Severe thunderstorm warning

See Severe weather terminology for a comprehensive article on this term and related weather terms.

A severe thunderstorm warning is issued when trained Skywarn spotters or a Doppler capable weather radar indicate a strong thunderstorm is producing dangerously large hail or high winds, capable of causing significant damage. It does not account for lightning or flooding. A warning must not be confused with a severe thunderstorm watch.

In the U.S., the National Weather Service defines a severe thunderstorm as having large hail being at least ¾ inch (19 mm) in diameter, high winds as being 58 miles per hour (93 km/h) or greater, producing tornadoes, or any combination of the three. In Canada, a severe thunderstorm has one or more of the following:

wind gusts of >90 km/h
hail with a diameter of >20 mm
rainfall of >50 mm in an hour or >75 mm in three hours
tornadoes

A severe thunderstorm warning means there is significant danger for the warned area. Occasionally, severe thunderstorms can and do produce a tornado without warning. While not all severe thunderstorms produce tornadoes, they can produce serious wind damage as severe as a tornado. A severe thunderstorm warning can also be upgraded to a tornado warning if a tornado is detected on radar or actually spotted. Generally, but not always, a severe thunderstorm watch or tornado watch will precede a warning.

Some storms, especially in the Great Plains, may produce massive hailstones the size of baseballs or even grapefruits, falling fast enough to kill a person by repeated blunt trauma. Everyone in the path of such a storm should take cover immediately, as it is very dangerous and possibly life-threatening.

In the United States, the National Weather Service issues warnings for tornadoes and severe thunderstorms in polygon shapes. Warnings are usually delineated by county or by a section of a county.

A similar warning is issued by Environment Canada's Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth.

Example of a Severe Thunderstorm Warning

An example of a Severe Thunderstorm Warning issued by the National Weather Service in the United States.


SEVERE THUNDERSTORM WARNING
OHC035-153-080300-
/O.NEW.KCLE.SV.W.0206.070808T0200Z-070808T0300Z/

BULLETIN - EAS ACTIVATION REQUESTED
SEVERE THUNDERSTORM WARNING
NATIONAL WEATHER SERVICE CLEVELAND OH
1000 PM EDT TUE AUG 7 2007

THE NATIONAL WEATHER SERVICE IN CLEVELAND HAS ISSUED A

* SEVERE THUNDERSTORM WARNING FOR...
  CUYAHOGA COUNTY IN NORTHEAST OHIO
  SUMMIT COUNTY IN NORTHEAST OHIO

* UNTIL 1100 PM EDT

* AT 950 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
  SEVERE THUNDERSTORM CAPABLE OF PRODUCING PENNY SIZE HAIL...AND
  DAMAGING WINDS IN EXCESS OF 60 MPH.  THIS STORM WAS LOCATED NEAR
  NORTH ROYALTON...OR ABOUT 11 MILES NORTH OF MEDINA...AND MOVING
  EAST AT 40 MPH.

* OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO
  MACEDONIA...TWINSBURG AND SOLON

THIS IS A DANGEROUS STORM. IF YOU ARE IN ITS PATH...PREPARE
IMMEDIATELY FOR DAMAGING WINDS...DESTRUCTIVE HAIL...AND DEADLY CLOUD
TO GROUND LIGHTNING. PEOPLE OUTSIDE SHOULD MOVE TO A SHELTER...
PREFERABLY INSIDE A STRONG BUILDING BUT AWAY FROM WINDOWS.

LAT...LON 4146 8139 4127 8139 4126 8138 4126 8140
      4126 8151 4125 8167 4128 8168 4128 8179
      4137 8182

$$

FITZ

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