Insurance claims of over $US2 billion are forecast from the deadly tornado that ripped through a suburb of Oklahoma City in May.

The system began to form two days prior to the event when warm air moving north from the Gulf of Mexico met a cold front from the Rocky Mountains, leading to thunderstorms.

During the next few days, 20 tornadoes formed in Oklahoma, Missouri, Texas, Kansas, Arkansas and Colorado but the Oklahoma tornado was by far the worst, being a mile wide and with winds of up to 320 kilometres an hour. 

Tornado activity had been particularly low this year until the event on May 20 and catastrophe modeller RMS says now there is much uncertainty about the US tornado outlook for 2013.  May is traditionally the worst month for tornadoes.

RMS says it was the third major tornado to hit the suburb of Moore since 1999.

Most of the homes in the area are of wood-frame construction and more vulnerable to high winds than masonry houses.

Although basements and underground shelters are the safest refuge from a tornado, Air Worldwide says many homes in Oklahoma have concrete slabs because the soil in the region makes basement construction too costly.