"As the southern and plains states heat up this spring, so will the chances for severe thunderstorms, including tornadoes," said Julie Rochman, IBHS president and chief executive officer in a statement. "Even relatively isolated tornadoes can be an incredibly destructive force of nature. The time to prepare is now - before tornado season is in full swing," she said.

At least five homes were destroyed after a tornado touched down in western Oklahoma on March 8, authorities said. There were no reports of injuries. Other homes were damaged, and power was out to about 900 residences in and around the city of Hammon in Roger Mills County, Oklahoma.

Tornadoes are rated according to the Enhanced Fujita (EF) Scale (an upgrade to the older Fujita Scale). An EF0 with winds ranging from 65 to 85 mph might damage trees and small buildings but it is not likely to cause substantial damage to structures.

An EF5, on the other hand, with winds in excess of 200 mph can tear buildings off their foundations. The positive news is that only a small percentage of tornadoes have winds in the EF3 through EF5 categories, said IBHS.

When a tornado warning has been issued, or conditions appear threatening, it is important to move to a safe place quickly. "While actual warning times for weaker tornadoes can be quite short, the National Weather Service warning tools have advanced to the point where warning times are averaging well over five to 10 minutes for more intense tornadoes," Ms. Rochman said.

She advised that making sure one has a working weather radio and identifying in advance the safest place to go are two excellent ways to prepare for a tornado

"Without a doubt, the safest place to wait out the storm would be an actual tornado shelter that has been built to meet either Federal Emergency Management Agency (FEMA) requirements or the International Code Council's ICC 500 standard," said Tim Reinhold, senior vice president of research and chief engineer at IBHS.

"When that kind of shelter is not available, the next best option would be a small, windowless room located near the middle of the building," Mr. Reinhold advised.