The tornado warning sirens blaring across the Great Plains, Midwest and Southeast this month leave little doubt that tornado season has arrived with a flourish.  It has created a path of destruction through communities from Oklahoma to Wisconsin to Georgia and is off to an unusually aggressive start.

As of April 24, tornadoes have killed 41 people and ripped apart countless neighborhoods and, the St. Louis Airport. Check out the van perched on the roof of the St. Louis Airport.

Today, April 27, NOAA’s National Weather Service meteorologists expect significant and potentially deadly severe weather including long-lived strong tornadoes, very large hail, and torrential rain to spread east from the central U.S. through Wednesday night.   There have been, according to preliminary estimates, about 250 tornadoes so far this month and, in all likelihood, more are still to come

With more than 450 reports of severe weather on April 26—including 50 tornado reports across Texas, Louisiana, Arkansas and Mississippi—the NWS Storm Prediction Center in Norman, Okla., is forecasting the development of more strong to violent, long-track tornadoes over parts of the Tennessee Valley and Southeast this afternoon and tonight. The areas most likely to experience this activity include northern Alabama, far northwestern Georgia, northeastern Mississippi and southern Tennessee. Elsewhere, severe storms are also possible from the lower Great Lakes region and Mid-Atlantic to the central and eastern Gulf Coast state. Those same experts note that drawing conclusions about the true size of, or reason for, an increase in tornado activity is difficult because historical statistics are unreliable due to changes in the way storms are tracked and measured.

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NOAA weather map for April 27, 2011 (

Although the average number of April tornadoes steadily increased from 74 a year in the 1950s to 163 a year in the 2000s, nearly all of the increase is of the least powerful tornadoes that may touch down briefly without causing much damage. That suggests better reporting is largely responsible for the increase.

There are, on average, 1,300 tornadoes each year in the United States, which have caused an average of 65 deaths annually in recent years.

The number of tornadoes rated from EF1 to EF5 on the enhanced Fujita scale, used to measure tornado strength, has stayed relatively constant for the past half century at about 500 annually. But in that time the number of confirmed EF0 tornadoes has steadily increased to more than 800 a year from less than 100 a year.

Though scientists believe that climate change will contribute to increasingly severe weather phenomena, including hurricanes and thunderstorms, there is little consensus about how it may affect tornadoes.

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It is starting to look a bit like the Wizard of Oz out there! Tornadoes form when warm moist air combines with powerful dynamic winds inside a thunderstorm, sending a funnel cloud spinning toward the ground. They are most common in spring, typically peaking in May.

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