Where Are All The Tornadoes?

March is typically when tornado season ramps up, with spring's unstable weather giving rise to thunderstorms and twisters. But this year has been relatively quiet so far.

By early April 2012, there had been 290 tornadoes, including 60 tornado-related deaths. But this year, there have only been about 137 reported twisters, resulting in two fatalities, National Weather Service (NWS) records show.

So why so few twisters? The main reason, so far, is that it has been a colder-than-average spring and late winter, at least for much of the central and eastern United States, which is where most of the country's tornadoes occur. This can be blamed largely on a stationary mass of warm air over Greenland and the North Atlantic that has blocked the normal flow of air from west to east and south to north, Greg Carbin, a meteorologist with the NWS' Storm Prediction Center, previously told OurAmazingPlanet.

This flow of air, known as a jet stream, usually brings more warm, moist air from the south as the Northern Hemisphere begins to heat up for the spring.

Instead, winds have predominated from the north, bathing the eastern two-thirds of the United States in Arctic air. This tamps down the formation of tornadoes, which depend on muggy air to fuel their creation and destructive power, Carbin said.

A Tale of Two Marches

 A comparison with last year is somewhat incomplete, since 2012's tornado season got off to a roaring start, thanks to plenty of moist, warm air brought north from the Gulf of Mexico, which helped fuel the month's 154 tornadoes, a cluster of which hit on March 2-3 and were the year's first billion-dollar disaster. March 2012 was the warmest March on record in the U.S. according to the NWS.

The average number of tornadoes for March is 76; this past month, there were only 17 reported, Carbin said. The month was also tied as the second-coldest March since 2000.

http://weather.yahoo.com/where-tornadoes-215130617.html