In a typical year, the bulk of tornado season in the United States comes and goes by early July. This year, the country has been lucky: As of Aug. 1, only 473 twisters had struck the lower 48 states.
That's barely half the average of 936 tornadoes tallied by that date during the past 10 years, says Joseph T. Schaefer, director of the National Weather Service's Storm Prediction Center in Norman, Okla. It's also the lowest count for that period since 1988.
This year's tornadoes also have proven less lethal than normal. Only 11 people had been killed by Aug. 1. Over the past decade, an average of 46 people had fallen victim to tornadoes by that date.
Why such a mild season? Large masses of cold air spilled south from Canada in February and March, the period when tornadoes typically begin to occur in the southeastern United States, says Schaefer. That, as well as a southerly jet stream that kept warm, moist air from moving north into the Great Plains, delayed the onse