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Severe Thunderstorm Alert – what it means

August 9, 2012   A thunderstorm that precedes a cold front is generally stronger and more dangerous than thunderstorms associated with warm, humid tropical air masses. Pushed by strong jet stream winds, the line of storms associated with the cold front is often called a squall line. Severe thunderstorms are fast moving and can create incredible violence as they pass over an area. Among their most devastating aspects are Downbursts.  A downburst is severe localized wind blasting down from the thunderstorm. A microburst is a downburst that covers an area less than 2.5 miles in diameter and lasts less than 5 minutes. This downward and outward burst of colder air creates damaging winds at or near the surface that are often mistaken for a tornado and can cause tornado-like damage. Wind shear is any sudden change of speed or direction in wind flow. Since a microburst is a sudden vertical drop of air, it produces considerable wind shear.  By definition, a severe thunderstorm is a thunderstorm that contains any one or more of the following three weather conditions:  Hail that is 3/4 of an inch or greater in diameter, Winds 58 miles per hour or greater and Tornadoes, along with very heavy (torrential) rain and ground lightning.  Supercell thunderstorms are fierce and can sometimes discharge a number of tornadoes. They are tremendously powerful and well-organized, containing rotating columns of rising air.  These storms are capable of maintaining severe thunderstorm strength for hours. They can also produce dangerous straight line winds, large hail and torrential rain and can spawn particularly strong tornadoes.  Thunderstorms in general are always preceded by a dark sky and sometimes a greenish light, and often accompanied by dark, low-lying clouds. They may be fast-moving or they may move very slowly, seeming to hover over one location for an extended period of time. Thunderstorms usually produce brief periods of heavy rain, typically between 30 minutes to an hour. They can last much longer than this, however, depending on the severity. Roughly 10 percent of thunderstorms are classified as "severe thunderstorms,"   Thunderstorms and lightning strikes How Far Away Is It? - An easy way to estimate the distance between you and a lightning strike is to count the number of second that pass after seeing the lightning flash and hearing the resulting thunder. Dividing the number of second that have passed by 5 gives the distance in miles. For example: 5 seconds - 1 mile away       10 seconds - 2 miles away    15 seconds - 3 miles away   20 seconds - 4 miles away 25 seconds - 5 miles away    30 seconds - 6 miles away  Boaters can use a compass to determine if they are in a storm's path. If the bearings plotted for the average location of ground strikes to your boat remain unchanged, you are in the storm path and need to change course!   How to Forecast Storms – (better than the Weather Channel)  The data used by the Weather Channel and other TV stations is available to you (without their re-packaging) on the internet or on your cell phone. Instead of momentary snippets of radar shots and jumping all over the country, you need tools that will instantly give you the live “BIG Picture” for here, where you are boating and now. Intellicast  ( www.intellicast.com ) has a great page you can bookmark for local weather and short and long term forecasts, but their gem is the “Interactive Weather Map”  found in the middle right of their local page…...  http://www.intellicast.com/Local/WxMap.aspx?location=USCT9975   This WHOLE COUNTRY map “stitches” all of the NOAA radars together into a single “zoomable” tool that will allow you to really see where storms are, how fast they are moving and when they are going to hit.  This will allow you to make intelligent decisions as to “if you can go out” and “how long you can stay out” before you need to head in to miss the storm.  This self forecasting approach is much better than listening to the weather radio (do you really need the tide info at the Battery in NYC ??) or trying to piece together the NOAA radar screen into a clear picture you can understand.  And, better yet, Intellicast has just come out with an App that puts all that info on your phone in one place. (see the online link or go to your phone’s app store).  The radar at http://radar.weather.gov/ is good, as is the newly revised http://mobile.weather.gov  site for gathering data.   Yes, there are many other sites and apps….. the point is, we now have more information and data at our fingertips than we’ve ever had before.  Find your favorite, learn how to access it and how to make educated decisions that will keep your family safe AND spend more time on the water. Listen to the weather reports! Learn to read the weather conditions. Heed these reports and the conditions. Stay off or get off the water when weather conditions are threatening.  Head in early if the storm moves faster than forecast. Sometimes you even may need to make the decision to ride the storm out in the Sound, rather than risk coming in through the rocky channel just as the storm hits.  Put on life jackets, keep your weight low, don’t touch metal objects, get in the lee of an island if possible and anchor if you must.  It’s all part of becoming a better, and more informed boater.  For learn more… Click Here  Capt. Rick Delfosse  rdelfosse@rexmarine.com  203-216-7800

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