Doyle Rice

USA TODAY

As of Tuesday, no Americans have been killed by a tornado so far this year. The USA hasn't seen such a safe start to a tornado season since World War I.

The last time the nation made it to this point in the year without a tornado death was 1915, reports meteorologist Harold Brooks of the National Severe Storms Laboratory in Norman, Okla. That year, the first tornado death occurred on May 6.

No year in recorded weather history has ever been free of tornado deaths. On average, tornadoes kill about 60 Americans each year. In 2011, tornadoes killed 553 people — 158 of them in the massive tornado that struck Joplin, Mo.

A year without a single tornado death is possible, Brooks says, "although very, very unlikely."

AccuWeather meteorologist Mike Smith agrees, saying that "we certainly work toward no deaths but, realistically, I don't think it is possible. I'd love to be proven wrong."

It also has been a quiet year overall for tornadoes, with only 20 reports of tornadoes of EF1 or greater strength so far — "by far the slowest start to any year back to 1953," says Greg Carbin, warning coordination meteorologist for the Storm Prediction Center in Norman.

Tornadoes are rated from weakest to strongest on the "Enhanced Fujita" or EF scale, with EF0 the weakest and EF5 the strongest.

No tornadoes of EF3 strength or higher have occurred so far this year. Tornadoes in the higher Fujita categories do a disproportionate amount of damage and cause most tornado deaths, the Weather Channel reports.

Most violent tornadoes are formed by powerful thunderstorms, which are characterized by rotating, prolonged, intense updrafts of air.

Cool weather has helped tamp down the tornado season this year.

"We have seen well below-normal temperatures continue in some areas of the country ... a continuation of the below-normal temperatures over the winter," Carbin says. "Winter cold is loosely correlated with below-normal tornado numbers."

Late April and May generally are the most active time of year for tornadoes, and some could rattle the Plains on Wednesday.

The Storm Prediction Center has placed an area from central Nebraska to northern Texas under a "slight risk" for severe weather Wednesday. Large hail, strong winds and tornadoes are possible. Cities such as Oklahoma City and Wichita, Kan., are in the slight risk area.