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C. Severe Thunderstorms and Lightning A severe thunderstorm is defined as a thunderstorm containing one or more of the following phenomena: hail 3/4 inch or greater, winds gusting in excess of 57.5 mph, and/or a tornado (National Oceanic and Atmospheric Administration, National Weather Service, 1994). Severe weather can include lightning, tornadoes, damaging straight-line winds, and large hail. Most individual thunderstorms only last several minutes; however, some can last several hours. Long-lived thunderstorms are called super cell thunderstorms. A super cell is a thunderstorm that has a persistent rotating updraft. This rotation maintains the energy release of the thunderstorm over a much long time than typical, pulse -type thunderstorms, which occur in the summer months. Super cell thunderstorms are responsible for producing the majority of severe weather, such as large hail and tornadoes (National Oceanic and Atmospheric Administration). Downbursts are also occasionally associated with severe thunderstorms. A downburst is a strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder (National Oceanic Atmospheric Administration). Strong squall lines can also produce widespread severe weather, primarily very strong winds and/or micro bursts. When a severe thunderstorm approaches, the National Weather Service will issue an advisory. According to the National Oceanic and Atmospheric Administration NWS (1994) two possible advisories are as follows: • Severe Thunderstorm Watch: Conditions are favorable for the development of severe thunderstorms. • Severe Thunderstorm Warning: Severe weather is imminent or occurring in the area. Indian River County Comprehensive Emergency Management Plan Basic Page 11