1. Definition

2. Criteria in various countries

3. Examples of severe thunderstorm warnings

4. See also

5. References

6. External links

A severe thunderstorm warning (SAME code: SVR) is issued by the National Weather Service when trained storm spotters or 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. Skywarn issues the severe thunderstorm warnings for the United Kingdom. Just as in the United States, lightning does not warrant a severe thunderstorm warning.^[1] In Australia, severe thunderstorm warnings are issued by the Bureau of Meteorology for all Australian states.^[2]

Definition

In the United States, the National Weather Service (NWS) defines a severe thunderstorm as having large hail of at least {{convert|1|in|cm}}, surface wind speeds of {{convert|58|mph|kph}} or greater, and/or a tornado.^[3] Prior to January 2010, the hail size for which a thunderstorm would be considered severe was {{convert|0.75|in|cm}}; public complacency due to overly frequent issuances of severe thunderstorm warnings and recent studies stating hail didn't produce significant damage on the ground until it reached one inch in diameter caused the upgrade in hail criteria.^[4]

In Canada, a severe thunderstorm is defined as having wind gusts of greater than {{convert|90|kph|mph}}, hail with a diameter of greater than {{convert|20|mm|in}}, rainfall of greater than {{convert|50|mm|in}} in an hour or greater than {{convert|75|mm|in}} in three hours, or tornadoes.

In Australia, the Bureau of Meteorology issues severe thunderstorm warnings advising of damaging winds (>90km/h), heavy rainfall, large hail (2cm or larger), and flash flooding. Severe thunderstorm warnings are given as either a broad-based warning, covering expected impact in a weather reporting area, or as a detailed warning, when a thunderstorm is within weather-watch radar range and includes a map depicting any existing thunderstorms and the forecast direction of movement for up to 60 minutes.^[2] Similar official warnings are also issued for tropical cyclones, severe weather including heatwaves, tsunamis, and bushfires.^[5]

A severe thunderstorm warning indicates the warned area is in impending danger. Occasionally, severe thunderstorms can and do produce tornadoes without warning.^[11] 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.^[12] 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.^[13] 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 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 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 ({{convert|2.75|in|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 outline warnings for tornadoes and severe thunderstorms in polygonal shapes for their map-based weather hazard products, 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,^[14] 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 the total area of the division is 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

Examples of severe thunderstorm warnings

Examples of Severe Thunderstorm Warnings issued by the National Weather Service in the United States:^[6]

WUUS54 KJAN 092210SVRJANMSC079-089-121-123-092315-/O.NEW.KJAN.SV.W.0216.140609T2210Z-140609T2315Z/BULLETIN - EAS ACTIVATION REQUESTEDSEVERE THUNDERSTORM WARNINGNATIONAL WEATHER SERVICE JACKSON MS510 PM CDT MON JUN 9 2014THE NATIONAL WEATHER SERVICE IN JACKSON HAS ISSUED A* SEVERE THUNDERSTORM WARNING FOR...  LEAKE COUNTY IN CENTRAL MISSISSIPPI...  EAST CENTRAL MADISON COUNTY IN CENTRAL MISSISSIPPI...  NORTHEASTERN RANKIN COUNTY IN CENTRAL MISSISSIPPI...  NORTHWESTERN SCOTT COUNTY IN CENTRAL MISSISSIPPI...* UNTIL 615 PM CDT* AT 510 PM CDT...A SEVERE THUNDERSTORM WAS LOCATED NEAR PISGAH...AND  MOVING NORTHEAST AT 35 MPH.  HAZARD...60 MPH WIND GUSTS AND QUARTER SIZE HAIL.  SOURCE...RADAR INDICATED.  IMPACT...HAIL DAMAGE TO VEHICLES IS EXPECTED. EXPECT WIND DAMAGE TO  ROOFS...SIDING AND TREES.* THE SEVERE THUNDERSTORM WILL BE NEAR...  LEESBURG AND PISGAH AROUND 515 PM CDT.  LUDLOW AROUND 530 PM CDT.  FARMHAVEN AROUND 535 PM CDT.  CARTHAGE AROUND 600 PM CDT.  THOMASTOWN AROUND 605 PM CDT.OTHER LOCATIONS IN THE WARNING INCLUDE REDWATER.PRECAUTIONARY/PREPAREDNESS ACTIONS...FOR YOUR PROTECTION MOVE TO AN INTERIOR ROOM ON THE LOWEST FLOOR OF ABUILDING.&&LAT...LON 3290 8972 3278 8939 3231 8981 3238 8993TIME...MOT...LOC 2210Z 208DEG 30KT 3243 8983HAIL...1.00INWIND...60MPH$$DL

Severe Thunderstorm WarningOKC003-039-043-045-059-093-129-151-153-250545-/O.NEW.KOUN.SV.W.0409.180625T0454Z-180625T0545Z/BULLETIN - IMMEDIATE BROADCAST REQUESTEDSevere Thunderstorm WarningNational Weather Service Norman OK1154 PM CDT SUN JUN 24 2018The National Weather Service in Norman has issued a* Severe Thunderstorm Warning for...  Ellis County in northwestern Oklahoma...  Woods County in northwestern Oklahoma...  Northern Roger Mills County in western Oklahoma...  Eastern Harper County in northwestern Oklahoma...  Northwestern Alfalfa County in northwestern Oklahoma...  Northwestern Custer County in western Oklahoma...  Dewey County in northwestern Oklahoma...  Woodward County in northwestern Oklahoma...  Western Major County in northwestern Oklahoma...* Until 1245 AM CDT.* At 1153 PM CDT, severe thunderstorms were located along a line  extending from 10 miles southeast of Aetna to 6 miles northwest of  Mooreland to 10 miles northeast of Roll, moving east at 40 mph.  These are very dangerous storms.  HAZARD...80 mph wind gusts and two inch hail.  SOURCE...Radar indicated.  IMPACT...Flying debris will be dangerous to those caught without           shelter. Mobile homes will be heavily damaged. Expect           considerable damage to roofs, windows, and vehicles.           Extensive tree damage and power outages are likely.* Locations impacted include...  Woodward, Alva, Cherokee, Shattuck, Arnett, Taloga, Mooreland,  Waynoka, Seiling, Vici, Gage, Leedey, Fargo, Carmen, Fort Supply,  Freedom, Aline, Camargo, Burlington and Sharon.PRECAUTIONARY/PREPAREDNESS ACTIONS...Prepare immediately for large hail and damaging winds. People outsideshould move immediately to shelter inside a strong building. Stayaway from windows.&&LAT...LON 3700 9813 3617 9864 3560 9902 3591 9999      3661 9980 3700 9945TIME...MOT...LOC 0453Z 288DEG 36KT 3695 9887 3652 9926 3592 9962HAIL...2.00INWIND...80MPH$$WR

See also

• Convective storm detection
• Severe thunderstorm watch
• Severe weather
• Severe weather terminology (United States)
• SKYWARN
• Supercell

References

External links

• [https://web.archive.org/web/20110601195138/http://www.bom.gov.au/catalogue/warnings/WarningsInformation_SW_STSW.shtml Bureau of Meteorology: Severe Thunderstorm Warnings]
• [https://web.archive.org/web/20060703204148/http://www.msc-smc.ec.gc.ca/cd/brochures/warning_e.cfm Meteorological Service of Canada]
• United States National Weather Service

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