NEW YORK (Reuters) - Insured severe weather losses have already topped $15.5 billion in the United States this year, three times the full-year average of the last 20 years, insurance brokerage Aon Corp said yesterday.Almost all of that comes from the outbreak of tornadoes and thunderstorms that struck the country in April and May, the company’s reinsurance unit Aon Benfield said in a report.Over the last 20 years, the United States has averaged just over $5 billion in annual severe weather losses, the company said and a potentially devastating hurricane season is just beginning, which could make losses soar.Eight storm outbreaks, including five that generated more than $1 billion in losses, drove most of the costs. The May 22 twister in Joplin, Missouri and the April 27 event in Tuscaloosa, Alabama now rank as the most expensive tornadoes in history, even adjusting for inflation.As is usually the case, economic losses were greater than insured losses. Aon pegged the economic loss figure at $21.65 billion.Combined with devastating earthquakes in Japan and New Zealand, flooding in Australia and unrest in the Middle East, insurers and reinsurers worldwide are estimated to have already lost more than $60 billion this year.That has sparked a turn in parts of the insurance market, with prices rising for certain kinds of risk in certain regions after years of weakness in the market. Most industry experts agree a major US hurricane impact this summer would turn a much larger segment of the market back to firm pricing.