Another round of snow was moving into the Seattle area Monday evening, threatening to snarl the Monday evening commute.
Another 2-5 inches of snow are possible in the greater Puget Sound area through Monday evening, but snow totals will be highly variable.
Winter Storm Warnings remain in effect for a wide area of Western Washington, including the greater Seattle Metro area until 10 p.m. Monday as a storm moves into the region combined with arctic air already entrenched.
Travelers should expect low visibility and potentially slick roads for the afternoon and evening commute. The snow is expected to fall in spurts through around 10 p.m. then taper off toward midnight as the storm moves away.
In addition, winds are expected to increase into the afternoon, gusting as high as 20-30 mph, making for low visibilities.
Multiple weather events in play this evening
A very complex weather situation is setting up as a very cold area of low pressure slides through Western Washington, with several major factors contributing to snow in the Seattle Metro area.
First up, a Convergence Zone is setting up shop right over the Seattle metro area. This is due to strong north winds from the Fraser Valley colliding with southerly winds being brought up by the passing storm. As the winds collide, it causes the air to rise and condense into clouds and, in this case, snow. Convergence Zones can be great snow-makers and the Seattle area needs to be on guard as we could accumulate 2-6" by the evening or more.
Second, the storm's center is bringing its own moisture along for the ride, so expect enhanced snowfall for other areas outside of the Seattle Metro through the evening through about 5-6 p.m. These cells could bring 1-3".
Third, cold winds continue to blast out of the Fraser River Valley, gusting as high as 50 mph and could strengthen to gusts of 60 mph this evening. This winds are running across the Salish Sea and into the eastern Strait of Juan de Fuca where they are running into the Olympic Mountains. This is the inverse rain shadow, per se, and this rising air is squeezing out all sorts of moisture along the northern Olympic Peninsula, where snow totals from Joyce to Port Townsend are reaching 6-12" and more is falling.
Fourth, these strong winds will continue to filter south into the Puget Sound region later, meaning we'll add gusty winds to the equation as high as 20-30 mph this evening, making for blowing snow a possiblity.
Fifth -- and this too is important -- a very cold, unstable airmass is moving in behind this storm. If you thought it was chilly outside now, there is even colder air behind it. In addition, this storm briefly pulled up a little warm air from the south -- thus the hail at Sea-Tac Airport earlier today and temperatures in Tacoma briefly warming into the mid 30s. But this "warm" air has added some fuel to the system and could cause stronger development of these individual cells.
Thus as we get later into the evening, the steady snows may taper off, but we'll see these roaming snow showers and they could be quite intense, with thundersnow a possibility. This would be more in the 7-10 p.m. time frame. These cells will be random in nature and fairly compact, geographically, but where they strike, they could put down a few inches in a very short amount of time with whiteout conditions (plus the thundersnow). But a few miles away, it might be nothing. So all areas need to be prepared for intense snow showers through the late evening.
Temperatures will be below freezing through the event, with temperatures dropping as we head further toward midnight with lows gradually dropping to near 20.
When does the snow end?
Everything should taper off by 10 p.m. to midnight, with some individual snow showers/thundershowers possible through the night.