An impressive overrunning snow event is on tap for much of western Washington Tuesday, January 17th with bands of snow showers moving inland across northwestern Washington. The (mainly) snow shower activity has increased in coverage as mid level a disturbance rotates around a weak low west of Cape Flattery. In addition to this, more organized (and heavier) shower bands are merging with a Puget Sound Convergence Zone (PSCZ) and are bringing moderate to heavy snow to the region around an Everett to Port Angles line. The PSCZ set up is characterized by flow around the Olympic Mountains (red arrows in image above) through the Strait of Juan de Fuca on the north and the Chehalis Gap on the south.
Associated with the with the mechanism in place forming the PSCZ is a snow/rain shadow downwind of the Olympic Mountains (dotted yellow arrow pointing to orange oval, south of the convergence zone). This area typically gets only a third to half as much precipitation as the rest of the local region.
Forecasts issued Tuesday morning called for about 5 to 10 inches of snow in the Seattle metropolitan area through Wednesday…with 1 -3 inches today and 3 – 7 inches tomorrow. Mostly like the kids are home from school for at least a couple of days.
Our VISIT Orographic Effects Session has more details on this as well as other convergence zones around the country: http://rammb.cira.colostate.edu/training/visit/training_sessions/satellite_interpretation_of_orographic_clouds/