What is a Severe Thunderstorm?
Of the estimated 100,000 thunderstorms that occur each year in the United States, only about 10 percent are classified as severe. Your National Weather Service considers a thunderstorm severe if it produces hail 3/4 inch in diameter or greater, and/or winds to 58 mph or higher, or tornadoes.
All thunderstorms are dangerous however. Every thunderstorm produces lightning, which kills more people each year than tornadoes, and heavy rain from thunderstorms can lead to flash flooding. Severe Thunderstorm Warnings however are only based on the expectation of large hail and/or strong winds.
If you experience a thunderstorm with heavy rain and lightning and wonder why your National Weather Service has not issued a warning, it is because large hail and/or damaging winds are not expected at that time. If spotters report damage from the storm or the potential for large hail or damaging winds are indicated by radar, a warning will be issued.Severe Weather Awareness
A downburst is a small area of rapidly descending air beneath a thunderstorm which can cause damaging winds in excess of 100 MPH. A downburst can cause more damage over a larger area than a weak tornado (F0, F1). The strong winds usually approach from one direction and are know at Astraight-line@ winds. Because of the strength of the winds and the resulting damage, the destruction caused by a downburst is sometimes mistaken for tornado damage.
Downbursts are often classified into one of two categories. A Macroburst is a downburst (strong downdraft or gust of damaging wind) which produces a swath of damage that is 2.5 miles wide or greater. A Microburst is a downburst which produces a swath of damage less than 2.5 miles wide.
A tornado is a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destructions with wind speeds of 250 mph or more.
SEVERE THUNDERSTORM WATCH: tells you when and where severe thunderstorms (thunderstorms with large hail and damaging winds) are more likely to occur. There may be times when warnings are issued for isolated thunderstorms in areas where a Watch is not in effect.
SEVERE THUNDERSTORM WARNING: issued when severe weather (hail 3/4 inch in diameter or greater, wind 58 mph or higher) has been reported by spotters or indicted by radar. Warnings indicate imminent danger to life and property to those in the path of the storm. Severe Thunderstorm Warnings are not issued for lightning because all thunderstorms contain dangerous lightning. Severe Thunderstorm Warnings are not issued for heavy rainfall. Flash Flood or Flood Warnings are issued if rainfall is expected to be heavy enough to cause for flooding.
TORNADO WATCH: Tornadoes are possible in your area.
TORNADO WARNING: A tornado has been sighted or indicated by weather radar.