Severe thunderstorm warning
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A severe thunderstorm warning (SAME code: SVR) is issued when trained storm spotters or a Doppler weather radar indicate that a thunderstorm is producing or will soon produce dangerously large hail or high winds, capable of causing significant damage.[1] In the United States, severe thunderstorm warnings do not account for lightning, a significant hazard in any thunderstorm, or flooding caused by a thunderstorm's extreme rainfall (a flood advisory or Flash Flood Warning is issued in these cases).[1] A similar warning is issued by Environment Canada's Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth.[2] Skywarn issues the severe thunderstorm warnings for the United Kingdom.[3]
Contents
Definition
In the United States, the National Weather Service (NWS) defines a severe thunderstorm as having large hail of at least 1 inch (2.5 cm), surface wind speeds of 58 miles per hour (93 km/h) or greater, or any combination of the those two criteria.[4] Prior to January 2010, the hail size for which a thunderstorm would be considered severe was 0.75 inches (1.9 cm); public complacency due to overly frequent issuances of severe thunderstorm warnings and recent studies stating that hail did not produce significant damage on the ground until it reached one inch in diameter caused the upgrade in hail criteria.[5]
In Canada, a severe thunderstorm is defined as having wind gusts of greater than 90 kilometres per hour (56 mph), hail with a diameter of greater than 20 millimetres (0.79 in), rainfall of greater than 50 millimetres (2.0 in) in an hour or greater than 75 millimetres (3.0 in) in three hours, or tornadoes.[2]
A severe thunderstorm warning indicates that there is impending danger for the warned area. Occasionally, severe thunderstorms can and do produce tornadoes without warning.[6] While not all severe thunderstorms produce tornadoes, they can produce serious straight line wind damage as severe as a tornado, which can actually cover a much wider area than a tornado usually does.[7] If a tornado is detected on radar or is sighted visually, a tornado warning will be issued either in replacement of or concurrently to the existing severe thunderstorm warning.[8] Generally, but not always, a severe thunderstorm watch or tornado watch will precede a warning.
If a tornado warning is issued, based on Doppler weather radar, it means that strong rotation has been detected within a thunderstorm. Usually, if a thunderstorm is producing only weak rotation, it will only yield hazardous weather warranting a severe thunderstorm warning. However, the public will usually be advised that this type of rotation has been detected and that the storm in question should be watched closely in the near future for further intensification. In the United States, local NWS forecast offices, particularly those in the Great Plains or Southeastern U.S., sometimes include the wording "Severe thunderstorms can produce tornadoes with no advance warning..." or a similar reference in their severe thunderstorm warnings, usually when there is a tornado threat or especially when a tornado watch is in effect.
Some storms, especially in the Great Plains, may produce massive hailstones the size of baseballs (2.75 inches (7.0 cm)) or larger which may fall fast enough to potentially kill a person by repeated blunt trauma.
Weather Forecast Offices (WFO) of the National Weather Service issue warnings for tornadoes and severe thunderstorms in polygon shapes, based on the projected path of a storm at the time of the warning's issuance as estimated by Doppler radar. Warnings were issued on a per-county basis before October 2007,[9] and they are now usually delineated on maps in polygon shapes and in text by a sections of counties, although entire counties are sometimes included, especially if they are small. Storm Prediction Center and other NWS products, as well as severe weather alert displays used by some U.S. television stations, highlight severe thunderstorm warnings with a yellow or orange polygon or filled county/parish outlines. Depending on the severity of the storm, some NWS offices may also request activation of the Emergency Alert System at their discretion.
Criteria in various countries
| Country | Tornado | Minimum hail size | Minimum wind speed | Minimum rainfall rate per hour | Source |
|---|---|---|---|---|---|
| United States | See Tornado Warning | 1 inch (25 mm) | 58 miles per hour (93 km/h) | See Flash Flood Warning | [1] |
| Canada | See Tornado warning | 20 millimetres (0.79 in) | 90 kilometres per hour (56 mph) | 50 millimetres (2.0 in) | [2] |
| Australia | All | 20 millimetres (0.79 in) | 90 kilometres per hour (56 mph) | "Conducive to flash flooding" | [10] |
| New Zealand | F1 or stronger | 20 millimetres (0.79 in) | 110 kilometres per hour (68 mph) | 25 millimetres (0.98 in) | [11] |
| Germany | All | 15 millimetres (0.59 in) | 105 kilometres per hour (65 mph) | 20 millimetres (0.79 in) | [12] |
| Mexico | All | 5 millimetres (0.20 in) | 89 kilometres per hour (55 mph) | 20 millimetres (0.79 in) | [13][14][15] |
| United Kingdom | All | 20 millimetres (0.79 in) | 55 miles per hour (89 km/h) | "Conducive to flash flooding" | [3] |
Examples of Severe Thunderstorm Warnings
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An example of a severe thunderstorm warning, this one being issued in Minnesota
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Examples of Severe Thunderstorm Warnings issued by the National Weather Service in the United States:[16]
WUUS53 KICT 020245
SVRICT
KSC053-105-169-020345-
/O.NEW.KICT.SV.W.0171.110602T0245Z-110602T0345Z/
BULLETIN - EAS ACTIVATION REQUESTED
SEVERE THUNDERSTORM WARNING
NATIONAL WEATHER SERVICE WICHITA KS
945 PM CDT WED JUN 1 2011
THE NATIONAL WEATHER SERVICE IN WICHITA HAS ISSUED A
* SEVERE THUNDERSTORM WARNING FOR...
EASTERN ELLSWORTH COUNTY IN CENTRAL KANSAS...
SOUTHEASTERN LINCOLN COUNTY IN CENTRAL KANSAS...
SALINE COUNTY IN CENTRAL KANSAS...
* UNTIL 1045 PM CDT
* AT 941 PM CDT...NATIONAL WEATHER SERVICE METEOROLOGISTS WERE
TRACKING A SEVERE THUNDERSTORM CAPABLE OF PRODUCING QUARTER SIZE
HAIL...AND EXTREME DAMAGING WINDS IN EXCESS OF 100 MPH. THIS STORM
WAS LOCATED NEAR WESTFALL...OR 12 MILES SOUTHEAST OF LINCOLN...AND
MOVING SOUTHEAST AT 35 MPH.
* LOCATIONS IMPACTED INCLUDE...
SALINA...BROOKVILLE...KANOPOLIS STATE PARK...NEW CAMBRIA...
SMOLAN...GLENDALE AND SALINA AIRPORT.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
THIS IS AN EXTREMELY DANGEROUS SITUATION WITH TORNADO LIKE WIND
SPEEDS EXPECTED. MOBILE HOMES AND HIGH PROFILE VEHICLES ARE
ESPECIALLY SUSCEPTIBLE TO WINDS OF THIS MAGNITUDE AND MAY BE
OVERTURNED. FOR YOUR PROTECTION MOVE TO AN INTERIOR ROOM ON THE
LOWEST FLOOR OF YOUR HOME OR BUSINESS. THIS STORM HAS THE POTENTIAL
TO CAUSE SERIOUS INJURY AND SIGNIFICANT DAMAGE TO PROPERTY.
&&
A TORNADO WATCH REMAINS IN EFFECT UNTIL MIDNIGHT CDT THURSDAY MORNING
FOR EAST CENTRAL KANSAS.
LAT...LON 3908 9792 3896 9792 3896 9737 3880 9737
3858 9805 3892 9833 3910 9803
TIME...MOT...LOC 0246Z 322DEG 31KT 3885 9802
WIND...HAIL 100MPH 1.00IN
$$
MCGUIRE
SEVERE THUNDERSTORM WARNING NEC037-039-119-141-167-302230- /O.NEW.KOAX.SV.W.0024.120330T2144Z-120330T2230Z/ BULLETIN - IMMEDIATE BROADCAST REQUESTED SEVERE THUNDERSTORM WARNING NATIONAL WEATHER SERVICE OMAHA/VALLEY NEBRASKA 444 PM CDT FRI MAR 30 2012 THE NATIONAL WEATHER SERVICE IN OMAHA HAS ISSUED A * SEVERE THUNDERSTORM WARNING FOR... COLFAX COUNTY IN EAST CENTRAL NEBRASKA... EXTREME SOUTHWESTERN CUMING COUNTY IN EASTERN NEBRASKA... EXTREME SOUTHEASTERN MADISON COUNTY IN EASTERN NEBRASKA... NORTHEASTERN PLATTE COUNTY IN EAST CENTRAL NEBRASKA... EXTREME SOUTHWESTERN STANTON COUNTY IN EASTERN NEBRASKA... * UNTIL 530 PM CDT * AT 441 PM CDT...TRAINED WEATHER SPOTTERS REPORTED A SEVERE THUNDERSTORM PRODUCING QUARTER SIZE HAIL. THIS STORM WAS LOCATED NEAR CRESTON...OR 19 MILES NORTH OF COLUMBUS...AND MOVING EAST AT 30 MPH. * LOCATIONS IMPACTED INCLUDE... CLARKSON...HOWELLS...LEIGH...CRESTON AND HUMPHREY. PRECAUTIONARY/PREPAREDNESS ACTIONS... SEVERE THUNDERSTORMS PRODUCE DAMAGING WINDS IN EXCESS OF 60 MPH... DESTRUCTIVE HAIL...DEADLY LIGHTNING AND VERY HEAVY RAIN. FOR YOUR PROTECTION...MOVE TO AN INTERIOR ROOM ON THE LOWEST FLOOR OF YOUR HOME OR BUSINESS. HEAVY RAINS FLOOD ROADS QUICKLY SO DO NOT DRIVE INTO AREAS WHERE WATER COVERS THE ROAD. && LAT...LON 4179 9748 4173 9691 4145 9702 4163 9753 TIME...MOT...LOC 2141Z 285DEG 24KT 4171 9743 WIND...HAIL <50MPH 1.00IN $$ KERN
See also
- Convective storm detection
- Severe thunderstorm watch
- Severe weather
- Severe weather terminology (United States)
- SKYWARN
- Supercell
References
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