Severe thunderstorms can cause loss of life, property damage, and large insured losses. While losses from individual severe thunderstorms in the United States are generally not as great as those from individual hurricanes or earthquakes, the annual aggregate losses from severe thunderstorms have accounted for approximately one half of all U.S. catastrophic insured losses since 1990.
At any given moment there are approximately 2,000 thunderstorms occurring around the world, and an estimated 100,000 occur annually in the U.S. Only a small proportion of these thunderstorms, however, meet National Weather Service (NWS) criteria and can therefore be classified as “severe.” As defined by the NWS, a severe thunderstorm is one that produces a tornado, and/or winds of at least 58 mph (50 knots), and/or hail measuring a minimum of one inch in diameter.
Tornadoes and straight-line winds can cause significant losses, but hailstorms can too. Each year in the U.S., hailstorms can cause several billion dollars in property damage. The largest and heaviest authenticated hailstone to fall in the U.S. was found in Vivian, South Dakota, on July 23, 2010. It was 8.0 inches in diameter, weighed nearly 2 pounds, and had a circumference of 18.62 inches. Size alone, however, does not determine damage potential. Rather, hail damage is determined by several factors. High winds, for example, can increase the kinetic energy of hailstones and blow them at angles significantly off the vertical, thus increasing the likelihood of broken windows and cladding damage.
Verisk plans to release a substantially updated severe thunderstorm model for the United States in summer 2022. The second of three AIR Current articles in our series on managing U.S. severe thunderstorm risk by Dr. Tim Johnson, Dr. Sarah Bobby, and Dr. Karthik Ramanathan discusses our innovative and first-of-its-kind hail vulnerability framework and how our updated model can distinguish the view of risk from one property to another.