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.

About an inch at 1300 ft in Bellevue (picture courtesy of Peter Benda)

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.

Snowy Sequim

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.
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NWS Snow Reports

Public Information Statement
National Weather Service Seattle WA
1056 AM PDT Fri Nov 3 2017

..Updated Snowfall Reports...

Location                     Amount    Time/Date       Elevation (ft.)      

..Washington...

..Clallam County...
3 SW Port Angeles            10.0 in   0700 AM 11/03   0                    
3 WSW Agnew                  5.0 in    0700 AM 11/03   0                    
5 SW Sequim                  4.0 in    0830 AM 11/03   0                    
1 SSE Port Angeles           2.8 in    0700 AM 11/03   0                    
1 SE Sequim                  2.0 in    0700 AM 11/03   0                    
Port Angeles                 1.6 in    0700 AM 11/03   0                    
1 W Port Angeles             1.5 in    0800 AM 11/03   0                    
4 ESE Port Angeles           1.3 in    0700 AM 11/03   0                    

..Jefferson County...
1 SW Chimacum                0.5 in    0700 AM 11/03   0                    
4 W Port Ludlow              0.4 in    0800 AM 11/03   0                    

..King County...
1 NW Snoqualmie Pass         1.4 in    1000 AM 11/03   3100                 
6 WNW Greenwater             1.0 in    0400 AM 11/03   3900                 
Tolt South Fork Reservoir    1.0 in    0815 AM 11/03   2000                 
Eastgate                     0.4 in    0600 AM 11/03   0                    
1 ESE Woodinville            0.4 in    0800 AM 11/03   0                    
1 ESE Woodinville            0.4 in    0925 AM 11/03   0                    
1 NNE Redmond                0.3 in    0800 AM 11/03   0                    
2 WSW Mercer Island          0.2 in    0700 AM 11/03   0                    
2 ENE White Center           0.1 in    0830 AM 11/03   0                    
1 ENE Seattle                0.1 in    0800 AM 11/03   0                    

..Kitsap County...
1 SW Bainbridge Island       0.1 in    0859 AM 11/03   0                    

..Lewis County...
Paradise                     14.0 in   0800 AM 11/03   5130                 
7 S Longmire                 1.0 in    0900 AM 11/03   3770                 

..Pierce County...
Rainier Paradise Ranger Stn  10.0 in   1000 AM 11/03   5427                 
Longmire                     2.0 in    0745 AM 11/03   2762                 
6 W Crystal Mountain         1.3 in    1000 AM 11/03   6410                 
10 WNW Mount Rainier         1.0 in    0900 AM 11/03   3160                 
4 SSW Crystal Mountain       1.0 in    0900 AM 11/03   5240                 

..San Juan County...
8 SE Eastsound               1.3 in    0800 AM 11/03   0                    
4 SSW Friday Harbor          1.0 in    0800 AM 11/03   0                    
5 E Friday Harbor            0.5 in    0605 AM 11/03   0                    
6 ESE Friday Harbor          0.5 in    0700 AM 11/03   0                    

..Skagit County...
18 W Mazama                  2.0 in    0200 AM 11/03   3930                 
5 N Sedro-woolley            1.5 in    0800 AM 11/03   0                    
Concrete Ppl Fish Stn        1.3 in    0640 AM 11/03   195                  
14 SE Diablo                 1.0 in    0900 AM 11/03   4320                 
1 E Mount Vernon             0.5 in    0715 AM 11/03   0                    
Mount Vernon                 0.3 in    0800 AM 11/03   0                    

..Snohomish County...
Brier                        0.2 in    0640 AM 11/03   0                    

..Whatcom County...
Wells Creek Snotel           17.0 in   0800 AM 11/03   4030                 
7 S Mount Baker              14.0 in   0700 AM 11/03   3520                 
12 NNE Hamilton              13.0 in   0800 AM 11/03   3040                 
4 SSE Glacier                12.0 in   0900 AM 11/03   4970                 
12 NW Diablo                 7.0 in    0700 AM 11/03   3630                 
1 E Maple Falls              4.5 in    0700 AM 11/03   0                    
3 SE Bellingham              4.5 in    0700 AM 11/03   0                    
2 SW Bellingham              4.3 in    0700 AM 11/03   0                    
Glacier                      4.1 in    0725 AM 11/03   0                    
2 S Bellingham               4.0 in    0700 AM 11/03   0                    
Mount Baker                  3.3 in    1000 AM 11/03   4210                 
3 NNW Sudden Valley          2.8 in    0700 AM 11/03   0                    
4 ESE Blaine                 2.5 in    0700 AM 11/03   0                    
8 NNE Concrete               2.0 in    0600 AM 11/03   689                  
1 W Lawrence                 2.0 in    0600 AM 11/03   0                    
4 ENE Deming                 1.8 in    0700 AM 11/03   0                    
1 SSE Lawrence               1.7 in    0925 AM 11/03   0                    
2 WNW Ferndale               0.7 in    0700 AM 11/03   0                    
Lynden                       0.3 in    0800 AM 11/03   0                    
Point Roberts                0.3 in    0700 AM 11/03   0                    

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.