Severe thunderstorm warning
Encyclopedia
Specific Area Message Encoding
Specific Area Message Encoding or SAME is the protocol used to encode the Emergency Alert System and NOAA Weather Radio's SAME Public Warning System in the U.S...
code: SVR) is issued when trained storm spotters or a Doppler
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
indicate a strong thunderstorm is producing dangerously large hail or high winds, capable of causing significant damage. In the United States, it does not account for lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
or flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
ing. A similar warning is issued by Environment Canada
Environment Canada
Environment Canada , legally incorporated as the Department of the Environment under the Department of the Environment Act Environment Canada (EC) (French: Environnement Canada), legally incorporated as the Department of the Environment under the Department of the Environment Act Environment...
's Meteorological Service of Canada
Meteorological Service of Canada
The Meteorological Service of Canada , also known as "The Canadian Weather Service", is a division of Environment Canada, which primarily provides public meteorological information and weather forecasts and warnings of severe weather and other environmental hazards...
from their offices in Vancouver
Vancouver
Vancouver is a coastal seaport city on the mainland of British Columbia, Canada. It is the hub of Greater Vancouver, which, with over 2.3 million residents, is the third most populous metropolitan area in the country,...
, Edmonton
Edmonton
Edmonton is the capital of the Canadian province of Alberta and is the province's second-largest city. Edmonton is located on the North Saskatchewan River and is the centre of the Edmonton Capital Region, which is surrounded by the central region of the province.The city and its census...
, Toronto
Toronto
Toronto is the provincial capital of Ontario and the largest city in Canada. It is located in Southern Ontario on the northwestern shore of Lake Ontario. A relatively modern city, Toronto's history dates back to the late-18th century, when its land was first purchased by the British monarchy from...
, Montreal
Montreal
Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...
and Dartmouth
Dartmouth, Nova Scotia
Dartmouth founded in 1750, is a community and planning area of the Halifax Regional Municipality, Nova Scotia. Located on the eastern shore of Halifax Harbour, Dartmouth has been nicknamed the City of Lakes after the large number of lakes located in the city.On April 1, 1996, the provincial...
.
Definition
In the U.S.United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
defines a severe thunderstorm as having large hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
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
Tornado warning
A tornado warning is an alert issued by government weather services to warn that severe thunderstorms with tornadoes may be imminent. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation...
), or any combination of the three. 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. In Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, 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.
A severe thunderstorm warning means there is significant danger for the warned area. Occasionally, severe thunderstorms can and do produce a tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
without warning. 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). If a tornado is detected on radar or sighted, a tornado warning
Tornado warning
A tornado warning is an alert issued by government weather services to warn that severe thunderstorms with tornadoes may be imminent. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation...
will be issued either in replacement of or concurrently to the existing severe thunderstorm warning. Generally, but not always, a severe thunderstorm watch
Severe thunderstorm watch
[File:Annual_severe_thunderstorm_watch_frequency_in_the_United_States.svg|thumb|right|300px|Map of average annual severe thunderstorm watches in the United States between 1999 and 2008.Legend:...
or tornado watch
Tornado watch
A tornado watch is issued when conditions are right for a tornado to form. Since any thunderstorm capable of producing a tornado is defined as severe, a tornado watch is also automatically a severe thunderstorm watch...
will precede a warning.
If a Tornado Warning is issued, based on Doppler
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...
weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
, 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
Great Plains
The Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...
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
Tornado watch
A tornado watch is issued when conditions are right for a tornado to form. Since any thunderstorm capable of producing a tornado is defined as severe, a tornado watch is also automatically a severe thunderstorm 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 cm)) or larger, falling fast enough to potentially kill a person by repeated blunt trauma
Blunt trauma
In medical terminology, blunt trauma, blunt injury, non-penetrating trauma or blunt force trauma refers to a type of physical trauma caused to a body part, either by impact, injury or physical attack; the latter usually being referred to as blunt force trauma...
.
In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
issues warnings for tornadoes and severe thunderstorms in polygon
Polygon
In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...
shapes. Warnings are usually delineated in text by county or by a section of a county.
Criteria in various countries
| Country | Tornado | Minimum hail size | Minimum wind speed | Minimum rainfall rate per hour | Source |
|---|---|---|---|---|---|
| United States | See Tornado warning Tornado warning A tornado warning is an alert issued by government weather services to warn that severe thunderstorms with tornadoes may be imminent. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation... |
1 inches (25.4 mm) | 58 miles per hour (93.3 km/h) | See Flash flood warning Flash Flood Warning A Flash Flood Warning is issued when a flash flood is imminent or occurring in the warned area. A flash flood is a sudden, violent flood after a heavy rain... |
|
| Canada | See Tornado warning Tornado warning A tornado warning is an alert issued by government weather services to warn that severe thunderstorms with tornadoes may be imminent. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation... |
20 millimetre (0.78740157480315 in) | 90 kilometres per hour (55.9 mph) | 50 millimetres (2 in) | |
| Australia | All | 20 millimetre (0.78740157480315 in) | 90 kilometres per hour (55.9 mph) | "Conducive to flash flooding" | |
| New Zealand | F1 Fujita scale The Fujita scale , or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation... or stronger |
20 millimetre (0.78740157480315 in) | 110 kilometres per hour (68.4 mph) | 25 millimetre (0.984251968503937 in) | |
| Germany | All | 15 millimetre (0.590551181102362 in) | 105 kilometres per hour (65.2 mph) | 20 millimetre (0.78740157480315 in) | |
| Mexico | All | 5 millimetre (0.196850393700787 in) | 89 kilometres per hour (55.3 mph) | 20 millimetre (0.78740157480315 in) | |
Example of a Severe Thunderstorm Warning
An example of a Severe Thunderstorm Warning issued by the National Weather Service in the United States.
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