A severe thunderstorm warning (SAME code: SVR) 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.[1] In the United States, it does not account for lightning or flooding.[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 warnings for the United Kingdom.[3]
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In the U.S., the National Weather Service defines a severe thunderstorm as having large hail being at least 1 inch (2.5 cm), high winds as being 58 miles per hour (93 km/h) or greater, producing tornadoes (though this would result in a tornado warning), or any combination of the three.[4] Prior to January 2010, the size in which hail in a severe thunderstorm would be considered severe was ¾ of an inch; public complacency due to overly frequent issuances of severe thunderstorm warnings and recent studies stating that hail does not produce damage until it reaches one inch in diameter caused the upgrade.[5] In Canada, a severe thunderstorm has wind gusts of >90 km/h (56 mph), hail with a diameter of >20 mm (0.8 inch), rainfall of >50 mm (2 inches) in an hour or >75 mm (3 inches) in three hours, or tornadoes.[2]
A severe thunderstorm warning means there is significant danger for the warned area. Occasionally, severe thunderstorms can and do produce a tornado without warning.[6] While not all severe thunderstorms produce tornadoes, they can produce serious straight line wind damage as severe as a tornado (and can actually cover a much wider area than a tornado).[7] If a tornado is detected on radar or sighted, 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 strong rotation has been detected. Usually if a severe thunderstorm is producing weak rotation, it will only be a Severe Thunderstorm Warning; however, the public will usually be advised that this rotation has been detected, and the storm should be watched closely.
In the United States, local National Weather Service 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 warning products, usually when there is a tornado threat or especially when a tornado watch is in effect, though sometimes the wording is included even if the threat of severe weather does not include a threat of tornadic activity.
Some storms, especially in the Great Plains, may produce massive hailstones the size of baseballs (2.75 inches (7.0 cm)) or larger, falling fast enough to potentially kill a person by repeated blunt trauma.
In the United States, the National Weather Service issues warnings for tornadoes and severe thunderstorms in polygon shapes. Warnings are usually delineated in text by county or by a section of a county.
| 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" | [9] |
| New Zealand | F1 or stronger | 20 millimetres (0.79 in) | 110 kilometres per hour (68 mph) | 25 millimetres (0.98 in) | [10] |
| Germany | All | 15 millimetres (0.59 in) | 105 kilometres per hour (65 mph) | 20 millimetres (0.79 in) | [11] |
| Mexico | All | 5 millimetres (0.20 in) | 89 kilometres per hour (55 mph) | 20 millimetres (0.79 in) | [12][13][14] |
| United Kingdom | All | 20 millimetres (0.79 in) | 58 miles per hour (93 km/h) | "Conducive to flash flooding" | [3] |
An example of a Severe Thunderstorm Warning issued by the National Weather Service in the United States.[15]
BULLETIN - EAS ACTIVATION REQUESTED SEVERE THUNDERSTORM WARNING NATIONAL WEATHER SERVICE TOPEKA KS 802 AM CDT MON JUN 2 2008 THE NATIONAL WEATHER SERVICE IN TOPEKA HAS ISSUED A * SEVERE THUNDERSTORM WARNING FOR... SOUTHERN REPUBLIC COUNTY IN NORTH CENTRAL KANSAS... * UNTIL 845 AM CDT * AT 800 AM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A SEVERE THUNDERSTORM CAPABLE OF PRODUCING BASEBALL SIZE HAIL...AND DAMAGING WINDS IN EXCESS OF 60 MPH. THIS STORM WAS LOCATED 4 MILES NORTHWEST OF COURTLAND...OR 22 MILES NORTHWEST OF CONCORDIA... MOVING EAST AT 35 MPH. * LOCATIONS IMPACTED INCLUDE... SCANDIA... NORWAY... BELLEVILLE... CUBA... AGENDA... LARGE DESTRUCTIVE HAIL WILL OCCUR WITH THIS STORM. FOR YOUR SAFETY MOVE INDOORS NOW...AND STAY AWAY FROM WINDOWS. STAY TUNED TO WEATHER RADIO OR LOCAL MEDIA OUTLETS FOR THE LATEST SEVERE WEATHER INFORMATION. LAT...LON 3990 9793 3991 9736 3965 9736 3971 9793 TIME...MOT...LOC 1302Z 272DEG 30KT 3980 9793 $$ CAVANAUGH
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