A Very Difficult Snow Forecast

The ingredients for lowland snow, somewhere in the region, are now evident:  cold air will be moving in aloft during the next days, an upper level trough is approaching, and a low pressure center will be approaching the Northwest coast.

How these pieces will come together is the issue and I will give you may take based on the latest forecast models.   Either Portland or Seattle is going to get a lot of snow, and I suspect it will be the former.

This will be a performance in three acts.

First Act:  Showers Coming Today

There is a band of showers moving in this morning and afternoon, as illustrated by the radar image at 10:40 AM

But at this point, the freezing level is relatively high (around 1400 ft), which means the snow level (the level where all the snow is melted), is around 400 ft (see observed temperatures below, from the Seattle SnowWatch website).  So most lowland residents will only see rain, perfects with a few flakes mixed in during the heavy showers). 

There will be snow showers in the mountains, near the Hood Canal, and for those of you living at higher elevations.  The UW forecast snow totals ending 4 AM Sunday, tells the story.

Act 2:  Cold Air Moves in As an Upper Trough Approaches

As in any good play, some of the protagonists reveal themselves in the second act.   An upper level trough will be building and moving south into our region (see upper level-500hPa-- map for Sunday at 4 AM).   Such a trough results in upward motion (which can produce clouds and precipitation) and can push cold air southward at low levels.

 In fact, the forecast surface chart at 10 AM Sunday shows cool air moving into our region (blue colors) and a weak low center of SW Washington.

The predicted configuration is close but not quite right to give a substantial snow event over western Washington.   The trough is extending too far offshore, the upward motions over us is weak, and the low center is not very strong.  But small differences in the trough position and shape could make a huge difference.  So let's try to tease out the uncertainty by looking at the total accumulated snowfall at Sea Tac Airport using the NWS GEFS multi-run ensemble.  There is some variability, but the average is about .5 inches.  Accumulation on the ground would be less.  And remember this is at Sea-Tac, which is relatively high (452 ft). (Note...time is in UTC, with 12Z/24 being 4 AM Sunday)

 The UW has another, and higher resolution ensemble (run by my group).  There is a lot of uncertainty, but the average is about .8 inches.

So the bottom line of the second act, is that some light snow is possible tomorrow, but it won't add up to much.  Perhaps .5-.75 inches falling out of the sky, with relatively little accumulation.

Third Act:  The Real Threat Emerges

In my plays, the real action occurs in the third act, and this weather situation is no different.  The tension is palpable.  The villain (or whatever you want to call it), will be modest low pressure system that will approach our coast from off the Pacific.  The U.S. GFS model and the UW model that is driven by it, takes the low center into southern Oregon (see below), leaving Puget Sound in dry, cold conditions (4 AM Monday is shown).

As shown by the 48-hr snowfall totals ending 4 AM Tuesday, this forecast produces little snow north of Olympia, but Portland gets several inches, and central/southern Oregon gets huge amounts of the white stuff.   

The vaunted European Center models brings the system a bit further northward--see below.

And the resulting 48-hr snow total from the European Model brings very light snow to Puget Sound and notably more to Portland (6 inches or more).  Salem and Eugene would be snowbound.

There is substantial uncertainty for Puget Sound snowfall, but it looks like we will get little, with Portland "enjoying' the brunt of the action in the third act.

Fourth Act:   More Snow Later This Week?

In a typical fourth act, there is a denouement, where all the plot twists are resolved.  But not in this play.   The European Center Ensemble prediction of accumulated snowfall, shows the potential next week on Tuesday and later.  The fuel and matches are around and our forecasts would not have to be off much to bring significant snow back into western WA.  But this blog is long enough, and this dramatist has to go outside and clean up debris in his yard.

For never-resting time leads summer on 
To hideous winter and confounds him there, 
Sap checked with frost and lusty leaves quite gone, 
Beauty o'er-snowed and bareness every where 

...William Shakespeare

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/02/a-very-difficult-snow-forecast.html