U.S. Braces for More Tornadoes and Flooding After Deadly Weekend

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Homes and businesses are wrecked in downton Vilonia, Ark., Monday, April 28, 2014 after a tornado struck the town late Sunday.

Image: Danny Johnston/Associated Press

By Andrew Freedman2014-04-28 11:28:21 -0400

After deadly tornadoes swept through Oklahoma and Arkansas, this weekend the threat for significant tornadoes shifts slightly to the east into Monday night, putting cities from Huntsville, Ala. to Memphis, Tenn. at risk.

There may not be as many storms as there were on Sunday, when a long-track tornado, which may have been multiple tornadoes from the same parent thunderstorm, caused widespread destruction in the towns of Mayflower and Vilonia, Ark. The death toll from Sunday's tornadoes stands at 17.

Luckily, central Arkansas is not in the crosshairs for tornadoes again on Monday. Instead, the Storm Prediction Center, which is the branch of the National Weather Service tasked with issuing tornado watches, has identified northern and central Mississippi, northwest Alabama, western and middle Tennessee, and far northeast Louisiana as the areas most likely to see a few intense tornadoes.

See also: Storm Chaser Describes Long Night of Rescue After Arkansas Tornado

“The greatest risk is forecast from southern Kentucky and Tennessee south into Mississippi and Alabama,” SPC said.

As of Monday morning, the SPC forecast a “moderate risk” of severe weather, including intense tornadoes, but forecasters cautioned that an upgrade to “high risk” may become necessary. If Monday were to be designated a high-risk day, it would be the first time there were back-to-back high risk days in the U.S. since May 2011.

As was the case on Sunday, the ingredients necessary for tornadoes to form are once again present on Monday. This include fast-moving air at different layers of the atmosphere, with the wind varying in speed and direction with height. This is known as wind shear, and it is a prerequisite to tornado formation because it causes the air to spin, allowing rotating thunderstorms, known as "supercells," to form and spawn tornadoes.

On Sunday evening, before the deadly tornadoes tore apart the towns of Mayflower and Vilonia, the amount of wind shear in northern Arkansas was almost off the charts.

Monday’s forecasts also show a high amount of wind shear will be present from Tennessee south to Mississippi and Alabama. There will also be plenty of moisture to support severe thunderstorms, and since the storms will move slowly, they could cause significant flash flooding from Louisiana to Indiana.

Typically, weather systems responsible for causing tornado outbreaks move relatively quickly from west to east across the U.S., with multi-day, multi-state outbreaks in nearly the same areas standing out in the historical record. In this case, the weather pattern across the U.S. is blocked by a large and unusually powerful dome of high pressure across southeast Canada.

Winds at about 30,000 feet above the surface on April 28, 2014, showing a highly contorted jet stream pattern.

The jet stream, which is a current of high-speed winds at about 30,000 feet above the surface, is contorted like a twisted slinky toy, diving southward over the west, then shunted north into Canada, only to plunge southeast across the northeast U.S.

The result is several days of severe, life-threatening weather — from flooding to tornadoes.

Some scientists think global warming is making such “amplified” jet stream patterns more likely to occur by warming the Arctic and altering the heat balance between the North Pole and the equator. This hypothesis, however, has not yet been proven in the scientific literature, and remains a subject of heated debate.

Topics: Arkansas, Climate, deadly tornadoes, Jet Stream, tennessee, tornadoes, tornado outbreak, U.S., US & World