Snow Expected over Western Washington

The first real blast of cold air will reach western Washington with substantial snow in the mountains, where the snow level will descend to approximately 1000 ft.   There could even be some snow reaching sea level in some very favored locations, such as Bellingham and Port Angeles, downstream of the Fraser River Gap in the Cascades.

In any case, this will be the coldest air over the region since last January, so be prepared.

During the past week, high pressure has dominated our region, producing warmer than normal conditions and lots of sun.    But everything is changing now.  By Thursday morning (5 AM, see below) the upper level ridge (high pressure) area will move westward towards the Aleutians, while a potent trough of low pressure is moving towards the Northwest.  If this was one month later, there would be a serious threat of snow over Seattle.  Real serious.

The forecast of sea level pressure (solid lines) and low-level temperatures (colors) shows very cold air (blue) over British Columbia and a low pressure center (associated with upper trough) over southern Vancouver Island.  An intense pressure difference (or gradient) is found north of the border.  This is when the JAWS music starts.

One day later (Friday, 5 AM), the low has moved to the mouth of the Columbia River and cold air starts pushing into northern Washington State.  With the large pressure difference along the border,  strong and cold northeasterly flow will be moving through the  Fraser River Valley into Bellingham and over the San Juan Islands.

With cold air moving in aloft and upward motion/precipitation from the upper level trough, the Pacific Northwest will not only be wet, but snow will fall in the mountains and in areas cooled by the Fraser River outflow.

Next, let me show you the 24h snowfall maps during the next few days.  For the 24h ending 5 PM Thursday, significant snow will fall in the Cascades, with some locations getting a foot.  Lots over NE Washington.

 The next 24h, with cold air coming through the Fraser River Valley,  shows light snow over some parts of NW Washington and heavier snow over the northern side of the Olympics (down to sea level) as northeasterly flow from the Fraser outflow winds ascends that mountain range.

The 72 hr total snow over the region is impressive, with higher elevations getting 1-2 feet.  Snow in eastern Oregon as well.  Skiers should be excited...we will get a good start on base for this winter.    Snow shoeing will be viable by the weekend.

And yes, don't forget the winds.  Expect strong (30-50 mph) northeasterly wind gusts from Bellingham, across the San Juans to the Olympics (see map for 8 PM Thursday).

The pattern this week is a classic one for a La Nina winter....it will be interesting to see if this configuration is repeated soon.

from Cliff Mass Weather and Climate Blog http://ift.tt/2ims1AS