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 Severe Weather Products

It is estimated that at any given moment nearly 2,000 thunderstorms are active on the earth. All thunderstorms have the potential for the violence to make them one of nature's great destroyers and killers.

Thunderstorms begin when relatively warm, humid air is forced upward. This can occur three different ways...
  1. Mechanical - A wedge of dense air undercutting warm, moist air (for example, a cold front).
  2. Orographic - Moist air that is forced over higher terrain such as the Olympics or Cascades.
  3. Thermal - Rising air caused by the sun's heating of the earth's surface.
As the air rises it expands and cools, and the water vapor it contains begins to condense into cloud droplets. Continued upward movement can produce a cumulonimbus or thunderstorm cloud. Once an individual thunderstorm reaches maturity, downdrafts caused by falling precipitation begin to destroy it.

Most thunderstorms consist of several cells, each lasting about 20 minutes. New cells may form, replacing older ones, giving thunderstorms the potential to last several hours or more. Strong gusts of relatively cool wind occur beneath many mature thunderstorm cells. Lightning always accompanies thunderstorms. Hail (ranging from pea size to that of a softball) and strong wind sometimes occur and cause considerable damage. Flash floods (produced by heavy rains), tornados and/or funnel clouds may also occur in thunderstorms.

Lightning strikes the earth about 100 times per second with and more than 100 Americans are killed each year by lightning strikes. Annual property losses--fire and other damage to structures, aircraft damage, livestock deaths and injuries, forest fires and other effects--are estimated to cost hundreds of millions of dollars.

Lightning is a huge electrical spark resulting from the generation and separation of electrical charges in a thunderstorm. The accumulation of an electrical charge in the lower portion of the cloud can induce the opposite charge on the earth's surface.

Electrical charges accumulate until the air, a poor conductor of electricity, can no longer prevent a flow of current. When the discharge occurs, it is along a lightning channel that typically connects elevated ground objects (such as buildings and trees) and the cloud base.

Thunder is the sound produced by an explosive expansion of air heated by a lightning stroke. When lightning is near, the thunder sounds like a sharp crack. More distant strokes produce growling and rumbling noises, a result of the sound being refracted and modified by the turbulent environment of the thunderstorm.

Because the speed of light is about a million times that of sound, the lightning bolt is visible before the thunder is heard. This makes it possible to estimate the distance (in miles) of the lightning stroke by counting the number of seconds between lightning and thunder and dividing by five.

When weather conditions in an area are expected to become severe, the Storm prediction Center (SPC) in Norman, Oklahoma issues a severe thunderstorm or tornado watch. This watch defines the area that severe weather may occur during the watch period.

A severe thunderstorm watch means the potential exists for thunderstorms to reach the severe threshold but the threat of tornados is limited. A tornado watch means there is a potential for severe thunderstorms and tornados.

When severe weather is occurring or imminent, a warning is issued by the local NWS office with warning responsibility for that area. The warning consists of the type of event, location affected, duration of warning, and a call-to-action statement. It contains limited information and is designed for rapid dissemination to the public. Shortly after the warning is disseminated, a severe weather statement is usually issued. The severe weather statement updates the initial warning and provides additional information.

 
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