Severe thunderstorm warning
A severe thunderstorm warning is an alert issued by national weather forecasting agencies to warn the public that severe thunderstorms are imminent or occurring.
The warning is issued when trained storm spotters or Doppler weather radar indicate that a thunderstorm is producing or will soon produce dangerously large hail and high winds, capable of causing significant damage. Flooding caused by a thunderstorm's extreme rainfall. 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. In Australia, severe thunderstorm warnings are issued by the Bureau of Meteorology.
Definition
In the United States, the National Weather Service defines a severe thunderstorm as having large hail of or larger, surface wind speeds of or greater, and/or a tornado. Prior to January 2010, the hail size for which a thunderstorm would be considered severe was ; 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.In Canada, a severe thunderstorm is defined as having wind gusts of greater than, hail with a diameter of greater than, rainfall of greater than in an hour or greater than in three hours, or tornadoes.
In Australia, the Bureau of Meteorology issues severe thunderstorm warnings advising of damaging winds, heavy rainfall, large hail, 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. Similar official warnings are also issued for tropical cyclones, severe weather including heatwaves, tsunamis, and bushfires.
A severe thunderstorm warning indicates the warned area is in impending danger. 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 tornado, which can actually cover a much wider area than a tornado usually does. 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 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 or larger which may fall fast enough to potentially kill a person by repeated blunt trauma.
Weather Forecast Offices 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, 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
Here is an example of a Severe Thunderstorm Warning issued by the National Weather Service Baltimore/Washington. The audio file to the right is for Minnesota.
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...
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
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...
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
$$
KLW
This is a Severe Thunderstorm Warning issued for a cluster of severe thunderstorms.
Severe Thunderstorm Warning
ALC003-FLC033-113-130200-
/O.NEW.KMOB.SV.W.0225.200713T0114Z-200713T0200Z/
BULLETIN - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Warning
National Weather Service Mobile AL
814 PM CDT Sun Jul 12 2020
The National Weather Service in Mobile has issued a
- Severe Thunderstorm Warning for...
West central Santa Rosa County in northwestern Florida...
Escambia County in northwestern Florida...
- Until 900 PM CDT.
- At 813 PM CDT, a cluster of severe thunderstorms was located near
HAZARD...60 mph wind gusts.
SOURCE...Radar indicated.
IMPACT...Expect damage to roofs, siding, and trees.
- Locations impacted include...
Beach.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
For your protection move to an interior room on the lowest floor of a
building.
Damaging winds and continuous cloud to ground lightning is occurring
with this storm. Move indoors immediately.
Lightning is one of nature`s leading killers. Remember, if you can
hear thunder, you are close enough to be struck by lightning.
Torrential rainfall is occurring with this storm, and may lead to
flash flooding. Do not drive your vehicle through flooded roadways.
&&
LAT...LON 3022 8750 3092 8781 3099 8770 3070 8706
TIME...MOT...LOC 0113Z 301DEG 31KT 3084 8779
HAIL...<.75IN
WIND...60MPH
$$
CGAL
On August 19th, 2019 at 1:31 AM MDT, the National Weather Service in Cheyenne WY has incorporated the "emergency" wording to a Severe Thunderstorm Warning. This is unofficial wording that is used when an extremely intense Severe Storm impacts a largely populated location. In this case, 3 inch diameter hail was imminent for areas around both Scotts Bluff, and Gering, Wyoming.
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