A blast of cold air is moving into the Puget Sound region from Central Canada that will make temperatures plummet by mid-week, according to the National Weather Service in Seattle.

There's an outside chance of snow, depending on how much moisture is in the air over the Pacific when the cold air mass arrives, said forecaster Jay Albrecht.

"I'm not willing to put my neck out and say we'll see snow," he said, putting the likelihood of snowfall at 30 percent or less. "It will get sharply colder, probably starting" Tuesday. By Wednesday, "we'll see temperatures where we're struggling to get to 40 (degrees) during the day" and Thursday and Friday, daytime temperatures are expected to be in the 30s, dipping down into the 20s at night, Albrecht said.

"We'll also be getting 15 to 20 mph wind that will add a little bite to it," he said.

Albrecht explained that there is a big ridge of high pressure in the upper atmosphere, about 1,000 miles off shore. That's going to bring in winds from the north and northeast; the Cascades will act as a bit of a buffer, warming up the -30 degree air as it moves west, he said.

Forecasting snow for the lowlands is a bit tricky, Albrecht said. "When you have cold, dry air over the Pacific, weird things can happen," he said. "It depends on if and where a little low pressure-system forms off the coast ..., which can shove Pacific moisture over the cold air coming from the northeast.

"When you clash moisture with cold, that's where you get your snow," he said. "We'll be watching it closely over the next several days."

Even if we don't get snow, "this will be the coldest air we've seen in the season and it could well be the coldest air we've seen in a decade," Albrecht said. "It's going to get a lot colder and that's pretty close to definite."

Sara Jean Green: 206-515-5654 or sgreen@seattletimes.com

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