December 06, 2016

Increasing Probability of Lowland Snow Late Thursday Afternoon Through Early Friday

Update Wed AM.   The latest runs are shifting the precipitation several hours later, with a start on the early evening on Thursday.  So the Thursday PM Seattle commute may be OK.  Each model run is shifting the low farther offshore and delaying the precipitation onset.  All runs suggest a large east-west snow variation, from little along the western slopes of the Cascades to serious amounts (roughly a half foot) over the area west and southwest of the Olympics (e.g., Hood Canal area).

It is becoming increasingly likely that we will see some significant snowfall over the lowlands late Thursday afternoon into early Friday morning, with amounts of up to 2 to 5 inches before the precipitation turns to rain.  But there will be a large variation of snow across the lowlands, with less near the western Cascade foothills.

My work tonight is made more difficult by major systems failures at the National Weather Service, slowing of Comcast internet (surprise), and the lack of availability of the UW WRF runs (probably due to the NWS problems).

Thursday afternoon, cold and dry air will be in position over western Washington and a strong warm front will be moving up the coast (see graphic for 4 PM showing low-level temperature, sea level pressure, and winds).  Where you see blue colors the air will be cold enough for snow to reach the surface.

Moisture associated with the warm front will be able to fall through the cold air as snow.   By the next  (Friday) morning, as the warm front and associated low moves northward, rain will spread into western Washington (the situation at 4 AM Friday is shown).

The latest run of the UW WRF forecast system shows snow over the region of various amounts.  For example, the 24-h snowfall ending 4 AM Friday indicates 2-6 inches over over the western Kitsap Peninsula and  more over the high terrain, and several inches inches over NW King County.  There is less over the eastern Seattle suburbs due to easterly flow down the western Cascade slopes (downslope flow causes drying).

The European Center snow depth forecasts for 4 AM Friday (below) shows 2-3 inches around Seattle, with less right next to the Sound and over the eastern suburbs.

Talking over easterly flow, there should be strong winds in the western Cascade foothills (see graphic of max gusts at 10 am on Thursday).   45 knots in some locations SE of Seattle.   Very strong winds over the eastern Strait and offshore water.

This situation is more threatening than Monday, since we will have cooler and drier air in place and the amount of precipitation is greater.  Furthermore, with low pressure approaching cold air will be pulled into the region through the Fraser River Valley.

The timing right now suggests that snow will reach the Seattle Metro area between 4 and 6 PM Thursday.

The snow forecasts are critically dependent on the amount of precipitation and the exact temperature structures over us... and the models have been shifting in their solutions somewhat.  Thus, there is still considerable uncertainty in the forecasts, which I will quantify using ensemble predictions tomorrow.   But the nearly all of the ensembles I have seen so far start this event off as snow.  All warm up the area by Friday morning.  The question now is really about the amount of snow and the exact timing.


  1. Potential is definitely there. I am a bit skeptical of the WRF though. Always seems to overdo snowfall amounts.

    1. It's estimates are actually on the low end the euro model had been saying 5-6" over seattle.

  2. Glad to see the old Cliff back. You were beatin yourself up too much over the wind-storm forecast. We got your back dude. Love the passion you bring to this!

  3. This has another bust written all over. I do believe everyone will see snow Thursday evening. Enough to have a snowball fight and stop Seattle traffic. That's a given. But ironically this time will be moisture. A stretching surface low with a weird looking spreadout occlusion with energy aimed south & north. Doesn't it look odd? Set up is classic but not this time. But I think everyone will be happy since there's a little of everything. If you're an extreme snow lover just visit our mountains,

  4. Cliff,

    Clearly this is a better setup for the Hood Canal region...this has been consistent from the models, NAM, GFS, Euro and easy to deduce given the strong easterly winds.
    However- it seems the GFS has been rather variable don't you think? Apparently the Euro has been far more consistent and has a more moderate coating of snow for the metro areas as well...I don't know what to think at this point for the metro regions.
    Hoping it is not another bust for them. It is time Seattle has a couple of inches. Crossing my fingers!

  5. "Increasing Probability..." in the title means that, more of a chance. Nicely written Professor Mass.

    I'm with "Matt the Troll" all the way on this one. Glad to see the insight.

  6. As a teacher that is in desperate need of a snow day, I am glued to your blog. Thank you for such detailed predictions. Could you do them on the hour? Kidding. Sort of.

  7. I am a new resident of Bellingham, Washington (moved here from Portland, Oregon in May 2016).

    What I want to know is how similar is Bellingham's weather compared to Seattle, which is where Cliff lives (I assume).

    Can I assume when these models call for 2-4 inches of snow in Seattle; about the same would go for Bellingham? Or would it likely be more as we are further north? Do we have the same gap winds that Seattle has?

    Thank you

    Mark Allyn
    Bellingham, Washington

  8. Fraser Valley outflow is once again making its presence well known here in north Whatcom County. Wind chill in single digits this morning and incessant northeaster. It's always unpleasant, even more so if it comes in tandem with, or on the heels of snow. We'll see....

  9. Definitely echo Matt the Troll sentiments and as far as weather goes I am with Ben Green on this. Where I live at 1100' on the side of a steep mountain and in the path of downslope winds that can get as big as Enumclaw, this could go any way it wants. No snow, 9" of snow or a blizzard with low accumulation. It is always interesting though to compare predictions with what hapoened after the fact and try to figure out why. I am counting on the downslope winds being a major factor. Thanks Professor.

  10. Fingers crossed for a bust, have a flight out of SEA at 640 am Friday. Looks like I can count on rough air but I like turbulence so that's OK. :-) And the Alaska pilots certainly know how to cope with snow. As always, the issue will be problems on the ground, either on the roads at 330 or 4 am when I leave home, or just general problems on the ground at the airport.

  11. Hey Mark the new Hamster!

    Good luck trying to translate a Seattle-based forecast to Bellingham/Whatcom weather. I live 15 miles northeast of town and conditions can be completely different by the time I get to work at WWU. So many microclimates, plus proximity to water (or foothills) do a number on predictions. The Fraser outflow can have a big effect away from the water too.

    It's really pretty much a crap shoot. Even though I obsess over weather data and monitor my own weather station, I usually plan for the worst and look out the window when I get up in the morning.

  12. In Auburn valley. by green river.

    East Winds just kicked up seemingly out of nowhere, sure feels eerie for some reason. Things going 'bump' in the backyard that normally get shielded from the wind by the house.

    My guess is the east winds will keep the snow away from puget sound/seattle/tacoma area tomorrow and it'll remain a snow event for kitsap and skagit only.

    If memory serves, every time we've had wind predicted with snow, the wind keeps the snow away.

  13. Truly remarkable. For nearly the past 5 years any forecast changes have leaned heavily toward less snow. Not surprisingly, models are now pointing to less snow than before. Good bye Winter Storm Warning, hello Winter Weather Advisory. Now I realize it takes some work to get snow around here, but come on. Again, I'd love to see an analysis of how little snow there has been in our area since January 2012 and how it stands out with other snow droughts the past 100 odd years. :)

  14. Snow in Seattle is like buying a house or getting a job. You just don't know until it actually happens. So far north and so difficult to get cold and snow.

  15. Guess I may need to seriously look at moving to a place where snow is more common AND PREDICTABLE. I'm bummed.

  16. Any suggestions about the best day - and best route - to drive to Hood River, OR? Thursday (original plan) or Friday? I-5/I-84 or I-90/Yakima/route 97? What's the best way to stay current with snow and road conditions especially considering that once a choice is made, the other route is no longer an option?


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

Wet, Then Extended Dry, Then Very Wet

During the autumn, as the jet stream strengthens and moves south, our weather becomes more variable, with very pleasant warm periods interru...