December 23, 2012

Snow in the Lowlands on Christmas Day?

The National Weather Service has special weather statement out right now about the potential for snow over the western Washington lowlands and some of the TV stations have already started the drumbeat.  As I will describe below, MOST of you will not see any snow accumulations, but some places (like western Kitsap County and Mason County, southwest WA) might get substantial accumulations.  And lets not forget eastern Washington and the mountains, both of which should enjoy snow on Christmas Day.

Snow plow near Silverdale.  Courtesy of Kitsap Sun.
We are close enough to the event now that I have some confidence in the general picture of what will happen...but snow forecasts are always hard ones.   Let me explain the meteorological situation and why Kitsap/Mason county are often snowy when the rest of the lowlands are wet.

The first thing to keep in mind is that the current temperatures are marginal for lowland snow.  We really don't have very cold air over us, with the snow level at 1500-2000 ft. The snow level will fall a bit during the next day (by about 500 ft).  Plenty cold for snow in the mountains and eastern Washington, but not for the west side.

After a relatively dry Monday, a fairly strong weather system will approach us.  As illustrated by the forecast map at the surface for 7 AM on December 25th, a warm front will approach that morning. The solid lines are pressure, the colors are temperature (white, blue and purple are colder), and surface wind barbs are shown as well (click on image to blow it up).  You will notice modestly cooler air over western Washington.  The front will be accompanied by precipitation and moderate to strong southeasterly flow.

Strong southeasterly flow will be forced to rise up over the Olympic Mountains--in fact the lift starts tens of kilometers upwind of the Olympics.   Thus, there will be heavier precipitation over Mason and Kitsap counties. 

Here are the winds, temperatures, and pressures at roughly 5000 ft at 7AM on Tuesday.  You can see the strong southeasterly flow and the cooling occurring over and immediately upstream of the Olympics.  The central and eastern Puget Sound are a bit in the rainshadow of the Cascades (the southeasterly flow sinks on the western slopes of the Cascades and sinking air kills precipitation).

 In marginal situations, places with less precipitation (like Seattle in this case) will be mainly rain (perhaps a few wet snow flakes mixed in on the hills), but areas with heavier precipitation rapidly turn to snow as melting of snow falling from aloft progressively cools the air air, pushing the snow level down to the surface.  (heavier winter precipitation around here virtually always starts as snow aloft).

  Let me show you the details!  We can take the model forecasts and plot the wind, dew point (blue line), and temperature (red line) with height.   Here is such a plot at Shelton, WA (Mason County, upstream of the Olympics) at 7 AM on the 25th. The x axis is temperature and the y axis is height in pressure (850 is about 5000 ft).  You can see the southeasterly winds at low levels and the temperature is almost exactly 0C (32F) in the lower atmosphere.   Why 0C?  Because snow is melting in that entire layer!   And the air is saturated (red and blue lines are on top of each other).  A Kitsap classic.

But what about Seattle?  Here is the same type of plot at the same time. The air is  saturated (temperature and red lines are not on top of each other) and the air temperature is above freezing for a few thousand feet above the surface.  Rain and disappointed kids.

   Lets look st the 24-h snow maps for periods ending 4 PM on Tuesday and Wednesday.  You can see the Kitsap and Mason county snows, and in fact it extend to Grays Harbor County as well.  Portions of SW Washington may see some light snow, as the higher suburbs east of Seattle (e.g., Somerset neighborhood in Bellevue, Issaquah Highlands).

 The next day, with warmer air, the snow is over over the west, but large parts of easterly Washington, including around Spokane and Wenatchee should be whitened up.

Here is the predicted snow for the next 72 hr.  The Cascades will get plenty, particularly over the southern sections (e.g., Crystal, Mt. Hood!). 

There are some uncertainties with this forecast (e.g., heavier precipitation more snow) and it would not shock me if Portland received a significant dusting.  But I would be surprised if the general picture shown above is not correct.  Temperatures are warming during the day, so if Seattle had a little wet snow during the initial stages of the event, it would turn to rain in a few hours.

Sobering fact:  less than 10% of Christmas days are white in Seattle...


  1. Hi Cliff,

    I was wondering if you'd take a look at the possibility of some freezing rain Christmas morning in the northern Willamette Valley, specifically the east Metro area and Troutdale.

    I'm seeing moderate to strong east winds Tuesday morning coming out of the gorge keeping temperatures low when the rain comes in. The models are giving more moderate temps in the upper 30's Tuesday morning, but I was curious what you think the chances are of the temps west of the gorge dropping close to freezing, which would make things interesting.

  2. Cliff

    I thought you would enjoy this economic link to the near future and your clever name.....Happy Holidays and thanks for all your wonderful posts all year long.

    Stocks: Investors hope for a 'Cliff-mas' miracle

  3. I'm not counting on a white Christmas, Cliff. I was born in the NW and raised on small farms east of the city and in my 64 years I've seen, I believe, 7 white Christmases, including 3 in a row in the foothills in '65, '66 and '67.

    But I am interested in the data closing in on a 48 inches of rain year in Seattle. No wonder we have so much snow in the mountains, what with an abundance of moisture in the atmosphere, plenty of lift, and just enough coolness in the lower levels to turn the water vapor to crystals.

    Upper Rainier, Baker, Glacier, Adams, and even St Helens must be awash in snow. I wouldn't be surprised to see the retreat of their glaciers arrested for a few years with the accumulation of ice they must have seen the last several winters.

    Alas, it won't last. But it's fun as a break in the march of warming.

  4. Snow was NOT predicted tonight yet here in Eatonville we are getting dumped on!!!! So, does that mean since it is predicted for Christmas day that we wont get any? Mother nature is soooooo fickel!

  5. Hi Cliff,

    I'd like to point out that the NWS has a "special weather statement" out rather than a "special weather station" ;)


  6. What about snow on the Sammamish plateau?

  7. My location is going to be blessed with a wonderful white christmas. The upper valleys of E. Washington has snow but most of the lower basin is brown.

    But Cliff, where has been the usual artic air?!? Not one intrusion of cold air from the north, yet. So without some assistance from our neighbors to the north, it will never snow much on the westside. So fortunate though, Yakima is running 8 degrees above average for the month and we still managed a great snowcover.

  8. Darn. I was hoping I would be the first to point out the "Cliff-mas" miracle to you:

    Also think I'll mention that I've been following you for about 4 1/2 years and on Dec 4, 2008, my now wife Kim and I planned our first (blind/eharmony) date going to hear you talk on campus that evening, but it was sold out, so we went to see an Arctic Birds show at the Burke Museum instead. She's a UW wildlife science grad, and we love your blog - following it is a fun cornerstone of our marriage, but no pressure:)

    Keep on keepin on tho! - Chris Chisholm, Wolf Camp and the Wolf College, Puyallup WA


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