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.
..cliff

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.