The National Weather Service (NWS) had defined severe thunderstorms as storms which produce tornadoes, winds of 28.5 m s-1or greater, or hail of 19 mm in diameter or greater. If wind estimates are not available, wind damage may be used to indicate that a severe thunderstorm has occurred. The NWS Storm Prediction Center (SPC) maintains a data base on severe thunderstorms that have been reported over the contiguous United States. Tornado data extends back to 1950, while wind and hail data is available back to 1955. Among the information recorded are the latitude , longitude, time, date, and number of deaths and injuries caused by the event.
An analysis of the temporal trends in the data both regionally and nationwide will be presented. For instance it will be shown that while reported tornado activity over the United States is increasing at an almost linear rate of 15 events per year, the annual number of strong and violent tornadoes reported is decreasing by about 1.5 per year. The data will also be examined with regard to the ENSO phase. Annual and monthly comparisons will be made for several different regions of the country. Similar analysis will be given for various sizes of hail reports and speeds of thunderstorm induced gusts. These data can be used as the bases for site specific analyses of the severe thunderstorm threat. The data base will be made available to the community via the SPC homepage