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4A A Severe Thunderstorms 4. 1 . 4. 1 Hazard Identification A severe thunderstorm is defined as a thunderstorm containing on or more of the following phenomena : hail 3/4 inch or greater, winds gusting in excess of 57 . 5 mph , and/or a tornado ( NOAH , NWS , 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 s per cell is a thunderstorm that has a persistent rotating updraft. This rotation maintains the energy release of the thunderstorm over a much longer time than typical , pulse-type t understorms , which occur in the summer months . Super cell thunderstorms are responsible for producing the majority of severe weather, such as large hail and tornadoes ( NOAA , NW , 2003 ) . Downbursts also are occasionally associated with severe thunderstorms . A d wnburst is a strong downdraft resulting in an outward burst of damaging winds on or near tie 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 ( NOAA , NWS , 2003 ) . Strong squall lines also can produce widespread severe weather , primarily very strong winds and/or microbursts . When a severe thunderstorm approaches , the NWS will issue an advisory . According to NOAA, NWS ( 1994 ) two possible advisories are as follows : `�Irr • Severe Thunderstorm Watch : Conditions are favorable for the development of severe thunderstorms . • Severe Thunderstorm Warning : Severe weather is imminent or occurring in the area . Historic Events. In 1997 , thunderstorms spawned 103 tornadoes , injured 121 people , and produced over $38 million in property damage statewide . According to FDCA' s on - line hazard maps , Indian River County averages more than 70 days with thunderstorms per year, with the most frequent occurrences being between the months of July and September. According to the NCDC , there have been a total of 52 t understorm events in Indian River County since 1953 , causing a total of $ 875 , 000 in property damage . On 13 March 1993 , a downburst in Vero Beach damaged eight homes and three commercial buildings . On 5 April 1995 , a thunderstorm damaged several homes in the Windsor Subdivision in Wabasso . On 26 June 1995 , a thunderstorm knocked down stadium lighting at Dodgertown in Vero Beach . In Sebastian , storms in May 1996 and August 2002 damaged a 20- passenger airplane and 3 moored vessels , respectively . In January of 1999 , a thunderstorm in Vero Beach caused $ 5 , 000 worth of damage to doors at the Vero Beach Municipal Airport . In the past 5 years , the City of Vero Beach has recorded one severe thunderstorm event; and the City of Sebastian has recorded two severe thunderstorm events . NCDC has recorded 37 incidents of hail in Indian River County . Tie average accumulation for these events being 1 . 15 inches . The City of Vero Beach has recorded six incidents of hail since 1999 , the Town of Fellsmere , four, and the City of Sebastian , three . 'ftw 4-32