January 12, 2012

The Upcoming Snow

 Friday Morning Note:
    I will do a major update on the weekend snow potential tonight.  Plus, a discussion of the heavy rain/flooding threat middle of next week that could start as snow.  I talked about the snow situation on KPLU this morning--you can read about it and listen to it at:  http://www.kplu.org/
 This is the first snow event in which we have the coastal radar and dual-polarization...should be fascinating to watch it unfold.

I waited to write this blog until I could "see the whites of their eyes"  and this case white is the right color.

It really looks like there will be some lowland snow late Saturday into Sunday...but not the heavy, widespread variety.  It is clear that the mountains will  get quite a bit during the next week and that the entire region will go through a much colder period starting later on Saturday.

As I have mentioned on a number of occasions, it is easy for the western lowlands to be mild and wet, since most weather systems come from the west, passing over the relatively warm Pacific (sea surface temps around 50F).   Sometimes we are cold and dry, when a large high pressure area builds to our west and north.   But to be cold AND wet is not easy and requires very specific conditions that are relatively rare.

The key pattern we look for is a big ridge over the eastern Pacific, with a trough moving southward on its eastern side.  The trough can't swing too much over the ocean or too much warming of low-level air will occur.  Here is the pattern for Sunday morning forecast by the UW WRF model (forced by the NWS GFS model):

You see the basic snow configuration, although it would be better if the flow did not swing out so much over the water.  This kind of trough provides upward motion--necessary for precipitation--and is associated with cool air.  Close enough to be a real threat.

Here is the surface forecast for 4 AM on Sunday for sea level pressure and lower atmosphere temps and winds:

Cool air (white and blue colors) has moved in behind a Pacific front (which will move through Saturday afternoon with rain.)  Generally low pressure centered over northern Washington, but ONSHORE flow at low levels (surface wind vectors shown).   This is important...to get really cold we need low pressure SOUTH of us and winds from the north, pulling cold air out of BC.  With onshore winds the area will be tempered at low levels even though it is cold aloft.  Implication:  the possibility of the nemesis of local meteorologists--snow levels of a few hundred feet.  I hate it.

At this point in time (4 AM Sunday), the main frontal precipitation is past and we are in the post-frontal shower regime.  Above roughly 500 feet, they will be snow.   And where the precipitation is heavy enough, snow falling into warmer air melts and cools it...driving the snow level down to the surface.

As you can see from the above forecast, the winds will be westerly on the coast and that could well lead to a Puget Sound convergence zone, which can produce a band of heavier precipitation between Seattle and Everett.  There could be an accumulation of several inches in such a convergence zone.  This situation occurred on December 18, 1990 where north Seattle got 10-14 inches, with virtually nothing 10 miles north or south of the band.  I doubt we will see that much....but a CZ zone event is quite possible on Sunday.  Here is the latest high resolution model forecast of snow for the 24-h end 4 PM on Sunday from the UW 4-km model:

2-6 inches over the coastal mountains, 6-12 inches over the Cascades, and several inches over the eastern side of the lowlands.  Less in the rainshadow of the Olympics and Cascades.  Even the UW 4-km model lack the resolution to get the convergence zone right--on Saturday the 4/3 km forecasts will be available for this time period.  And there will be snow over the eastern side of Eastern Washington.

So the bottom line:  a very good chance of some lowland snow, but not a uniform heavy snowfall...snow in showers and potential for some accumulation in a convergence zone and the higher eastern suburbs.  And with all snow forecasts, there is substantial uncertainty, although several ensemble systems and other forecasts suggest that we can have a lot of confidence in the larger scale patterns.

The timing (Sunday AM) of this snowfall could not be better for Seattle DOT--surely the supplications of Mayor McGinn to higher powers have been answered.  I should note that the Mayor's office and SDOT have made a great many preparations for a snow event, including supporting the development of the new snow forecasting application, SNOWWATCH, which I will describe in detail on a future blog.

Forecast for later periods indicate the potential for more snow and perhaps flooding rains.  But more on that later...

PS:  If you live in Mountlake Terrace or a nearby community, please keep you eye out for my lost dog..see the picture in the right panel.  Thanks.


  1. The isobars look pretty close together for Saturday....is it going to be windy?

  2. Great post! Now I don't have to explain to my friends how complicated it is to get snow here.

  3. Cliff,

    You pointed out that if the trough moves out over the ocean it can warm too much in the low levels essentially killing the chances of snow in the valleys and lowlands.

    Isn't it also true that if the trough stays too much over land the air moving in tends to be too dry for precipitation?

    I was wondering what you consider to be a good balance. Something that provides a NNW flow that skirts the coast to bring in some moisture but not so much that it gets warmed by the sea surface temperatures?

  4. We live on a steep steep hill that doesn't get plowed, so any time you mention the possibility of snow, I rush to Costco. Thanks for the heads up! :)

  5. Finally, the S word in the forecast! Yea!!!

  6. So are we in the Methow Valley going to be victims of the rainshadow/snowshadow and miss out on that fun white stuff?

  7. Ack! This post contains two nasty phrases ~ "winds from the north, pulling cold air out of BC" and "flooding rains" ~ both of which spell nightmare for those of us farming up on the Canadian border.

    Really, our weather is so drastically different from yours, and even from those ten miles south, that I never know what to make of these forecasts. I'd love a pleasant winter snowscape, but...well, let's be honest. More likely we'll deal with a week of snow showers, black ice, and wind. Just enough "winter" to cause trouble, but not enough to satisfy!

  8. I'm certainly looking forward to the opportunity to break out my snow gloves and plan on building another snow-cat this year. :) (I don't settle with plain 'ol snowmen. No sir!)

    I really hope the forecast works out in favor of all the Snow lovers out there! I missed it terribly during the holidays when all there was to be had was rain. Much sadness indeed :( (not that I mind rain, of course).

  9. Looking forward to your radio spot on 88.5. Gee - I wonder what the subject will be :)

    It looks like two different lows may be coming through this weekend after we cool off and the high res models indicate quite a bit of activity. The coastal radar will be really nice to see what SW flow and showers are offshore on their way in coming around the SW part of the Olympics.

    Even more interesting is the over-runnning event for late Tuesday which could be a rare guaranteed snow situation but depends on how far north it pushes the cold air mass and potential change to heavy rain. Good idea to hold off on that until the right time!

  10. Looking forward to your radio spot on 88.5. Gee - I wonder what the subject will be :)

    It looks like two different meso lows may be coming through this weekend after we cool off and the high res models indicate quite a bit of activity. The coastal radar will be really nice to see what SW flow and showers are offshore on their way in coming around the SW part of the Olympics. Justin

  11. Let's get Cliff's review of the evil dogsitter unfiltered on Yelp. Are you a Yelp user? Go to Cliff's review http://www.yelp.com/biz/dede-harris-pet-nanny-seattle#hrid:4-UZwxVKzBYE26dFPallpA go down to the bottom, click on the grey text "6 filtered," enter the Captcha text, scroll down to Cliff's review. Finally. give it a useful and then friend Cliff. This will take longer to read than to do, I promise. :-)

  12. Cliff ~ I went into the "weatherwiz" site and looked at the GFS model. Now, I am certainly not an expert, and long range models can change by the minute, but they show that we will be flirting with cold and a lot of moisture for at least a couple of weeks, with a couple of possible good blustery/windy days in between. What's your take on this? Are we in for a couple of crazy weeks?

  13. Looking at the 12z ECMWF and GFS there is a greater likelihood that Weds even Thursday could be all snow from Olympia North. Still to early to tell but things are trending that way and will need to be watched. This could end up being a significant snow event but time will tell.

  14. Quite a difference this Friday morning on the European vs GFS models for next weekend (Jan 20th and beyond).Looks quite cold if the ECWFM plays out.Finally, some interesting weather for everyone at the UW and NWS.

  15. Re: JewelyaZ's suggestions. I'm not sure if it's been mentioned but we figured that Dr. Mass' comment gets filtered because one one-star review was made to one business. Perhaps another way to unfilter is to make a couple of positive reviews to other businesses to make you a legit Yelper. If time allows, perhaps you can share with us Yelp ratings of your favorite restaurants in town? Just a thought

  16. Why no NWS updates? They haven't updated since 4 am and keep pushing out the next update. I am no weather expert so I rely on very basic tools. Does that lack of updating mean nothing has changed since 4 am or that they haven't a clue?? Or do they do this all the time and I just don't notice?

  17. Curious how accurate you feel this site is in terms of upper level flow/Jet Stream predictions?


  18. Thank you for the link to KPLU...it's very helpful if we miss your Friday segment.


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

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