A Week Of Wild Weather: Decoded Science Names Severe Weather Outbreak Dromedary

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Twin tornadoes in 1965. Photo courtesy of NOAA

These twin tornadoes occurred in 1965. Photo courtesy of NOAA

The twin tornadoes in Nebraska, tornadoes and flooding in South Dakota, and hail in Minnesota can all trace their history to a slow-moving weather system that involves contrasting air masses and what remains of the polar vortex in a chicken-and-egg argument.

Decoded Science considers the outcome to be significant enough to be accorded the status of a severe weather outbreak and has named it Dromedary.

The Parents Of Dromedary: The Jet Stream And The Gulf Of Mexico

Since winter ended its onslaught on the eastern United States, a strong low-level flow has driven very warm and humid air into first Dixie, then the southern plains, and more recently the northern plains and midwest. Meanwhile, the remnant of the winter polar vortex is continuing to bring stronger-than-usual middle level winds across the Rockies and into the plains and midwest.

Fronts, the demarcation lines of contrasting air masses, lie along the strongest part of the jet stream. The temperature gradient (change of temperature with distance) at the surface causes a pressure gradient (change of pressure with distance) above it.

The past winter’s extreme cold in the eastern United States was often blamed on the strong polar vortex over eastern Canada and the midwest, but it could easily be thought of as the other way around. Without getting into the chicken-and-the-egg, the current situation is caused by the strong jet stream coupled with the temperature and humidity contrast at the edge of the air mass that originated in the Gulf of Mexico.

Axis Of Action: Northern Plains Into The Midwest

Though the entire warm air mass is somewhat unstable, the major tornadic activity has taken place in Nebraska and South Dakota, with flooding from thunderstorms in Minnesota and Wisconsin.This is where the warm, humid air meets drier and cooler polar air, and where the jet stream is most pronounced. Whether tornadoes or only strong thunderstorms form depends on the vertical wind structure. Vertical wind shear (turning of the wind with height) can start the rotation necessary for tornadoes to form.

Thunderstorms — even some severe ones or tornadoes — can occur any place  in the warm air mass. Daytime heating produces individual updrafts that are strong enough to cause a local overturning of the atmosphere. These thunderstorm cells are scattered and have no particular pattern. Along the boundary of the warm air mass, with a dry flow of air above, the conditions are right for the most violent weather. As warm air moves over cold at the warm front, or cold air undercuts warm at the cold front, widespread instability is likely.

The forecast for Thursday evening. The highest probability of severe weather is along the fronts. Forecast courtesy of NOAA

The forecast for Thursday evening. The highest probability of severe weather is along the fronts. Forecast courtesy of NOAA

Where Are The Tornadoes? Is Nebraska The New Kansas?

Though it is true that on an annual basis Kansas has more tornadoes per unit of area than any other state, this statistic is misleading, because the location of the maximum varies with the season.

In the spring, when the warm air first makes a foray into the mainland, severe weather outbreaks such as Aardvark and Beaver affect the southern plains and southeast.

As the season progresses, the action moves into the central plains; Cougar affected that area.

By summer, the warm air has normally spread northward so that severe weather outbreaks like Dromedary affect the northern plains and upper midwest. Later in the summer, the severe weather will move south again.

Monday’s activity was centered in Nebraska, and yesterday the activity moved into South Dakota and Minnesota. Today’s severe weather will progress slowly east to Wisconsin, Iowa, Illinois, and the Ohio valley. Meanwhile a curl in the jet stream will reinvigorate action in the plains, most likely in Kansas.

The Twin Tornadoes

The rare phenomenon of two powerful tornadoes within a mile of each other affected Pilger, Nebraska on Monday. The potential energy in the atmosphere, a result of contrasting air masses constantly replenished by the differential heating of equator and pole, is large relative to the energy of a tornado. On rare occasions, two powerful tornadoes can exist in close proximity.

Dromedary Departs. What’s Next?

The jet stream pattern that produced Dromedary is forecast to relax by Friday, and the threat of severe weather will diminish. Currently the forecast calls for a weaker jet stream for the next week, and though some severe thunderstorms will undoubtedly occur in the warm air, the worst of this severe weather outbreak will be over after today.

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