December 09, 2023

First Lowland Snow over Western Washington

It is snowing right now in some favored locations in western Washington....yes, snow falling to sea level.

Take a look at a video provided by Greg Johnson of Skunk Bay Weather, whose cam is located on the northern Kitsap Peninsula.  I warn you, you will be looking for some hot chocolate before you are finished.


Or consider the Bremerton Airport, which at 1:30 PM was completed snowbound!


The lowland snow is mainly limited to the area southeast of the Olympic Mountains--mainly the Kitsap County area--and has been forecast for days.    Below is the UW model forecast from yesterday afternoon for the 3-h snowfall ending 1-PM.   Not bad at all.


So why snow southeast of the Olympics, something that happens all the time?

It has to do with precipitation intensity and cooling by melting snow.

This morning was cool throughout the region, but a bit too warm for snow to reach sea level.  Temperatures at 8 AM were in the mid-30s to lower 40s around the region and folks were not thinking of snow (see 8 AM temps below).


But by 1 PM the situation had changed substantially in a very interesting way:  temperatures had COOLED over the Kitsap Peninsula and southeast of the Olympics but remained the same or warmed in most other locations (see below).

Why would temperatures COOL during the daytime?  No cold front was moving in.  No cold air was entering through the passes!


The answer has to do with melting snow and it can cool the air and bring the freezing level down.

Pretty much all serious winter precipitation around here starts as snow aloft, snow that melts in the lower atmosphere near the surface.

A warm front is moving in right now and in front of the front, there is not only precipitation but strong southeasterly flow (flow from the southeast).    To demonstrate this to you, here is the forecast for 7 AM this morning showing you low-level winds, sea-level pressure, and low-level temperatures (shading, blue is cold, yellow is warm).  I indicated the rough location of the warm front with a red line.


Ahead (east) of the front the winds were southeasterly (from the SE) and as the front approached the Olympics the air was forced upward by the Olympics...with this upslope extending tens of miles upstream of the barrier (over Kitsap County).

More upslope means more precipitation, which is produced by upward motion.  More precipitation means more snow falling into the warmer air below.  The heat needed to melt the snow causes cooling and the freezing and snow levels plunge towards the surface.   The lower atmosphere cools and eventually the snow level reaches the surface.

That is why it is snowing on Kitsap and not anywhere else.  Other locations cooled aloft, but not enough to get snow to the surface. Again, because the precipitation intensity was not strong enough.

I can show you the cooling aloft from the falling snow by using the aircraft observations at SeaTac airport, which is not that far away from the snowy action.  Time is on the x-axis and height is given in pressure (hPa or mb) on the yY-axis (sorry). 850 hPa is about 5000 ft.

 The freezing level dropped from around 5000 ft at 9 AM to less than half that at noon as the precipitation came in (blue arrows show you the change).  

Anyway, the warm front will soon move in, but until it does, some flakes will be falling southeast of the Olympics.











5 comments:

  1. The models for snowfall were close to accurate over Kitsap, but I think they missed on the evaporative cooling. My WeatherUnderground forecast near Suquamish was 40F/39F. It was 39F around noon, dropped to 33F during the flurries, and is back up to 36F 3 hours later.

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  2. "The heat ... causes cooling ..." Great stuff!

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  3. In line with the previous post, 12/09 was an exceptionally dark day around Bellingham. Measurable daylight for my solar sensor began at 8:39AM and ended at 3:38PM. During that time the maximum irradiance value I measured was just 19W/m^2 between 2:20PM and 2:22PM, a little more than 7% of normal for this time of year and the lowest such value in my record. The total solar energy for the day was a mere 0.26MJ/m^2, a bit over 6% of normal.

    The only other day in my record which was similarly devoid of sunlight was 12/19/21 which was an essentially infinitesimal 0.65% darker than 12/9/23 with regard to total solar energy.

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  4. Also regarding the previous post:

    Cliff, I'm in Bellingham and the most daily solar energy I've measured at my location is ~26MJ/m^2. Is the difference in these values between my location and the Seattle AgWeather site attributable to the difference in latitude all else equal? Or could there be some kind of measurement error going on?

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  5. Love learning about those dynamics - keep it coming. Glacier ("last town before Mt Baker") is squarely in the upslope zone, and your descriptions of cause and effect are first rate. I think that the daily CoCoRaHS maps are extremely revealing; day-to-day they illustrate how terrain affects the atmospheric flows (and v versa) from the Pacific ...well, "incoming" from every direction. The fronts we experience are self-evident given the precip data (both rain and snow). Weather watchers: If you're not familiar with the CoCoRaHS maps, you might want to add the link below to your bookmarks. There's a constant wealth of observation data here:

    https://maps.cocorahs.org/

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