Tropical typhoon will cause cold temps, snow in Chicago next week

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Blame a tropical typhoon for an arctic blast that will hit us next week.

Super Typhoon Nuri, which has been churning the Pacific Ocean with 180 mph winds, is expected to kick a couple of dents in the jet stream.  And that will bring arctic air rushing into the Chicago area early in the week, with highs below freezing in some places and lows in the teens.  Measurable snow is a good bet too.

"Temperatures like that would be normal for mid- to late December," said Ricky Castro, a meteorologist with the National Weather Service.

The chain of events — or as the weather service calls it, "a fascinating and somewhat unusual evolution" — begins over the Bering Sea off Alaska.  That's where the remnant of Nuri is expected to start buckling the jet stream, sending warmer air toward the polar region and plunging arctic air over Canada and into the Midwest.

The jet stream will actually bend in two places, Castro said.  Once over Alaska and another over the Chicago area. How far down the jet stream dips will determine how cold and snowy it will be around here.  "It's possible we could have highs below freezing," Castro said.

There could be "accumulating snow" north of the cold system, but Castro said it is too early to tell where and how much. But a good cover of snow could make it even colder, he said.

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The cold will stay for pretty much the whole week, or as the weather service puts it, "leaving our region in the icy grips of an early season cold snap."

You can use the dreaded polar vortex — the swirling mass of air over the poles — to help explain what is going on.

"Obviously, polar vortex became a buzzword last winter," Castro said, referring to the culprit that sent seemingly endless waves of arctic air our way. " You don't have to use that term to explain things next week.  The polar vortex is always there.  We'll get pieces of it, not the whole vortex over us."

Next week's weather could be an opening shot of another long, bitter winter.  Maybe.

Meteorologists have been watching for an El Nino weather system, which can reverse the direction of Pacific trade winds and pull warm water to the surface.  El Ninos in the Pacific can moderate winters in Chicago.
But the El Nino has been weak this season and there are signs of a "negative arctic oscillation" up north that could send cold air spinning our way.

"There are plenty of signs pointing toward a colder winter," Castro said.  "But it's early yet."

--Chicago Tribune staff report



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