Light Snow Reaches the Lowlands of Western Washington

Snow revved up in the mountains yesterday, but this morning the snow level fell to near sea level over several locations of western Washington.

The upper-level flow pattern was a classic for western Washington snow with high pressure offshore and a sharp trough of low pressure reaching the Northwest coast (see upper level map at 5 AM).    This pattern brings both upward motion (which produces precipitation and clouds) and helps push cold air into the region.

If this was a month or two later, we would have been worrying about a major lowland snow event.

Cooler air and higher pressure first moved into British Columbia yesterday, and as a result cool winds started blowing in the Fraser River Valley northeast of Bellingham, where northeasterly winds revved up yesterday (see below).  Winds gusted to 39 knots (45 mph) there, and temperatures plummeted from the upper 40s to 31F.  And yes, it is snowing there.

The is cool, but not super cold---still very early in the fall/winter season, of course.  The cool air pushed south overnight, with northerly flow ascending  the northern Olympics, enhancing snow in places like Port Angeles and Sequim (see map at 6 AM).  No rain shadow for these weather-spoiled folks.

The freezing level (the height where the temperature falls to freezing) dropped all day yesterday, and the latest observations at Sea-Tac airport suggest it is at about 1300 ft (950 hPa pressure)--see the time-height cross section below (red lines are temperature, x-axis is time, y-axis is height in pressure).

Heavy precipitation showers can push the freezing and snow levels (the lowest level of snow) down to near sea level, and we do have unstable air and showers over the region (see radar at 7:04 AM)

The model forecasts had suggested this snow interlude.  Here is the 24-h snow amount ending 5 PM today from last nigh's run.  Not bad!  And the mountains are going to be hit hard today.  Note the substantial upslope snow on the north side of the Olympics.

Today the lowland precipitation will quickly end as the area of showers moves through.  So Seattle and other lowland mayors can rest easy.  Furthermore, road surfaces are relatively warm. 

But the snow fun is not over.    Another upper trough and associated low moves in Saturday night and Sunday, with some significant lowland snow, particularly south of Seattle (see forecast below).  Olympia gets hit hard. Lots of uncertainty with this...so keep tuned.

This is a La Nina winter and this situation is classic for a La Nina year.  The Cascades will have a good start to a winter base for skiing and other snow fun.  I mean several feet above 5000 ft.



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