November 28, 2022

It Will Snow over Central/Northern Puget Sound But It Won't Last Long

I will have an update at noon....  Some light snow showers are over western WA right now....

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The most powerful high-resolution meteorological tools are now available for the snow event tomorrow night.   Parts of the lowlands will get snow--up to several inches in places.

But it won't last.

And the situation tomorrow is not a good one for accumulating lowland snow in our region.

The Golden Rule of Snow

Snowfall is NOT snow depth.  

This is going to be very important for the upcoming event.   And this subtlety is often missing from the analyses of some amateur weather websites.

A key issue is the temperature of the ground and particularly roadway surfaces.  Below are this morning's values from the city of Seattle's Snowwatch website, with the temperatures in the boxes showing the roadway surface temperature.   Most are in the upper 30s to near 40F.


The air above us right now has a freezing level of about 1300 ft (see below from SnowWatch below for SeaTac Airport).  Snow melts below that level,  taking about 1000 ft to reach the snow level, beneath which it is all rain.  So the current snow level is about 300 ft.

 
With showers moving through this morning, some of the higher hills in the region got some snow (see below).  And the Cascade passes have been hit hard, with chains required at Stevens Pass right now.

Around 1000 ft ASL in Bellevue.  Picture courtesy of Dr. Peter Benda

Tomorrow's Snow Event

    As noted in my previous blog, a front will approach our region tomorrow afternoon and evening, bringing precipitation.  Cool air will be in place.... just on the edge for being cool enough to allow snow to reach sea level.

But there is something else that will come into play.  Cooling by melting and evaporation.

As precipitation falls into drier air below, it can cool by evaporation, helping to drive the snow level to the surface.   Snow falling into above-freezing air also melts...and that takes energy.   The result is even more cooling, with the amount of cooling dependent on precipitation intensity.

On the other side, warm ocean air will be moving in behind the front!

Now that you are all trained up....let me show you some graphics I rarely present on this blog: forecast precipitation maps that show whether the precipitation reaching the surface is rain (black and gray shades) or snow (colors).

And to make it even better, these forecasts are from the highest-resolution UW system (1.3 km grid spacing).

First, here is the forecast for the 3-h total precipitation ending at 4 PM Tuesday.  Snow around the Olympics Peninsula, but heavy rain offshore and light rain over the interior.


For the 3 hr ending 10 PM Tuesday, the rain (and strong southerly winds have reached the coast), while snow intensifies over the Olympics and the Kitsap Peninsula, where southeasterly upslope flow intensifies the precipitation.  Some light snow from Seattle downtown northward.


The next three hours (ending at 1 AM) are the center of the snow fun for northern Puget Sound (see below).  The higher elevations from Lynnwood to Everett could get several inches of snowfall (NOT SNOW DEPTH).

But note the rain (black colors and gray) rapidly moving in from the south.


Depressingly,  the same graphic ending 7 AM shows rain sweeping in over the lowlands, but snow continuing at higher elevations.


The 24-h snowfall total ending 4 AM Wednesday shows the snow north of Seattle and Kitsap.


The snow depths (in metric units...sorry) at 4 AM Wednesday are far less impressive, with only limited lowland areas getting to .1 meter (3.7 inches)....and that would be rapidly melting at the time.


I told my class that I was pretty certain Wednesday would not be a snow day at the UW.  A very good event for skiers though....


15 comments:

  1. With this persistent northeast wind I'd bet north of Bellingham gets hammered by snow

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  2. Interestingly, the latest ECMWF ensembles have anywhere between 1.5 and 9 inches of snow depth by midday Thursday in Bellingham, with most going for around 6 inches.

    I can tell you the cold Frasier outflow if whipping through Whatcom County right now and it certainly feels like we will get some kind of snowstorm.

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    Replies
    1. I agree with you. It's 28 here now in Sudden Valley and I tend to always expect us to get snow as we usually do. Cliff mostly talks about Puget sound area. :)

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  3. North wind the last few hours with very light intermittent snow above the canal at 500'.
    Temps dropping now and 33⁰, with clouds around also. Thinking it'll be a snowy afternoon and evening tomorrow.

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  4. Rain is better than snow. Especially for KC Metro Users and Drivers

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  5. If anything models will back off even more tonight on the snow amounts, as they often do once we get closer to the event.

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  6. Dewpoint is rapidly falling- now down to 20 degrees in Mill Creek. I shiver for my grapefruit tree. La Nina, go home!

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  7. I've got a -9 here in Kelowna right now. Some lake effect snow. Gonna get hairy tomorrow

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  8. 26.1 degrees at Arlington Airport at 9:30PM and dropping. I hope this holds

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  9. People in Portland are always Jonesing for snow, but seem to forget that less than one inch paralyzes the city. Now that there's a shortage of snow plow drivers, ODOT has warned that people should expect severe delays for any snowfalls this winter. Awesome.

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  10. Maybe today will be a snow day at UW...

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  11. Probably not going to last, but just looked outside of my office window here at home and saw it's snowing here in Tacoma! Yes, at 7:34AM Tuesday. The snow is not covering the streets, but is dusting the cars, the grass and a bit of the sidewalks along with rooftops. Quite pretty if you ask me, but I suspect it'll likely not last as the ground is still too warm, and it's also early in the season still.

    I think we will see a dusting here in T-Town but be cold and it'll likely be gone by end of day perhaps.

    Meanwhile, it's pretty while it's coming down as the day is still very young. As Cliff has mentioned, we would need at least an atmospheric river from the SW overriding the cold front from up north from the Frazer River, and perhaps spillover from Eastern Washington east to get snow, which is if I recall, was what happened in the big snow of 1985, which landlocked in place for a while, and many areas of the sound got upwards of a foot, or more of the white stuff before it cleared out, dripping down the dry, but very cold air from up north to allow the snow to hang around for about a week before moderating.

    I remember that snow as the morning it began, I was driving to school out at what was then known as Ft Steilacoom Community College, now Pierce College and when I left around 11AM, there were already several inches on the ground and it was still coming down good, with near whiteout conditions at times coming home. At that time, my parents and I were living in the NW part of Tacoma in the Skyline subdivision.

    Currently, the snow is still coming down, but it is now finer flakes than the larger ones I first saw, but may be picking up a little again. Even the weather app that comes with Windows 10 is saying snow.

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  12. Snowing in East Bremerton at 300 feet. 32°

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  13. NWS has issued a high wind warning bullseye centered NW of Anacortes in Padilla Bay 30-40 mph gusts 55 mph expected. What produces this centering effect so often just out of Admiralty inlet. Any future blog post appreciated.

    ..HIGH WIND WARNING REMAINS IN EFFECT FROM 4 PM THIS AFTERNOON
    TO 7 AM PST WEDNESDAY...

    * WHAT...Southeast winds 30 to 40 mph with gusts up to 55 mph
    expected.

    * WHERE...San Juan County, Western Whatcom County, Western
    Skagit County and Admiralty Inlet Area.

    * WHEN...From 4 PM this afternoon to 7 AM PST Wednesday.

    * IMPACTS...Damaging winds will blow down trees and power lines.
    Widespread power outages are expected. Travel will be
    difficult, especially for high profile vehicles.

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  14. There was over an inch on my driveway in Haller Lake when I checked last night around 11PM. Snow was falling hard, and we heard/saw more than one large tree branch break off into the street.

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