Today is the start of our Severe Weather Awareness Day for the Mid-South. We are going to start with the topic of severe storms.
What is a Severe Thunderstorm?
A severe thunderstorm is a thunderstorm that produces one or more of the following:
1. hail that has a diameter of one inch (quarter size) or larger
2. winds greater than or equal to 58 mph
3. tornadoes.
If a storm does not contain any of the following characteristics, it is not severe. It does not matter how bad lightning strike or heavy rainfall, if it does not contain either these qualifications then it is not severe. There are about two thousand thunderstorms going on around the world at a given time but only a small percentage is actually severe. 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.
There are a few types of severe thunderstorms such as pulse storms, multicell storms, MCS, and even supercells. Severe thunderstorms co-exist due to three synoptic airstream's
1- low-level southerly flow from 925mb up to 850mb
2- mid-level southwest flow from 925mb to 700mb
3-jet stream flow at 300mb
The four primary ingredients for severe thunderstorms are: instability, shear, lifting, moisture. The severity of a storm is dependent upon two things: instablity and shear. Instablity determines strength of storms and shear determines storm types. The life of a thunderstorm depends upon the strength of the updraft.
What is the Difference between a Watch and a Warning?
A severe thunderstorm watch means that conditions are favor-able 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 thunder-storm has either been indicated on radar or witnessed by storm spotters. Your local NWS Forecast Office issues severe thunder-storm 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 trans-ported above the freezing level, they combine with tiny airborne particles, such as dirt, salt, volcanic ash, etc., and freeze on contact, forming tiny ice par-ticles. Once the hail stones are heavy enough to overcome the upward force of the updraft, they fall out of the cloud. If large enough, hail can inflict significant damage to automobiles, buildings, crops, and even people.
NWS Hail Criteria
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
Safety Tips
-Have a plan. Prepare ahead of time so you and your family know what actions to take when severe weather occurs.
-Get indoors! There is no safe place outdoors during a thunderstorm.
-Stay informed! When severe weather threatens, stay tuned to NOAA Weather Radio, local television and radio stations, or the NWS homepage online at www.srh.noaa.gov/ for up to date information on the weather situation. Click on the office that serves your area.
-Know what county you are in. When a warning is issued, the threatened area will be identified by the counties that contain it.
-Have a NOAA Weather Radio. This is the best way to receive the latest and most up to date weather information from the National Weather Service.
***Please be sure to pause the background music in order for you to really hear the audio from the video below. To do just that, scroll down to the very bottom of the page and you will be able to pause or stop the background music.***
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