Severe thunderstorm warning
This article contains too many or overly lengthy quotations for an encyclopedic entry. (December 2019)
A severe thunderstorm warning (SAME code: SVR) is a severe weather warning product issued by regional offices of weather forecasting agencies throughout the world to alert the public that severe thunderstorms are imminent or occurring. A severe thunderstorm warning is issued when Doppler weather radar, trained storm spotters or local emergency management personnel indicate that a thunderstorm is producing large hail and high winds capable of causing significant damage, and is expected to continue producing severe weather along the storm's projected track. Flooding is also sometimes caused by torrential rainfall produced by a thunderstorm. (In these cases, a flood advisory or flash flood warning may be issued to alert the public of the flooding threat.)
A severe thunderstorm warning indicates the warned area is in impending danger from hail or wind speeds meeting warning criteria as well as from lightning and hydrological impacts associated with the storm cell. Severe thunderstorms can and do produce tornadoes without warning. While not all severe thunderstorms produce tornadoes, they can produce serious straight line wind damage as severe as a lower-category tornado or hurricane, and can actually cover a much wider area than that exceeded by a tornado's comparatively relative narrow path width.
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. Generally, but not always, a severe thunderstorm watch or tornado watch (or a regional equivalent thereof) 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.
Regional basis and criteria
In the United States, the National Weather Service (NWS) defines a severe thunderstorm as having large hail of one inch (2.5 cm) in diameter or larger, surface wind speeds of 58 mph (50 kn; 93 km/h) or greater, and/or a tornado. 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 did not produce significant damage on the ground until it reached one inch in diameter caused the upgrade in hail criteria. Severe thunderstorm warnings are issued by the National Weather Service through the agency's local Weather Forecast Offices, which utilize WarnGen software integrated into the Advance Weather Interactive Processing System (AWIPS) to generate the warning statement; it is then disseminated through various communication routes accessed by the media and various agencies, on the internet, to NOAA satellites, over NOAA Weather Radio and, depending on the storm's severity and at the office's discretion, via activation of the Emergency Alert System to local broadcast media and Wireless Emergency Alerts for cellular phones.
Local NWS forecast offices outline warnings for tornadoes and severe thunderstorms in polygonal shapes for map-based weather hazard products distributed to the main agency, individual forecast office websites and the Storm Prediction Center (including open-source APIs available for free use to public weather websites and mobile apps), based on the storm's projected path as determined by Doppler radar at the time of the warning's issuance. In NWS text products, warnings are usually illustrated by individual counties, parishes or other county-equivalent jurisdictions (sections or the entirety thereof, and in list format if it covers more than one jurisdiction), particularly dependent on the jurisdiction's total land area. Prior to October 2007, warnings were issued by the National Weather Service on a per-county basis. SPC and NWS products as well as severe weather alert displays used by some television stations typically highlight severe thunderstorm warnings with a yellow or orange polygon or filled county/parish/equivalent jurisdiction outline.
The NWS has the option of adding enhanced wording to severe thunderstorm warnings and update statements issued as a Severe Weather Statement (SVS)—"particularly dangerous situation" (PDS), "severe thunderstorm emergency", or, as used by some Central and Southern Region offices as indicative PDS wording, "this is a very dangerous storm"—when an extremely intense severe thunderstorm expected to impact a densely populated area contains very large hailstones and/or intense straight-line winds capable of causing major structural damage (especially to roofs, windows, siding and structures of poor construction such as outbuildings and mobile homes), and severe injury if not death to people and animals exposed outdoors (from either repeated hail-induced blunt trauma or wind-generated debris). Although it is an unofficial alert product, a "severe thunderstorm emergency" statement—an extension of the tornado emergency statements issued to warn of a violent tornado impacting populated areas—is the highest and most urgent level relating to non-tornadic severe thunderstorms. When a threat of tornadic activity exists (especially if a tornado watch is in effect), local NWS forecast offices, particularly those in the Great Plains or Southeastern U.S., will sometimes include a safety precaution action statement indicating "Severe thunderstorms can produce tornadoes with no advance warning..." or similar wording in their severe thunderstorm warnings to advise the public to keep abreast of possible tornado development and any tornado warnings that may be issued.
Since April 28, 2021, the NWS has added categorical damage threat indicators for higher-end hail and/or wind events at the bottom of text products for severe thunderstorm warnings (including update statements to existing warnings issued as Severe Weather Statements):
- Considerable – to be used when hail of 1.75 inches (4.4 cm) or larger and/or winds at or above 70 miles per hour (110 km/h) is indicated by radar or observed.
- Destructive – to be used when hail of 2.75 inches (7.0 cm) or larger and/or winds at or above 80 miles per hour (130 km/h) is indicated by radar or observed.
Similar to the indicators used for tornado warnings since the 2013 implementation of its Impact Based Warning system, the categorical criteria—which will be applicable to all NWS Weather Forecast Offices—was introduced to further explain to the public of the dangers of some thunderstorms, especially those that occur in the Great Plains, that can produce massive hailstones of baseball size (2.75 inches [7.0 cm]) or larger that may fall fast enough to severely injure if not kill a person by repeated blunt trauma, and winds that can cause damage equivalent to hurricanes and tornadoes (especially to roofs and weaker structures such as outbuildings and mobile homes). When deemed necessary, initial warnings and Severe Weather Statement updates containing the "destructive" tag will be disseminated as Wireless Emergency Alerts.
In Canada, a severe thunderstorm is defined as having wind gusts of greater than 90 km/h (56 mph), hail with a diameter of greater than two centimetres (0.79 in), rainfall rates of greater than 50 millimetres (2.0 in) in one hour or greater than 75 millimetres (3.0 in) in three hours, or tornadoes. Environment and Climate Change Canada issues severe thunderstorm warnings through regional offices of the Meteorological Service of Canada based in Vancouver, Edmonton, Winnipeg, Toronto, Montreal and Dartmouth for specified municipalities and census subdivisions, sometimes including areas adjacent to a particular warned thunderstorm that are not in the immediate approximate path but may experience severe weather from adjacent thunderstorms during the warning timeframe. Warnings are disseminated to the public through broadcast and online media outlets, and Weatheradio Canada; depending on storm severity and regional office discretion, the warning may require activation of the National Public Alerting System (Alert Ready) (French: Système national d'alertes à la population [En Alerte]) and feeding provincial alerting systems (such as Alberta Emergency Alert and SaskAlert) to distribute the alert to local broadcast media and cellular phones.
In United Kingdom, a severe thunderstorm is defined as having hail in excess of two centimetres (0.79 in) and straight-line winds in excess of 89 km/h (48 kn; 55 mph). The Met Office issues de facto severe thunderstorm warnings through general-purpose weather warning products disseminated by the National Severe Weather Warning Service (classified as yellow, amber or red, depending on storm severity and proximity to the time of expected onset of storm impacts).
In Australia, a severe thunderstorm is defined as having damaging winds in excess of 90 km/h (49 kn; 56 mph), large hail of two centimetres (0.79 in) or larger, heavy rainfall, and flash flooding. Severe thunderstorm warnings are issued by regional offices of the Bureau of Meteorology (BOM) based in Melbourne Docklands, Adelaide, Darwin, Perth and Brisbane. BOM-issued severe thunderstorm warnings are outlined as either a broad-based warning, covering expected impacts within 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.
In New Zealand, a severe thunderstorm is defined by the Meteorological Service of New Zealand (MetService) (Māori: Te Ratonga Tirorangi) as having large hail of 20 millimetres (2.0 cm; 0.79 in), damaging winds of 110 km/h (59 kn; 68 mph), rainfall rates of at least 25 millimetres (0.98 in) per hour and/or tornadoes producing winds above 116 km/h (63 kn; 72 mph) (rated F1 or stronger on the Fujita Scale). MetService issues severe thunderstorm warnings to alert regions expected to experience severe weather phenomena meeting regional criteria and utilize the Common Alerting Protocol (CAP) to generate the warning statement. The warning is then disseminated to the public through terrestrial television and radio stations, online media, mobile app and SMS messaging through Emergency Mobile Alert (EMA); if a tornado has been sighted or indicated by radar within the warned area during the severe thunderstorm warning's duration, civil defense sirens, if present, are usually activated within the region to be affected to inform people who are present outdoors of the approaching tornado threat.
In the Philippines, a severe thunderstorm is defined by the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) as having strong winds, large hail, torrential rainfall, tornadoes, flash flooding and/or landslides. PAGASA, through its regional services divisions, issue thunderstorm advisories to alert regions and/or provinces expected to experience severe weather phenomena meeting regional criteria within a period of two hours. Warnings are disseminated to the public through terrestrial television and radio outlets, online media, and via SMS messages through the National Disaster Risk Reduction and Management Council's Emergency Cell Broadcast System.
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|||
|Canada||See Tornado Warning||20 millimetres (0.79 in)||90 kilometres per hour (56 mph)||50 millimetres (2.0 in)|||
|Australia||All||20 millimetres (0.79 in)||90 kilometres per hour (56 mph)||"Conducive to flash flooding"|||
|New Zealand||F1 or stronger||20 millimetres (0.79 in)||110 kilometres per hour (68 mph)||25 millimetres (0.98 in)|||
|Germany||All||15 millimetres (0.59 in)||105 kilometres per hour (65 mph)||20 millimetres (0.79 in)|||
|Mexico||All||5 millimetres (0.20 in)||89 kilometres per hour (55 mph)||20 millimetres (0.79 in)|||
|United Kingdom||All||20 millimetres (0.79 in)||55 miles per hour (89 km/h)||"Conducive to flash flooding"|||
Examples of severe thunderstorm warnings
Severe Thunderstorm Warning VAC059-153-600-683-685-140130- /O.NEW.KLWX.SV.W.0529.190814T0058Z-190814T0130Z/ BULLETIN - IMMEDIATE BROADCAST REQUESTED Severe Thunderstorm Warning National Weather Service Baltimore MD/Washington DC 858 PM EDT Tue Aug 13 2019 The National Weather Service in Sterling Virginia has issued a * Severe Thunderstorm Warning for... The City of Fairfax in northern Virginia... Central Fairfax County in northern Virginia... The City of Manassas Park in northern Virginia... Central Prince William County in northern Virginia... The City of Manassas in northern Virginia... * Until 930 PM EDT. * At 858 PM EDT, a severe thunderstorm was located near Bull Run, or near Centreville, moving east at 25 mph. HAZARD...60 mph wind gusts and quarter size hail. SOURCE...Radar indicated. IMPACT...Damaging winds will cause some trees and large branches to fall. This could injure those outdoors, as well as damage homes and vehicles. Roadways may become blocked by downed trees. Localized power outages are possible. Unsecured light objects may become projectiles. * Locations impacted include... Centreville, Reston, Fairfax, Vienna, Mantua, Burke, Linton Hall, Oakton, Chantilly, Wolf Trap, Merrifield, Bull Run, Newington, Manassas, West Springfield, Sudley, Manassas Park, Fairfax Station, Ravensworth and Clifton. PRECAUTIONARY/PREPAREDNESS ACTIONS... For your protection move to an interior room on the lowest floor of a building. && LAT...LON 3871 7753 3881 7755 3895 7729 3875 7720 TIME...MOT...LOC 0058Z 250DEG 21KT 3879 7746 HAIL...1.00IN WIND...60MPH $$ KLW
This is a Severe Thunderstorm Warning issued for a cluster of severe thunderstorms.
Severe Thunderstorm Warning ALC003-FLC033-113-130200- /O.NEW.KMOB.SV.W.0225.200713T0114Z-200713T0200Z/ BULLETIN - IMMEDIATE BROADCAST REQUESTED Severe Thunderstorm Warning National Weather Service Mobile AL 814 PM CDT Sun Jul 12 2020 The National Weather Service in Mobile has issued a * Severe Thunderstorm Warning for... Southeastern Baldwin County in southwestern Alabama... West central Santa Rosa County in northwestern Florida... Escambia County in northwestern Florida... * Until 900 PM CDT. * At 813 PM CDT, a cluster of severe thunderstorms was located near Bay Minette, moving southeast at 35 mph. HAZARD...60 mph wind gusts. SOURCE...Radar indicated. IMPACT...Expect damage to roofs, siding, and trees. * Locations impacted include... Ensley, Gonzalez, Bay Minette, Pace, Lillian, Molino and Perdido Beach. PRECAUTIONARY/PREPAREDNESS ACTIONS... For your protection move to an interior room on the lowest floor of a building. Damaging winds and continuous cloud to ground lightning is occurring with this storm. Move indoors immediately. Lightning is one of nature`s leading killers. Remember, if you can hear thunder, you are close enough to be struck by lightning. Torrential rainfall is occurring with this storm, and may lead to flash flooding. Do not drive your vehicle through flooded roadways. && LAT...LON 3022 8750 3092 8781 3099 8770 3070 8706 TIME...MOT...LOC 0113Z 301DEG 31KT 3084 8779 HAIL...<.75IN WIND...60MPH $$ CGAL
On August 19, 2019 at 1:31 a.m. MDT, the National Weather Service in Cheyenne, Wyoming incorporated the "emergency" wording into a Severe Thunderstorm Warning. In this instance, the "severe thunderstorm emergency" statement was included to denote that three-inch (7.6 cm) diameter hail (as confirmed by storm spotter reports) was imminent in portions of Scotts Bluff, Banner and Sioux Counties in Nebraska, specifically areas around the towns of Scottsbluff and Gering.
� 187 WWUS55 KCYS 160731 SVSCYS Severe Weather Statement National Weather Service Cheyenne WY 131 AM MDT Fri Aug 16 2019 NEC007-157-165-160815- /O.CON.KCYS.SV.W.0342.000000T0000Z-190816T0815Z/ Scotts Bluff NE-Banner NE-Sioux NE- 131 AM MDT Fri Aug 16 2019 ...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 215 AM MDT FOR SCOTTS BLUFF...NORTHEASTERN BANNER AND SOUTHEASTERN SIOUX COUNTIES... ...THIS IS A SEVERE THUNDERSTORM EMERGENCY FOR SCOTTSBLUFF AND GERING!... At 129 AM MDT, a severe thunderstorm was located near Scotts Bluff National Monument, or near Scottsbluff, moving east at 40 mph. 3 INCH DIAMETER HAIL IS IMMINENT! PREPARE NOW FOR DESTRUCTIVE HAIL! This is a very dangerous storm! HAZARD...Three inch hail and 60 mph wind gusts. SOURCE...Trained weather spotters. IMPACT...People and animals outdoors will be severely injured. Expect shattered windows, extensive damage to roofs, siding, and vehicles. Locations impacted include... Scottsbluff, Gering, Mitchell, Terrytown, Minatare, Wildcat Hills State Recreation Area, Lake Minatare Campground, Wildcat Hills Campground, Scotts Bluff National Monument, Hubbard Hill, Scottsbluff Airport, Lake Minatare, Lake Alice, Melbeta and McGrew. PRECAUTIONARY/PREPAREDNESS ACTIONS... For your protection move to an interior room on the lowest floor of a building. This is a dangerous storm. Prepare immediately for large destructive hail capable of producing significant damage. People outside should move to a shelter, inside a strong building and away from windows. && LAT...LON 4185 10394 4233 10379 4230 10344 4209 10345 4209 10340 4200 10340 4198 10336 4156 10337 4170 10388 TIME...MOT...LOC 0729Z 288DEG 34KT 4189 10374 HAIL...3.00IN WIND...60MPH $$ Lyons
- Convective storm detection
- Severe thunderstorm watch
- Severe weather
- Severe weather terminology (United States)
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