Even in Tornado Alley in springtime, it's unusual to get two storm systems as powerful as the one that spawned a deadly tornado in Arkansas and another that was working its way through Oklahoma and North Texas on Tuesday, a meteorologist said Tuesday.
At their simplest, storms occur when masses of cold air and warm air collide -- and they occur pretty often in the central United States. But what gives Tornado Alley its name is the complicated mix of humid air off the Gulf of Mexico; cooler, dry air from the north; and jet streams that scream across the continent from east to west.
The combination contributed to storms Monday that killed 10 in Arkansas, and was in place again Tuesday -- setting up areas from Texas to Tennessee for tornadoes, hail, high winds and flooding rains.
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Even by Tornado Alley standards the succession of bad weather is unusual.
"What's kind of interesting, it's basically in the same place for two days in a row. That doesn't happen very often," said Greg Carbin, the warning coordination meteorologist for the Storm Prediction Center in Norman, Okla. "Such rapid succession doesn't give any time for a break."
The Storm Prediction Center, part of the National Weather Service, provides long- and short-term forecasts that emergency managers nationwide use to gauge the threat of troubling weather. It predicted a moderate risk of severe weather for Monday, and by the end of the day Arkansas had suffered through tornadoes and the Mid-South saw flooding rains.
For Tuesday, forecasters said there was a high risk of severe weather in an area from Dallas to Memphis, with a moderate risk from southern Oklahoma and central Texas into western Kentucky -- predicting weather a lot like Monday's, but in a broader area.
Weather patterns have to line up just so to designate a high risk area.
"As we get closer to the event, the confidence builds that this is really the threat area," Carbin said.
Storms two weeks ago killed 45 in a stretch from Oklahoma to North Carolina, starting with two people killed in tiny Tushka, Okla., April 14. The weather has been largely unsettled over the Southern Plains and the South since.
Carbin said April and May are historically the most active months.
"The reason is that we get the combination of very strong winds in the jet stream and the contrast between warm springtime air masses and cooler, drier air masses. It's that transition from winter to spring that provides ingredients for severe storms," he said.
He said the jet stream comes in from the Pacific Northwest, dives southeast to the Four Corners region, arcs around the Southern Plains and Red River Valley before heading north to the Mississippi Valley.
Within the jet stream are disturbances that are crucial for the development of severe storms.
"You really do add fuel to the fires," he said.