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March-April 2012

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Weather Front

Collision-Borne Storms

It is rare, but according to a University of Wisconsin-Madison professor of atmospheric and oceanic sciences, when two jet streams collide over the ocean, colossal rain and tornados hit the Midwest. Jonathan Martin's study suggests that when the polar jet stream and subtropical jet stream that forms over the Pacific merge, they not only pick up moisture from the Gulf of Mexico, but they also produce strong vertical circulation that can lead to the kind of conditions seen in Nashville last spring of 2010, when 10 to 20 inches of rain fell during intense storms.

The subtropical jet stream is a high-altitude band of wind that runs around 30 degrees north latitude. The polar jet stream is normally hundreds of miles to the north. According to Martin, during the spring and fall, organized complexes of tropical thunderstorms over Indonesia push the subtropical jet stream north where it converges with the polar jet stream and forms what he calls a “superjet.”

Jet streams in the northern hemisphere blow from the west at roughly 140 mph, and are surrounded by a circular whirlwind that looks something like a tornado pushed on its side. The circulating wind at the bottom of the jet stream blows from the south. On the north side, the circulating winds turn vertical, lifting and cooling the air until the water vapor condenses and feeds precipitation.

KIMBRA CUTLIP is a freelance writer and former assistant editor for Weatherwise.

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