Local mitigation strategy (LMS) - Volusia County Government

SECTION 5: HAZARD PROFILES5.6 Thunderstorm5.6.1 BackgroundThunderstorms are common throughout Florida and occur throughout the year. Althoughthunderstorms generally affect a small area, they are very dangerous given their ability to produceaccompanying hazards including high winds, hail, and lightning which all may cause serious injury ordeath, in addition to property damage 6 .According to the National Weather Service, more than 100,000 thunderstorms occur each year, thoughonly about 10 percent of these storms are classified as “severe.” A severe thunderstorm occurs whenthe storm produces one of three elements: 1) Hail of three-quarters of an inch; 2) Tornado; 3) Winds ofat least 58 miles per hour.Three conditions need to occur for a thunderstorm to form. First, it needs moisture to form clouds andrain. Second, it needs unstable air, such as warm air that can rise rapidly (this often referred to as the“engine” of the storm). Finally, thunderstorms need lift, which comes in the form of cold or warm fronts,sea breezes, mountains, or the sun’s heat. When these conditions occur, air masses of varyingtemperatures meet, and a thunderstorm is formed. These storm events can occur singularly, in lines, orin clusters. Further, they can move through an area very quickly or linger for several hours.The National Weather Service collected data for thunder events and lightning strike density for the 30-year period from 1948 to 1977. A series of maps were generated showing the annual average thunderevent duration, the annual average number of thunder events and the mean annual density of lightningstrikes. Figure 5.5 illustrates thunderstorm hazard severity based on the annual average number ofthunder events from 1948 to 1977. Volusia County falls into the range of an average of 110 – 120thunderstorm events annually.6 Lightning and Hail are discussed in detail as separate hazards in this section.Volusia County Multi-jurisdictional Local Mitigation StrategyFebruary 20105:25