Severe thunderstorms have been the largest annual aggregated risk in the US insurance industry over the past decade, topping hurricane losses, according to a Willis Re report.

The average annual loss from “severe convective storms” was $US11.38 billion ($14.90 billion), compared with $US11.28 billion ($14.77 billion) for hurricanes.

The report, Managing Severe Thunderstorm Risk, was compiled with Columbia University to help understand the El Nino-Southern Oscillation’s (ENSO) impact on tornado and hail frequencies, and to introduce the concept of ENSO-conditioned event rates.

It says severe thunderstorms are more likely to occur in the US following La Nina and less likely following El Nino.

Willis Re EVP Prasad Gunturi says severe thunderstorms can have a significant impact on property insurance companies, and it is important to understand why they occur.

“We are working to develop a climate-conditioned severe convective storm event set for portfolio probabilistic loss estimates,” he said.

“We hope the climate-conditioned view of risk can help companies understand, manage and mitigate the regional and year-over-year variability in severe convective storm losses.”

Columbia physics and applied mathematics expert Michael Tippett says research shows ENSO and other climate signals modulate the frequency of tornado and hail activity. “We’re excited to be using that research as a scientific basis for making long-range – up to a month – forecasts of the meteorological factors that go along with severe convective storms.”