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Why sunny in Seattle means snowy in Chicago

Gary Davis
A skateboarder cruises past the Space Needle in Seattle during a sunny day on Feb. 1, 2011.

You might have wondered -- as you gazed out your sunny window and listened to news of record cold and snow sweeping the midwest and East coast -- is there a connection?

Yes, there is.

"Our weather often is the just the opposite of what it is in the eastern part of the united states," says Cliff Mass, professor of Atmospheric Sciences at the University of Washington.

"When we are cold, like it was just before Thanksgiving, they tend to be warm," says Mass.

The reason we're yin when they're yang, and vice versa, has to do with the jet stream and "ridges" of high and low pressure in the sky, as Mass explains it.

A pressure ridge is parked over the Pacific Ocean, near our coastline. That's sending the jet stream north.  For every ridge, there must be a valley (or "trough" in weather parlance), and a valley of low pressure is over the midwest. And, that's sucking the jet stream, and stormy Arctic air, back south, to the middle of the U.S.

"So the persistent ridging over us, bringing dry, cool weather this week over the NW, brings the opposite to those poor folks east of the Rockies. Want dry weather with lots of sun...forget Florida...head to Seattle," writes Mass on his weather blog.

Sometimes, it's the other way around, and the stormy trough is parked over Washington state, while the rest of the country gets the mild ridge.

The models, at this point, show a little weakening of the ridge and some light rain for us this weekend, but more sunny and clear weather next week.

It's unusual to have a ridge last this long, but not unprecedented. The winter of 1976-77 had a similar pattern, with lots of dry days in western Washington. In January of 1977, a legendary blizzard hit New York, and it snowed in Miami for the first time in history. Today's (Wednesday Feb. 2nd) Seattle Times headline read:

"Once in a lifetime storm hits millions" (though it's changed in the latest online version).

The one downside here in the NW: The Cascades had barely any snowpack in 1977. This year, the snowpack is running at about 70% of normal.

Keith Seinfeld has been KPLU’s Health & Science Reporter since 2001, and prior to that covered the Environment beat. He’s been a staff reporter at The Seattle Times and The News Tribune in Tacoma and a freelance writer-producer. His work has been honored by the American Association for the Advancement of Science (AAAS) and the Knight Science Journalism Fellowships at Massachusetts Institute of Technology.
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