The National Weather Service said a storm gathering strength in Canada is expected to bring anywhere from two to seven inches of snow to the lowest elevations of Southwest Washington and Northwest Oregon by Wednesday afternoon into Thursday. The Coast Range and Cascade foothills could see significantly more.

“It’s looking more and more that we may get snow, with two to four inches a good bet in the Portland metro area,’’ said Clinton Rockey, a forecaster for the

.”Everything looks good to get the air mass cold enough and supportive enough for snow.”

Rockey said two essential ingredients for snow in the lowlands (and they have to be precise or this snow souffle falls flat) are starting to align.

First, a low pressure system packing cold air is going to spin counterclockwise out over the Pacific Ocean from British Columbia. That system will be out over the water long enough to pick up moisture, but no long enough get too warm. Meteorologists call it a “short trajectory over water.”

In the meantime, bitterly cold arctic air now in place over the Canadian Rockies will come knocking at the back door, filtering into the Columbia River Basin, where it will shoot down the conduit of the Columbia River Gorge to Northwest Oregon.

As in past storms, where you are will be the biggest factor in determining just how much snow you’ll get at your house, Rockey said.

“The flow is starting to set up where the showers start to train,’’ Rockey explained. “Think of the showers like boxcars on a train that travel over the same location on the tracks. Each shower goes over the same area again and again. Suddenly, you have four inches where the showers hit, and nearly nothing nearby. There looks to be some heavy bands of snow showers.”

The Oregon coastal mountains could see up to a foot of snow Wednesday into Thursday, with five to seven inches in the northern parts of Clark County. Rockey said late Wednesday may not be a good time to drive over the coast range to the Oregon Coast.

And what of the timing?

“During mid- to late-afternoon is when we may see things start going,’’ he said. “But there may not be a whole lot of anything until rush hour in Portland. The better stuff comes in Wednesday night.”

While

are rare in Portland, they do occur, Rockey said. During a four day period from February 18-21, 1993, Portland was walloped by 6.6 inches of snow, but some places saw up to a foot or more. During that same period Salem measured 14.1 inches of snow.

The granddaddy of snowfalls for late February happened in Eugene from Feb. 20-25, when 22.7 inches of snow fell, which led to a week of good sledding.

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