Many Americans still think of twisters as almost always occurring in "Tornado Alley" — an area that roughly includes Nebraska, Kansas, Oklahoma and Texas.
Mother Nature cleared up this misconception between April 14 and 16, as scores of tornadoes ripped through the South, hitting North Carolina, Alabama, Georgia, Mississippi and Virginia the hardest.
In a separate storm on April 22, a tornado hit just north of St. Louis, peeling off a large section of roof from a concourse at Lambert International Airport and slamming the area with baseball-size hail. Nobody was killed.
On Monday, four people were killed when yet another tornado ravaged Vilonia, Ark. The state was expecting more severe storms and tornadoes Tuesday night after the National Weather Service issued a high risk warning that stretched from northeast of Memphis to just northeast of Dallas.
The South is smack in the middle of its tornado season, according to John Snow, a meteorologist at the University of Oklahoma. Though tornadoes can occur any day of the year and in any state, every region has an annual period that most often produces conditions for severe thunderstorms and tornadoes, Snow says. It starts in March in the South, sometimes referred to as "Dixie Alley." It then moves north to the Great Plains in May and June, following the same progression as spring.
Two or three tornado-producing thunderstorms will often occur over a small region during the season. Every four or five years, tornado conditions cover a bigger area and create 20 or 30 severe storms. But this month's spate of storms, which have caused nearly 50 deaths, is "a once-in-10-or-20-years type of event," Snow said.
Tornadoes happen all over the world, but most often in North America. Between 900 and 1,400 tornadoes occur each year in the United States, followed by China with 100.
It's difficult for the atmosphere to put together the right combination of extreme conditions to produce even one tornado, according to Greg Carbin, a meteorologist with National Oceanic and Atmospheric Administration's Storm Prediction Center in Norman, Okla.
"Any tornado is extreme," Carbin says. "People want to talk about 'normal tornadoes,' but a tornado is not normal in any sense of the word."
Tornado-producing thunderstorms require cold air, warm air, wind and moisture. The mid-April outbreak was created when dry, cold air from the north, powered by a strong jet stream, collided with warm, humid air originating from the Gulf of Mexico and, as it traveled east, from the Atlantic Ocean.
Almost 300 reports of tornadoes were received during that rampage through the South. But multiple witnesses reported the same one, and the National Weather Service is still figuring out exactly how many twisters occurred, Carbin says. He expects the final count to be about 135 or 145 — "and that's still a big three-day number."
The worst recorded tornado outbreak in U.S. history was in 1974, when 148 twisters touched down in 13 states, stretching from Alabama to Michigan. The 16-hour event caused 330 deaths, according to NOAA.
One reason that people usually picture tornadoes occurring in the center of the country is that Plains twisters tend to be more photogenic. The Southeast often gets "rain-wrapped tornadoes," Carbin says, which are harder to see than dramatic funnel clouds roaring across the flatlands. Trees and a diverse terrain can also obscure the view.
"The one in Raleigh was pretty ominous-looking, but it was hard to say you were looking at a tornado with the rain around the vortex," Carbin said of the storm that caused five deaths in North Carolina's capital.