Well, that snow event from Wednesday was surely a great example of just how variable the weather can be in Western Washington, and why it is so difficult to forecast snow around here.
The Convergence Zone that formed was fairly narrow -- just a few miles across from north to south. And it wasn't a gradual transition from snow-covered streets to bare pavement.
My wife commutes from Mukilteo to Edmonds and reported that snow and slush covered the roadway on Highway 99 from the SR-525 interchange to about 164th Ave. SW in Lynnwood. In fact, 164th reported several snow-related problems with spun out cars and buses.
However, conditions rapidly improved as you went south, and by 196th Ave SW the roads were completely bare and you'd never know it snowed -- just two miles away. And of course, as you headed into Seattle, there was nary a flake to be found at all on Wednesday.
So with that in mind, here are some reported snowfall totals from around the region:
- Duvall: 4.5"
- Bonney Lake: 3.0"
- Mill Creek: 2.7"
- Everett (Silver Lake): 2.5"
- Enumclaw: 2.5"
- Puyallup: 2.5"
- Mukilteo: 2.0"
- Eatonville: 2.0"
- Edgewood: 1.5"
- Friday Harbor: 1.5"
- Canyon Park: 1.0"
- Mount Vernon: 1.0"
It's not uncommon to have such a stark edge to a Convergence Zone. That's because the mechanism that creates the zone also causes drier weather on either side of the zone.
The zone is formed by colliding winds at the surface. As those winds rise, they condense and form into clouds and rain/snow/thunder/whatnot. But once those winds reach the upper levels, they diverge and sink back down toward the surface on either side -- think of a large fountain and how the water falls back down to the ground on the edges after being shot into the air.
But sinking air dries as it falls, and thus there is a typical dry slot on either side of the Convergence Zone -- even some clearing sometimes. Thus, the much nicer weather over Seattle today even with the raging snow in Lynnwood.
Want to see more on the Convergence Zone? Take a look at this time lapse video taken from the Univ. of Washington Atmospheric Sciences building and watch how the winds in the morning were out of the south (camera faces west) but then switches to the north as the zone nears and passes by:
And here is a radar loop that shows the Convergence Zone forming and snaking around the North Sound.