Tornado watch
Encyclopedia
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
. In most cases, the potential exists for large hail and/or damaging winds in addition to the tornadoes.
A tornado watch must not be confused with a tornado warning
: a watch does not imply that the severe weather is actually occurring, only that conditions have created a significant risk for it. If severe weather actually does occur, a tornado warning
or severe thunderstorm warning
will be issued.
, 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 are usually issued in the format of " __ miles (north and south; east and west; either side) of a line from __ miles (direction) of (City, State), to __ 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 appear as a red outline. In addition, a list of all counties included in its area of responsibility is issued by each NWS forecast office for each watch.
TORNADO WATCH NUMBER 227
NWS STORM PREDICTION CENTER NORMAN OK
615 PM CDT FRI MAY 4 2007
THE NWS STORM PREDICTION CENTER HAS ISSUED A
TORNADO WATCH FOR PORTIONS OF
WESTERN AND CENTRAL KANSAS
WESTERN AND NORTHERN OKLAHOMA
THE EASTERN TEXAS PANHANDLE AND NORTHWEST TEXAS
EFFECTIVE THIS FRIDAY NIGHT AND SATURDAY MORNING FROM 615 PM
UNTIL 200 AM CDT.
TORNADOES...HAIL TO 3 INCHES IN DIAMETER...THUNDERSTORM WIND
GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
AREAS.
THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 80 STATUTE
MILES EAST AND WEST OF A LINE FROM 25 MILES WEST NORTHWEST OF
RUSSELL KANSAS TO 75 MILES SOUTH SOUTHEAST OF CHILDRESS TEXAS.
FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
OUTLINE UPDATE (WOUS64 KWNS WOU7).
REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
TORNADOES AND 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.
OTHER WATCH INFORMATION...CONTINUE...WW 226...
DISCUSSION...WIDELY SCATTERED STORMS/SUPERCELLS POSSIBLE IN VERY
UNSTABLE AIR MASS ALONG SLOWLY RETREATING DRY LINE THIS EVENING.
STRENGTHENING ASCENT/WIND FIELD ASSOCIATED WITH APPROACHING FOUR
CORNERS UPPER TROUGH MAY ENHANCE TORNADO POTENTIAL AFTER DARK AS LLJ
INCREASES TO AROUND 50 KTS.
AVIATION...TORNADOES AND 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
550. MEAN STORM MOTION VECTOR 24030.
...CORFIDI
along with possible destructive winds and hail, enhanced wording with the words This is a particularly dangerous situation (PDS) can be added to the watch; this is occasionally issued.
, the criteria used are the same and watches are issued by regional offices of the Meteorological Service of Canada
of Environment Canada
in Vancouver
, Edmonton
, Toronto
, Montreal
and Halifax, on a county or regional basis.
Severe thunderstorm watch
[File:Annual_severe_thunderstorm_watch_frequency_in_the_United_States.svg|thumb|right|300px|Map of average annual severe thunderstorm watches in the United States between 1999 and 2008.Legend:...
. In most cases, the potential exists for large hail and/or damaging winds in addition to the tornadoes.
A tornado watch must not be confused with a 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...
: a watch does not imply that the severe weather is actually occurring, only that conditions have created a significant risk for it. If severe weather actually does occur, a 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...
or 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...
will be issued.
Tornado watches in the United States
In the United StatesUnited 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 are usually issued in the format of " __ miles (north and south; east and west; either side) of a line from __ miles (direction) of (City, State), to __ 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 appear as a red outline. In addition, a list of all counties included in its area of responsibility is issued by each NWS forecast office for each watch.
Examples of United States tornado watches
URGENT - IMMEDIATE BROADCAST REQUESTEDTORNADO WATCH NUMBER 227
NWS STORM PREDICTION CENTER NORMAN OK
615 PM CDT FRI MAY 4 2007
THE NWS STORM PREDICTION CENTER HAS ISSUED A
TORNADO WATCH FOR PORTIONS OF
WESTERN AND CENTRAL KANSAS
WESTERN AND NORTHERN OKLAHOMA
THE EASTERN TEXAS PANHANDLE AND NORTHWEST TEXAS
EFFECTIVE THIS FRIDAY NIGHT AND SATURDAY MORNING FROM 615 PM
UNTIL 200 AM CDT.
TORNADOES...HAIL TO 3 INCHES IN DIAMETER...THUNDERSTORM WIND
GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
AREAS.
THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 80 STATUTE
MILES EAST AND WEST OF A LINE FROM 25 MILES WEST NORTHWEST OF
RUSSELL KANSAS TO 75 MILES SOUTH SOUTHEAST OF CHILDRESS TEXAS.
FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
OUTLINE UPDATE (WOUS64 KWNS WOU7).
REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
TORNADOES AND 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.
OTHER WATCH INFORMATION...CONTINUE...WW 226...
DISCUSSION...WIDELY SCATTERED STORMS/SUPERCELLS POSSIBLE IN VERY
UNSTABLE AIR MASS ALONG SLOWLY RETREATING DRY LINE THIS EVENING.
STRENGTHENING ASCENT/WIND FIELD ASSOCIATED WITH APPROACHING FOUR
CORNERS UPPER TROUGH MAY ENHANCE TORNADO POTENTIAL AFTER DARK AS LLJ
INCREASES TO AROUND 50 KTS.
AVIATION...TORNADOES AND 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
550. MEAN STORM MOTION VECTOR 24030.
...CORFIDI
PDS watches
In the event that a tornado watch is likely to lead to a major tornado outbreakTornado outbreak
While there is no single agreed upon definition, generally at least 6-10 tornadoes produced by the same synoptic scale weather system is considered a tornado outbreak. The tornadoes usually occur within the same day, or continue into the early morning hours of the succeeding day, and within the...
along with possible destructive winds and hail, enhanced wording with the words This is a particularly dangerous situation (PDS) can be added to the watch; this is occasionally issued.
Tornado watches in Canada
In CanadaCanada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, the criteria used are the same and watches are issued by regional offices of the 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...
of 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...
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 Halifax, on a county or regional basis.