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....