November 04, 2017

Will it Snow Tonight and Tomorrow Morning in Western Washington?

The answer is yes, but the amounts and distribution will be complex, elevation/location dependent, with considerable uncertainty.

The issue, as it almost always is with western WA lowland snow, is temperature.  Near sea level, the temperatures will be marginal, something compounded by the fact we are early in the winter season.  If this was January, I would calling Seattle's DOT folks to rev up the plows and sand trucks.

The general set up is nearly ideal.  An upper level trough will be coming in from the northwest (see map at 4 AM tomorrow), bringing cool air and precipitation.

Associated with the upper trough, a tight low pressure center will move to the SW tip of Washington, producing a pressure pattern that draws cold air into Washington, particularly through the Fraser River Valley.  Perfect.  The trouble is that the air in BC is not super cold and the temperatures (shown by the color shading) are on the warm side from Seattle southward.  Marginal for snow below 500 ft.  The only way to overcome this is heavy precipitation, with lots of cooling due to melting from snow falling in from above.

The precipitation will occur mainly between midnight and 9 AM Sunday. 

Here is the UW WRF snow forecast for the 24 hours ending 4 PM Sunday.   Huge variations.  The central and southern Cascades get hammered, with over a foot in many locations.  With northeasterly flow from the Fraser River Valley headed for the Olympics, the northern slopes will get snowed in, with Port Angeles and Sequim getting a piece of that.    There will be snow over central Puget Sound, but it will be very light near the Sound and will increase substantially for elevations about 500 ft (maybe 1-3 inches, higher amounts above 1000 ft).  Upslope conditions will provide more snow around the Hood Canal.

What are the ensembles (running many forecasts) say?  Here is the output from the National Weather Service SREF (short-range ensemble forecast) system that shows a plot of three-hour snow amounts at Sea Tac for the various members of the ensembles.   A lot of variability (therefore uncertainty), ranging from a trace to a few inches.  But most are going for something. 

The other NWS ensemble (GEFS) also has snow at Sea Tac  for most of its members (see below), with an ensemble mean (average) of about .7 inch.
My take from all this is the following.  The mountains will get plenty of snow (feet), solidifying a good start to the base yesterday.   Thanksgiving skiing beckons.   Near the Sound and sea level (below 500 ft), there will be some very light snow (trace to .5 inches).  But go higher (above 500 ft)  and away from warm water and amounts will get to 1-3 inches.   Like on the hills above Bellevue.   Several inches near the Hood Canal and around Sequim/Port Angeles.

As shown by the ensembles, there is considerable uncertainty.  Modest changes in precipitation intensity will make the difference between rain and snow (rain for lighter intensities).   

And keep in mind it will be hard for the snow to stick to roadways in contact with the ground, since we are so early in the season.  The latest road temperatures shown in the Seattle SNOWWATCH web site (boxes, see below), indicate that road temperatures are now in the low to mid-40sF.
Anyway, enjoy the unusually early snow over the lowlands.  And remember to change the clocks tomorrow as well.  Another hour to enjoy a hot drink and watch the flakes fall.


  1. We've still got a lot of snow here on top of Bell Hill near Sequim. Had to go out and clear low branches from their snow load all across my property. The Spruces and Leland Cypress were bent to the ground.

    And good thing I got it off after reading this blog entry and realize we may get a redo tonight.

    Two Fraser River outflows with good moisture and upslopes in one week! We're normally lucky to get two per winter, and some winters none. Not sure if this sets the script for the rest of the winter, but if so, this one's gonna be REAL.

  2. Always love reading these posts! Thanks for the update :)

  3. Another 3 or 4" at elevation near Sequim. It's the "coat every twig" style of snow, which makes for a beautiful morning. That's about 8" between Fri and today, very little of which melted between then and now.

    Labs are in heaven.

    We put out our house-warmed hummingbird feeder at the first hint of dawn, and immediately collected a bunch of Anna's around it. Weird to see snow and hummers together. Tough on them too. One collapsed beneath our feeder to flop, wings out, half buried in the snow on the railing. We thought it was dead, so my wife went out to collect it and it surprised her by shaking off the snow to rise and fly off. Hope he makes it. Nothing else to eat except at feeders, and they freeze up at night if you leave them out.

  4. On-piste skiers, take note: Locally, the only foot-plus snowfall has been at Mt Baker. Stevens, Summit and Crystal received two to six inches overnight and not much more out there until Wednesday. But it's early!

  5. Your prediction was spot-on. We had very light flurries through the night (in Glacier), but they didn't amount to much in terms of actual precipitation.

    I wasn't terribly surprised by this extremely early snow-spell. The past summer was notably cooler than many I've seen here, near Mt Baker. Our nighttime lows (at least here) were so unusual that I kept a separate spreadsheet. I'm not suggesting a trend, but given how snowy last winter was we stocked extra firewood.

  6. This is the best kind of snow: big flakes blowing around in the wind, melting upon contact with the road but providing a lot of wintry atmosphere before they get there.

  7. Woke up to bright white Skagit Valley sprinkel, back to normal now.

  8. @John Marshall, don't forget to put a little protein powder into the hummer feeder during the winter. We think of them as nectar feeders only, but they do eat spiders and other insects as well, which aren't usually as readily available in the PNW fall/winter.

  9. Can you touch on why the jet stream pretty much teleported from Northern BC to Washington in 1 day? It looks like it will remain in North Cali, Oregon and Washington for the foreseeable? In Revelstoke it feels more like January but without the snow. Went from warm with showers to very cold and very dry in a 24 hour period with only a dusting of snow in between.


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