U.S. property-casualty catastrophe losses for the second quarter nearly doubled in both frequency and insured property damage compared to the first quarter, according to an estimate by Property Claim Services.

PCS, a unit of Jersey City, N.J.-based Insurance Services Office, said that it estimates just over $6 billion in property losses from 1,223,500 claims for the quarter. The claims averaged $5,000 each and stemmed from 16 catastrophes in 27 states, PCS said.

The costliest event, which was caused by the effects of severe weather over seven states from Colorado and Wyoming to Minnesota, occurred in May. PCS estimated insured property damage at $850 million for that event, and Gary Kerney, PCS assistant vice president, added that 135,000 claims were filed.

Texas saw the most insured losses of any state for the quarter, at $1.08 billion, according to PCS estimates. Minnesota, Kansas, Arkansas and Oklahoma rounded out the top five most severely affected states.

All of the second-quarter catastrophes resulted from “thunderstorms producing damaging winds, large hail, tornadoes and in some cases flooding,” Mr. Kerney said.

According to a chart provided by PCS, which lists second-quarter catastrophe frequency and related insured losses dating back to 1999, the second quarter of 2008 experienced 10 more events and $3.7 billion more in insured losses than the same period in 2007. Both the insured loss estimate and event frequency were higher in the second quarter of 2008 than in any other second quarter listed on PCS' chart.

PCS said that it defines a catastrophe as an event that causes $25 million or more in insured property losses and affects a significant number of policyholders and insurers.