North Puget Sound Lowland Snow from the Other Type of Convergence Zone

 The first significant lowland snow over western Washington occurred last night, ranging from a trace to about an inch.

How it happened is quite interesting....let me explain. 

The snow started falling late yesterday evening and was associated with a narrow convergence band that was evident in the weather radar (see below).  The band stretched northwest to southeast and started near Sequim.

The band brought heavy precipitation in places and extended into the Cascades.

Why Sequim?  The driest location in western Washington!

As we will see, this precipitation band was associated with a convergence zone, in which low-level air streams came together, forcing upward motion and precipitation.

But it was different from the "classic" Puget Sound convergence zone we often talk about.

The figure below shows a schematic of the typical Puget Sound convergence zone. Northwesterly winds from off the Pacific are forced around the Olympics and then converge over Puget Sound, producing upward motion, clouds, and precipitation.   

Most of the time, this is the convergence zone we are talking about.

But there is another way to get low-level convergence over the western interior:   air moving southward down the Strait of Georgia can converge with air moving eastward in the Strait of Juan de Fuca, producing a band of clouds and precipitation.

Something like this occurred last night.  A strong, sharp upper trough moved through, causing the northerly winds in the Strait of Georgia to be particularly strong.   That air converged with air moving eastward down the Strait to create a low-level convergence zone that stretched from Sequim to north Everett.

Want the proof?  Here are the winds late last evening.  You can see the two air streams coming together.

The high-resolution UW WRF model predicted this situation (see the model forecast winds for yesterday evening below).  Shading is wind speed (in knots), and wind vectors are plotted.

You can see the converging airflows.

 

The precipitation band was narrow and well defined, as shown by the 24-h precipitation totals (below), with some locations in Snohomish County receiving over an inch of liquid precipitation.

The heaviest precipitation was associated with light snow (see totals below).

Why such modest snow?  

Because temperatures were very marginal (too warm) for snow.  Only in places with a lot of precipitation (and thus a lot of cooling from melting and evaporation) could the snow level be driven down to the surface.  Initially, precipitation fell as rain and then gradually evolved into snow and snow pellets.