Snowy Washington State

The snow event yesterday had major impacts (generally negative) for transportation, but major benefits for water resources and late-season skiing.

It provides a highly educational lesson about snow and freezing levels.

As I write this Saturday morning, both Snoqualmie and Stevens Passes have reopened after being closed much of yesterday.   There have been major snow dumps in the mountains, with Snoqualmie getting 42.5 inches and Stevens receiving 34.5 inches on Thursday and Friday.

A big problem for the ski areas has been the major power outages, which are now resolved at Snoqualmie.

To get an idea of how the regional snow water situation has changed, here is the percentage of normal (1991-2020) of snow water equivalent of the regional snowpack.  A month ago, there were significant areas of red (less than 50% of normal) over the northern portion of the state.

Now the red is gone. Western Washington has gone from 54% of normal to 66%, and the Yakima Basin from 41% to 63%.    There has been a large and significant improvement in the amount of water stored in our regional snowpack.

This snowpack improvement, coupled with our above-normal reservoir levels, bodes well for water supplied this summer.  For example, the Yakima Basin storage is hugely ahead of schedule and will easily fill (see levels below, blue is the current year, red is normal).

Yesterday's lowland snow over western Washington was fascinating.    Before I discuss it, I want to remind you of some snow science 101:  the difference between freezing level and snow level (see below)

Nearly all of our precipitation starts as snow aloft.  It falls until it hits the freezing (or melting) level at which the temperature is 32F.  It takes about 1000 ft for all the snow to melt below the freezing level.  The elevation of total melting is called the snow level, below which the precipitation is all rain.

Between the freezing level and snow level, there is a mixture of melting snow and rain.   On Friday, much of western Washington was in this layer, with a lot of wet snow falling.

Snow that melted rapidly on the relatively warm roadways.

During Friday (and this morning), the freezing level has ranged from 1200 ft to 300 ft, which resulted in a lot of mixed precipitation reaching the surface.    If you were lucky enough to live above 1000 ft in the hills above Bellevue, you enjoyed over a foot of snow.

The rapid melting of snow between roughly 500 and sea level was evident if you looked at some of our hills:  very white on the top, with little snow at the bottom (see some images below).

Just for fun, I drove from near sea level to around 350 feet in north Seattle on Friday afternoon.

Near sea level, there was no snow on the grass and a few flakes mixed in with some rain.

A different story at 350 ft.   Grass and roofs were covered with snow, but most of the snow melted on the warm pavement.

Elevation was not only a key element.  Where the precipitation was heavier, the greater amount of snow falling from aloft caused more cooling (it takes energy to melt snow), which drove the snow level lower, resulting in more snow at the surface.   

There was a band of heavier precipitation over the South Sound that produced a band of heavier snow (see below, showing snow depth measurements on Friday).  As much as 4 inches in Federal Way!    Not very impressive by mountain standards.😉