Tornadoes: Why there? Why now?

A devastating tornado that ripped through south-central states of Arkansas, Oklahoma and Iowa Sunday has killed at least 18. Why weather conditions in spring are conducive for tornadoes in the central and south US, say experts.

Angie Davis/Reuters
A doorway and shattered tree remain on the grounds of a house a day after a tornado hit the town of Vilonia, Arkansas April 28, 2014. Workers searched for survivors on Monday in the rubble left by a wave of tornadoes that ripped through the south-central United States a day earlier, killing at least 18 people in Arkansas, Oklahoma and Iowa.

On Sunday, powerful tornadoes ripped through Arkansas, Oklahoma, and Iowa, leaving at least 18 dead.

The tornado that slammed into Vilonia, about 10 miles west of the state capital, on Sunday evening grew to about half a mile wide and was among a rash of tornadoes and heavy storms that rumbled across the center and south of the country overnight,” the Associated Press reported.

In April 2011, the same city was hit by a tornado that killed four people. According to the National Oceanic and Atmospheric Administration, March usually marks the beginning of “increased seasonal tornado activity,” a period that runs until the beginning of the summer. But it all depends on which part of the country you are.

For example, in the southern and central parts of the country, the weather conditions in spring are ripe for tornado formation, says Harold Brooks, a senior researcher at the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory in Norman, Okla.

Tornadoes develop from thunderstorms. There are certain ingredients needed for a thunderstorm, says Dr. Brooks. First, there needs to be warm, moist air near the ground. Then there has to be cold and relatively dry air between 3 and 7 kilometers above the ground. Under the right conditions, the warmer air near the surface will rise up like a balloon into the colder air prompting the formation of a thunderstorm.

If these updrafts occur in a spot where winds are changing speed and direction, it can create a horizontal rotation in the air. Rising air causes the spinning air to tilt, creating the tornado's characteristic funnel cloud.  

In the US, warm, moist winds blowing from the Gulf of Mexico meet the cold, dry winds from the Rocky Mountains, creating ideal conditions for tornadoes to form. During spring, this tends to happen most often in Texas, Oklahoma, Nebraska and Kansas, the states that comprise the core of "Tornado Alley."

Arkansas lies outside of this core, but it still gets its fair share. Like its neighbor Oklahoma, it lies at the intersection of winds from the Gulf and the Rockies.Tornado activity in Oklahoma is only slightly greater than Arkansas, and more confined to the spring season, Brooks says. 

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