From the ABC 7 Weather team

A split jet stream brings severe weather to the U.S. (MAP)

November 14, 2011 - 03:14 PM
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A jet stream that's forked like a snake's tongue is pumping up the risk of tornadoes and hailstorms in the Midwest.

Tornadic weather over the Midwest on Monday, Nov. 14, 2011, can be blamed on a seasonal split in the jet stream. (NOAA)

While D.C. is having a grand ol' weather day, a swatch of land stretching from Illinois through Indiana to Ohio to three other states has fallen under tornado watches and warnings. What's causing this discontent in the atmosphere? Blame it on a disturbed jet stream, a river of wind that can scream along at up to 450 m.p.h. several miles above in the sky.

The jet stream typically flows in a single thick band traveling west to east. But around this time during La Nina years, the stream can become split, with one vein coursing through the north part of the country and the other through the south. A split jet stream is a harbinger of severe weather; look above and you'll see why.

The blue in NOAA's map from today represents wind speed at a height of about 3.5 miles, with the darkest blue/purple areas indicating places where gusts top 100 m.p.h. The fast winds denote the location of the jet stream. The stream begins to split in the Pacific Northwest: One branch loops down into Mexico while the other stays the course at the top of the U.S. Where they rejoin is where the problems start. The northern stream is holding a load of frigid air that it carried in from Canada, whereas the southern one is damp and warm thanks to a nice vacation in the Gulf of Mexico. The combination of these disparate atmospheric conditions forms the recipe for severe storms, and is the reason that one tornado and quarter-sized hail have already been reported in Champaign County, Ill.

Here's another view of the same phenomenon, taken at 2 p.m. EST today by the GOES-13 satellite:

midwest tornado november 2011

Says NOAA:

The atmospheric instability can also be seen in the disarray of cloud structures – especially the wispy high cirrus clouds. The banding patterns seen in these clouds are also associated with areas of turbulence. In addition, the high over-shooting cloud tops that are characteristic of severe convective weather can be seen forming over central Indiana.

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