We're well into tornado season in the United States. In an average year, more than 200 twisters would have been sighted by now, but so far only about 50 have been reported.

Tornadoes require a special combination of atmospheric conditions. This time of year, they usually form in association with strong low-pressure systems emerging from the southwestern states. Southerly winds ahead of such storms draw warm moist air out of the Gulf. To the north of the storm, cold air is pulled southward, while behind the storm dry air from the deserts is drawn east. With a strong jet stream overhead, severe thunderstorms can form that then sometimes produce tornadoes. So far this year, these ingredients have simply not come together.

Just to give you an historical sense of what has happened tornado-wise this time of year - last week was the anniversary of the "Superoutbreak," when the biggest tornado blitz on record spread across 13 states from Mississippi to Michigan. On April 3 and 4, 1974, 148 twisters formed in just 16 hours.