Severe thunderstorm warning

Severe thunderstorm with a clear slot near main updraft core. Typically a tornado, if present, will form in this area.

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.[2] Skywarn issues the severe thunderstorm warnings for the United Kingdom. Just as in the United States, lightning does not warrant a severe thunderstorm warning.[3] In Australia, severe thunderstorm warnings are issued by the Bureau of Meteorology for all Australian states.[4]

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, and/or a tornado.[5] 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 hail didn't produce significant damage on the ground until it reached one inch in diameter caused the upgrade in hail criteria.[6]

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]

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.[4] Similar official warnings are also issued for tropical cyclones, severe weather including heatwaves, tsunamis, and bushfires.[7]

A severe thunderstorm warning indicates the warned area is in impending danger. Occasionally, severe thunderstorms can and do produce tornadoes without warning.[8] 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.[9] 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.[10] 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 (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 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,[11] 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

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" [12]
New Zealand F1 or stronger 20 millimetres (0.79 in) 110 kilometres per hour (68 mph) 25 millimetres (0.98 in) [13]
Germany All 15 millimetres (0.59 in) 105 kilometres per hour (65 mph) 20 millimetres (0.79 in) [14]
Mexico All 5 millimetres (0.20 in) 89 kilometres per hour (55 mph) 20 millimetres (0.79 in) [15][16][17]
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

Examples of Severe Thunderstorm Warnings issued by the National Weather Service in the United States:[18]

WUUS54 KJAN 092210
SVRJAN
MSC079-089-121-123-092315-
/O.NEW.KJAN.SV.W.0216.140609T2210Z-140609T2315Z/

BULLETIN - EAS ACTIVATION REQUESTED
SEVERE THUNDERSTORM WARNING
NATIONAL WEATHER SERVICE JACKSON MS
510 PM CDT MON JUN 9 2014

THE 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 A
BUILDING.

&&

LAT...LON 3290 8972 3278 8939 3231 8981 3238 8993
TIME...MOT...LOC 2210Z 208DEG 30KT 3243 8983

HAIL...1.00IN
WIND...60MPH

$$

DL

Severe Thunderstorm Warning
OKC003-039-043-045-059-093-129-151-153-250545-
/O.NEW.KOUN.SV.W.0409.180625T0454Z-180625T0545Z/

BULLETIN - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Warning
National Weather Service Norman OK
1154 PM CDT SUN JUN 24 2018

The 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 outside
should move immediately to shelter inside a strong building. Stay
away from windows.

&&

LAT...LON 3700 9813 3617 9864 3560 9902 3591 9999
      3661 9980 3700 9945
TIME...MOT...LOC 0453Z 288DEG 36KT 3695 9887 3652 9926 3592 9962

HAIL...2.00IN
WIND...80MPH

$$

WR

See also

References

  1. 1 2 3 "Severe Thunderstorm Warning". NOAA National Weather Service Glossary. Retrieved 2010-01-10.
  2. 1 2 3 Environment Canada. "Severe Thunderstorm Warning". Prairie and Northern Region Weather Watch and Weather Warning Criteria. Archived from the original on 2007-11-19. Retrieved 2010-01-10.
  3. 1 2 "SkyWarn UK Warning criteria". SkyWarn UK. Retrieved 21 December 2011.
  4. 1 2 "Australian weather and warnings information". www.bom.gov.au. Archived from the original on 2017-01-07. Retrieved 2017-01-06.
  5. ↑ "Storm Prediction Center Frequently Asked Questions (FAQ)". www.spc.noaa.gov. Retrieved 2018-09-09.
  6. ↑ McClure, Mike (2009-05-26). "New One Inch Hail Criteria for Severe Thunderstorm Warnings" (PDF). Weather Home Companion. National Weather Service Quad Cities, IA/IL. Archived from the original (PDF) on December 15, 2010. Retrieved 2010-12-15.
  7. ↑ "National Weather Services - Bureau of Meteorology". www.bom.gov.au. Retrieved 2017-01-06.
  8. ↑ Howerton, Paul (2008-06-12). "Severe Thunderstorm Warning". Iowa Environmental Mesonet NWS Product Archive. Retrieved 2010-01-10.
  9. ↑ "Severe Thunderstorm". NOAA National Weather Service Glossary. Retrieved 2010-01-10.
  10. ↑ Howerton, Paul (2008-06-12). "Severe Thunderstorm Warning". Iowa Environmental Mesonet NWS Product Archive. Retrieved 2010-01-10.
  11. ↑ NOAA Warning Decision Training Branch (2008-02-26). "Why Storm-Based Warnings?". Storm-Based Warnings. Archived from the original on 2010-01-24. Retrieved 2010-01-11.
  12. ↑ Bureau of Meteorology (October 2004). "Bureau of Meteorology: Severe Weather Warning Education". Severe Weather Warning Education. Archived from the original on 2012-11-27. Retrieved 2010-02-06.
  13. ↑ Meteorological Service of New Zealand Ltd. "Severe Thunderstorm Warnings and other NZ Weather Warnings from MetService.com". Severe Thunderstorm Criteria. Retrieved 2010-02-06.
  14. ↑ Deutscher Wetterdienst. "Wetter und Klima - Deutscher Wetterdienst -- Warnungen". Unwetterkriterien (in German). Retrieved 2010-02-06.
  15. ↑ Servicio Meteorológico Nacional (2010-01-30). "Granizo". Servicio Meteorologico Nacional - Mexico - Glosario G (in Spanish). Archived from the original on 2010-12-13. Retrieved 2010-02-06.
  16. ↑ Servicio Meteorológico Nacional (2010-02-05). "Escalas empleadas" (in Spanish). Archived from the original on 2013-02-21. Retrieved 2010-02-06.
  17. ↑ Servicio Meteorológico Nacional (2010-02-05). "Aviso de Tormentas Intensas y Granizo" (in Spanish). Archived from the original on 2013-02-21. Retrieved 2010-02-06.
  18. ↑ McGuire, Mick (2011-06-02). "NWS Wichita 2011 Severe Thunderstorm Warning #171". National Weather Service Wichita, KS. Retrieved 2012-02-03.
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