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.
