Sunday, 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...

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