Each day this week, we'll post information on various aspects of severe weather for Severe Weather Awareness Week 2011. Feel free copy, paste, or share this information. Today, we discuss severe thunderstorms and hail.
What is a Severe Thunderstorm?
A severe thunderstorm is a thunderstorm that produces one or more of the following: hail that has a diameter of one inch (quarter size) or larger, winds greater than or equal to 58 mph, and tornadoes. About 10% of all thunderstorms in the U.S. meet severe criteria.
Severe thunderstorms can occur at any time of year, although the most common time of occurrence is during the spring months of March, April, and May. In addition, pulse-type thunderstorms during the summer months can produce high winds, frequent lightning, and torrential downpours.
There is also a secondary season of organized severe weather in Mississippi, in November and early December.
What is the Difference between a Watch and a Warning?
A severe thunderstorm watch means that conditions are favorable for severe thunderstorms to develop. These are issued by the Storm Prediction Center in Norman, OK, typically before severe weather is developing.
A severe thunderstorm warning means that a severe thunderstorm has either been indicated on radar or witnessed by storm spotters. Your local NWS Forecast Office issues severe thunderstorm warnings when severe weather is developing or occurring.
How Is Hail Formed?
Hail is formed when water droplets are drawn into an area of strong upward moving air, known as an up-
draft, of a storm. Once the water droplets are transported above the freezing level, they combine with tiny airborne particles, such as dirt, salt, volcanic ash, etc., and freeze on contact, forming tiny ice particles. These ice particles are light enough that they remain suspended in the cloud, where they undergo processes that allow them to combine with other supercooled water droplets and grow into hail stones. Once the hail stones are heavy enough to overcome the upward force of the updraft, they fall out of the cloud. By definition, hail stones are 5 millimeters or larger, and can inflict significant damage to automobiles, buildings, crops, and even people.
Mississippi's primary season for significant hail (1 inch or larger) runs from March to May, but hail is possible year-round.
As of January 5, 2010, the National Weather Service has changed the severe hail criteria from 3/4 inch to 1 inch. Why the change? There are a couple of reasons. Research has shown that significant damage caused by hail does not occur until the hail diameter reaches one inch (approximately quarter size) or larger. Requests from NWS partners such as emergency managers and the media further prompted the decision to increase the severe hail criteria due to the fact that more frequent severe thunderstorm warnings may act to desensitize the public from their meaningfulness. Such an effect could cause people not to take caution during such a warning, which could lead to unnecessary damage or injury. The change means that severe thunderstorm warnings for hail will be issued less frequently so that a greater emphasis can be placed on more substantial hail threats.
Hail Size Estimates
Pea - 1/4 inch
Penny - 3/4 inch
Quarter - 1 inch
Half Dollar - 1 1/4 inches
Golf Ball - 1 3/4 inches
Tennis Ball - 2 1/2 inches
Baseball - 2 3/4 inches
Grapefruit - 4 inches
**Source: National Weather Service - Jackson, MS Forecast Office, Mississippi Severe Weather Awareness Week 2011 Brochure which can be viewed/downloaded by clicking the link.
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