January 04, 2022

Warm Front Snow Event for Western Washington

 It might seem ironic....the approach of a warm front can produce substantial snow over the lowlands of western Washington and in our mountains.

And such an event is predicted to occur later tomorrow, with Northwest Washington particularly vulnerable to both snow and freezing rain.  The mountains will also get hit hard with several feet of snow.

Today's Situation

    The freezing level over Puget Sound is now at roughly 1900 ft right (see a plot of temperatures from planes coming in and out of Sea-Tac Airport.  The snow level is about 1000 ft below that (snow melts between the freezing and snow levels, below the snow level only rain is observed)

Where precipitation is heavy, the snow and freezing levels can descend hundreds of feet as a result of melting snow.

A weak system is moving in today, resulting in rain over the lowlands with one exception:  around Bellingham where cool flow out of the Fraser River Valley will develop this afternoon and evening.

The snowfall prediction from the very high-resolution UW weather model for the period ending 4 AM Wednesday is shown below.   Decent snow in the mountains (around 6 inches) and a few inches over NW Washington.  Cold eastern Washington will be all snow.


A Strong Warm Front Approaches

A very well-defined warm front will approach on Wednesday......unusually strong for our region.   Below is the forecast sea level pressure, surface winds, and 800-m temperatures (colors) for Wednesday evening at 7 PM.    The red line indicates the warm front.   The front is associated with low pressure (a trough) and there is a large temperature change on its northern side.  

Cool air (blue) is over the land and you will note a huge pressure difference (gradient) over the Cascades, with higher pressure to the east.  This pressure gradient will force easterly winds in the passes and cool, NE winds out of the Fraser River Valley.

That is going to be important.


The strong warm front will be associated with heavy precipitation that will further cool the air over land by melting, driving the snow level to near the surface.   Snow will be falling everywhere, but the warming will turn the snow into rain rapidly over the western lowlands except for NW Washington.

Below is the forecast snowfall (NOT SNOW DEPTH) for the 24-h encompassing the front passage (ending 4 AM Thursday).

Wow.  The western slopes of the Cascades and Olympics get hit hard. Snow everywhere!  Plenty of snow over eastern Washington where it stays cold.  A few inches over Puget Sound.


Most of this will be late afternoon and overnight in the west and snow will be wet and much will melt quickly on the warm ground.  And as the warm front moves through, overnight temperatures will SOAR into the 40s across the west, except for those poor folks in NW Washington, where 4-6 inches could fall in places. 

To illustrate the temperature changes, below are the predicted temperatures at 10 AM Wednesday.   Cold in eastern Washington and cool in the west (blues are below freezing).  You can see the cool air coming out of the Fraser River Valley across Bellingham and the San Juans.

One day later, temperatures have surged in the west (yellows are in the upper 40s).  Eastern Washington stays cool.  Sections of NW Washington stay cool.



So expect lots of weather action later tomorrow...with heavy precipitation, lots of mountain snow, light snow over the lowlands that melts rapidly.  More snow over NW Washington. Winds will be strong and gusty over NW Washington....and strong winds will hit the coast as the warm front moves through.

Enjoy.




17 comments:

  1. Seems like good dynamics for hood canal and Kitsap, Cliff. Model shows 4-6 inches here as well. The cold air always holds out a bit longer. That’ll go nicely on top of our current snowpack. Not sure why NWS is forgetting us again with no winter storm watch like foothills and whatcom. Maybe a different model shows something less here, but these warm fronts are normally pretty good for hood canal snow

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    1. Yeah the Canal area gets good snow in these events, even last night got 3" to add to the foot from earlier this week. 34 degrees and very light misty mix falling right now after a daytime high of 35. I'm guessing we will have a typical event where it turns to rain everywhere but the snow machine revs up for 6-8" here.

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  2. As is said about PNW weather, "If you don't like it now, just wait."

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  3. How do you define NW Washington?

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    1. Whatcom/san Juan/parts of Skagit county. Correct me if I am wrong but I should know as I live in Whatcom.

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    2. Locations north of the rain shadow according to my sixth grade teacher.

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    3. More like north of convergence zone. But Northernskyi is correct…northern Skagit County, San Juan Islands, and Whatcom County.

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    4. More like north of convergence zone. But Northernskyi is correct…northern Skagit County, San Juan Islands, and Whatcom County.

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  4. Any more snow in the long term forecasts for our actual, normal snow window (late Jan and mid Feb)?

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  5. got 57" on ground in Cle Elum Elev 2100

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  6. I enjoy your blog and wondered if you know of a similar blog for the Sacramento area in California?

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  7. Live at south end of Lake Cushman at 790' Readings this morning at 23" on ground, but more worrisome than a few more inches of more snow is over 3.5" of water equivalent already on the ground/roofs with up to 2" inches of rain coming in. Many "bargain" carports already down. "Snow" loads already pushing structural limits for the older homes.

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  8. Washington has two NWs - Bellingham, and Neah Bay - confusing. There may be a convention to call the second the Olympic Peninsula.

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  9. I live in Yakima and this snow event is hitting the upper levels of Accuweather’s snow accumulation predictions. It also is predicting temps in the upper 30s for this afternoon with rain. I hope the temps stay colder as the temperature map above predicts. Rain on top of 16-18” of snow could be disastrous!

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