Damaging Thunderstorm Activity in the United States

Abstract

Thunderstorm-caused property losses resulted in 892 property insurance catastrophic events during 1949–98 and amounted to $87 billion (1998 dollars) in the United States. Catastrophes were events causing > $1 million, and the loss values over the 50-yr period were adjusted for temporal changes in inflation, shifts in insurance coverage, and population. Thunderstorm catastrophes were most common in the south and central climatic regions and infrequent in the west and northwest regions. The eastern two-thirds of the nation experienced 87% of all losses. The number of catastrophes causing losses of $35 million or more increased rapidly in all regions between 1955 and 1980, and nationally went from 33 events during 1949–53 to a peak of 84 occurrences during 1994–98. Some of this increase reflects increasing societal vulnerability to damage from increased density of structures and wealth, plus demographic shifts to storm-prone areas in the southeast and south where the greatest increases in storms occurred. National losses from storm catastrophes had a U-shaped 50-yr distribution with $19 billion in losses during 1949–53, $2 billion to $5 billion during the 5-yr periods from 1959 to 1988, and then increasing to $15 billion in 1994–98. The loss distributions were also U-shaped on the east coast and Midwest, but losses elsewhere in the nation increased continually from 1949 to 1998. The median loss of catastrophes in each 5-yr period, a measure of storm intensity, exhibited a slight U-shaped distribution nationally, but in the western half of the nation the intensity values decreased from high early values to minimums in the 1970s and then increased thereafter. Certain findings reveal that societal shifts were major factors in the time distributions of losses and catastrophe frequencies. Areas with rapid population growth and wealth including California, Florida, Colorado, and the southwest, all experienced rapid increases in storm losses that began during the 1960s and 1970s. In many areas temporal fluctuations in the catastrophe frequencies and losses also agreed with time shifts in the number of thunderstorm days reported by observing stations, the only independent measure of storm activity. Increases in thunderstorm activity since the 1950s in the western half of the nation and along the Gulf Coast relate well to the upward shifts in catastrophe frequencies and losses in these areas. However, thunder-day incidences decreased with time over most of the Midwest and northeast where catastrophe losses increased, further reflecting the influence of societal factors on the temporal distribution of catastrophe losses.

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