Tornadoes: Why Here? Why Now?

March 29, 2007 — -- People across the nation's midsection were cleaning up the damage today -- and bracing for more -- after more than 60 tornadoes tore through the plains states along a giant front that stretched from Texas to the Dakotas.

How could this happen? And why does the nation's midsection seem to suffer this way all too often?

Meteorologists said it was a classic case -- cold air from Canada, colliding with warm, muggy air from the Gulf of Mexico. Thunderstorms form along the boundary, many of them quickly taking on a spiral shape as the warm air tries to rise over the cold.

That's what happened in the Midwest yesterday and overnight. Weather maps show a stripe of damage stretching due north from the Texas Panhandle.

"It doesn't matter if it's November or January," said Bernie Rayno, a meteorologist with AccuWeather, the private forecasting service in Pennsylvania. "If the conditions are favorable for tornadoes, they can form at any time."

The fact is that tornadoes can happen anywhere on the planet, at any time of year. But the U.S. plains are the world's best -- or worst -- breeding ground for them.

A multitude of factors conspire. The land is flat, offering little resistance to storms moving over it. The Gulf of Mexico generates warm, steamy air year-round. And the prevailing jet streams tend to bring cooler, dry air from the north to clash with it. Nowhere else on Earth are the conditions so ripe for tornadoes.

It gets worse in the spring, when the sun warms the Gulf air and makes the storms more severe. This year, according to NOAA's Storm Prediction Center in Norman, Okla., there were 29 confirmed tornadoes in January, 89 in February, and then a jump to at least 182 in March, with the month not quite over.

That March number, if it is not reduced in follow-up reports, will be well above the average of 85 tornadoes for March in the last three years.

Forecasters blame changing weather patterns worldwide. In January, jet streams from the Pacific blocked many of the cold fronts that would have come south and spawned tornadoes. By February, the winds had shifted, and the number of tornadoes took a surprising jump.