is located in Southern Ontario on the northwestern shore of Lake Ontario. A relatively modern city, Toronto's history dates back to the late-18th century, when its land was first purchased by the British monarchy from...
, Montreal
Montreal
Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...
and Dartmouth
Dartmouth, Nova Scotia
Dartmouth founded in 1750, is a community and planning area of the Halifax Regional Municipality, Nova Scotia. Located on the eastern shore of Halifax Harbour, Dartmouth has been nicknamed the City of Lakes after the large number of lakes located in the city.On April 1, 1996, the provincial...
, Nova Scotia
Nova Scotia
Nova Scotia is one of Canada's three Maritime provinces and is the most populous province in Atlantic Canada. The name of the province is Latin for "New Scotland," but "Nova Scotia" is the recognized, English-language name of the province. The provincial capital is Halifax. Nova Scotia is the...
.
Example of a severe thunderstorm watch
URGENT - IMMEDIATE BROADCAST REQUIREDSEVERE THUNDERSTORM WATCH NUMBER 589
NWS STORM PREDICTION CENTER NORMAN OK
400 PM CDT SUN AUG 12 2007
THE NWS STORM PREDICTION CENTER HAS ISSUED A
SEVERE THUNDERSTORM WATCH FOR PORTIONS OF
SOUTHWEST IOWA
EXTREME NORTHWEST AND NORTH CENTRAL MISSOURI
EFFECTIVE THIS SUNDAY AFTERNOON AND EVENING FROM 400 PM UNTIL
1100 PM CDT.
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 55
STATUTE MILES NORTH AND SOUTH OF A LINE FROM 20 MILES SOUTH OF
OMAHA NEBRASKA TO 35 MILES NORTH OF KIRKSVILLE MISSOURI. 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.
OTHER WATCH INFORMATION...CONTINUE...WW 588...
DISCUSSION...BOW ECHO MCS APPEARS TO BE ORGANIZING W OF OMAHA...AND
THIS CONVECTIVE COMPLEX WILL LIKELY PROGRESS ESEWD ALONG A
BAROCLINIC ZONE NEAR THE MO/IA BORDER. MLCAPE VALUES OF 4000-5000
J/KG AND MODEST DEEP LAYER SHEAR ALONG THE BOUNDARY WILL HELP
MAINTAIN STORM INTENSITY INTO THE EVENING. RELATIVELY WIDESPREAD
DAMAGING WINDS WILL BE POSSIBLE ALONG THE PATH OF THIS
SYSTEM...ALONG WITH LARGE HAIL.
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 600. MEAN STORM
MOTION VECTOR 29035.
...THOMPSON
Example of Severe Thunderstorm Watch with PDS
URGENT - IMMEDIATE BROADCAST REQUESTEDSEVERE 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