Tornados and hail in the Ohio River Valley and Midwest this month may rank among the costliest severe weather events in US history, with total economic and insured losses likely to be billions of dollars, Aon says.

The May 14-17 storms produced 56 tornadoes, including a powerful twister that severely damaged thousands of buildings in St Louis, Missouri. 

Peak wind speeds of 245km/h hit western and northern parts of the city limits, and nearby Madison County in Illinois was also affected.

Several counties in Wisconsin reported hail of more than 5.1cm in diameter, while wind picked up loose farmland soil, creating the largest dust storm in Chicago since 1934, Aon says in a report on the severe convective storms. 

A separate slow-moving low-pressure system over the central Great Plains caused damage from May 18-20.

Aon says hailstones of at least 10.2cm in diameter and wind gusts exceeding 113km/h were recorded in Oklahoma and Kansas.

Much of the economic and insured losses are expected to come from the St Louis tornado, with city officials initially estimating about $US1.6 billion ($2.5 billion) in damage.

“Moreover, as more assessments are completed in the coming weeks, the May 14-17 event may even potentially rank among the costliest severe weather outbreaks in US history on a price-inflated basis,” Aon says.

Total economic and insured losses from the May 18-20 severe weather may also reach into the billions.

The Storm Prediction Centre says there were 924 tornado reports this year to May 20, just below the record for the same period – 963 – set in 2011.  

BMS Group VP and senior meteorologist Andrew Siffert said on LinkedIn that the May storms could cost the insurance industry more than $US5 billion ($7.8 billion), making it the most expensive severe weather event so far this year.  

For the year to May 15, severe weather insurance losses had reached about $US15 billion ($23 billion), just above the year-to-date average, he said.

Mr Siffert said more than 5000 homes were affected in St Louis – which last experienced a significant tornado in 1959 – and that could account for $US1 billion ($1.6 billion) in losses.

“With much attention focused on the tornado damage, the insurance industry can’t lose sight of the fact that hail will also be costly.”