March 21, 2018

Unusually Cold Air and Even the Chance of Light Lowland Snow will Hit the Pacific Northwest

By late March, the threat of lowland snow is normally over, temperatures are warming, and one starts thinking of gardening and hiking.

But during the next few days our relatively normal temperatures of the past week will cool substantially and some portions of lowland Washington and Oregon may get some light snow.

The reason?  A deep upper-level trough moving southward into the eastern Pacific.

The current (5 AM Wed)  upper level map at around 30,000 ft (see below, with wind speeds shown by color and wind parallel to the height lines) already shows substantially troughing (lower pressures or heights) over the eastern Pacific, with a jet stream (stronger winds, yellow colors) well south of the Northwest. Strong southwesterly flow will bring a lot of moisture into California and large amounts of precipitation there.  The jet stream is associated with a large horizontal temperature contrast (cold north, warm south of it), and thus we are on the cold side.

But the situation is about to get more exciting for us as the low to our north greatly amplifies and moves southward (see the same map for 11 AM Friday below).  The jet stream (yellow areas) will still be south of western Washington and Oregon and northern California will remain wet.

Now lets turn to a surface chart (sea level pressure, 3000 ft temperatures--colors, and surface winds). The cold blue temperatures show a lower atmosphere that is cold enough for snow to fall below roughly 1000 ft.  The problem for getting snow at sea level with such a pattern is that the flow is onshore (the ocean is a WARM 50F).

So what do the model snow forecasts show?  Here is the 24 total snowfall (not accumulation) ending 5 AM Friday. Some very light snow (flurries) near sea level and maybe an inch or so on the Kitsap where upslope flow will produce greater intensities (which helps bring snow to the surface).  Plenty of snow in the mountains and particularly the Olympics.

The next 24hrs?  Snow in the mountains extending down to roughly 1000 ft.

Looking at the total over the next 72 hours will bring smile to all skiers, snow-shoers and thus enjoying winter recreation.  Our regional mountains, already with an above-normal snowpack, will get a substantial hit of new snow (1-2 feet at higher elevations).   Northeast Washington gets more snow.  

The bottom line:  we are now guaranteed of going into the summer with a very healthy snowpack.  And with our reservoirs in excellent shape, water supplies for the population and agriculture are secure.

Such a strong end to the season is typical of La Nina years, and this one is not disappointing.


  1. The snowfall model is rather odd:Heavy snow right along the N.Oregon coast, as well as significant snow in the Columbia Basin.Rather unlikely, although there could be large variations depending on whether little vort maxes spin up,or if the PSCZ gets going.

  2. Friday morning @ 9:30 - light snow + rain in the Maple Leaf neighborhood (91st & Roosevelt). The snow is not sticking. It is 32 degrees on my front porch.

  3. Heavy snow is falling in Burien right now and a cumulus nimbus yesterday... can't get anymore PNW than this!


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