Snow could fall in Western Washington Monday

Sun. Dec. 20, 7 p.m. -- A fast-moving and very dynamic weather system will bring the potential of some wet snow across a good part of Western Washington on Monday, particularly from late morning through early evening. Some mainly-minor accumulations are possible, mainly above 300 feet in elevation away from water on grassy or elevated surfaces, though there are some uncertainties we’ll face going into Monday.

The weather setup is pretty clear: a quickly-moving low pressure system moves from west to east across Western Washington Monday morning with plenty of rain and warm southerly wind out ahead of it. That sure isn’t the recipe for snow! However, as this low moves away to the east, the counter-clockwise circulation of air around the low will bring down a colder north wind out of British Columbia. Snow can occur when that happens -- if there’s moisture left to produce it.

There are a few questions meteorologists face: How much of the moisture will even be left for snow when the colder air from Canada surges south? Is that Canadian air even cold enough for snow? Will there be any “convergence zone” across the central Sound (including near Seattle) for heavier snow? Will the warm ground melt all the snow on contact or could it stick and become slushy for a time?

Here are some thoughts by area as of Sunday evening, but as this is a very dynamic weather system with temperatures only marginally cold enough for lowland snow, these details are likely to change some on Monday as the event evolves.

WILL SNOW FALL? Unless you’re at the coast, you have a pretty good shot to see some snow falling on Monday, even if it’s mixed with rain. However, getting snow accumulation that sticks around for any length of time or causes travel problems will probably be a pretty isolated event.

PORT ANGELES AREA: It appears the best chance for significant snow affecting population centers Monday will be along the northern Olympic Peninsula. Some forecast models are suggesting more than six inches of snow could fall near Port Angeles. This is possible, though the warmth of the surface might well melt a good amount of that snow. However, these events where a “surge” of cold air blasts out of Canada can produce localized heavy snow nearer the mountains just south of the Strait and even in town. While no winter weather advisories or warnings are in effect as of this writing, folks around Sequim and Port Angeles -- anyone along Highway 101 in northern Clallam County from about Lake Crescent east to Port Townsend -- should prepare just in case for snowy roads Monday with impacts possibly lasting into Tuesday if heavy snow falls.

EVERETT NORTH TO BORDER: Locations above 300 feet east of the water have the best chance of a slushy 1-3 inches of brief accumulation on Monday morning through mid-afternoon Monday as the cold air surges south. Elsewhere, the window of opportunity for snow looks rather brief as the moisture will be drying up by late in the day. In many cases, grassy areas might stay white for a while if and where snow falls, but the roads just stay wet or have a manageable amount of slush for a time. The chance for any accumulation on the islands looks low.

I-90 CORRIDOR INCLUDING SEATTLE-BREMERTON-BELLEVUE: This forecast could well be trickiest of all. As the cold air moves south, we could have a southward-moving band of heavier snow or rain/snow mix that gets “stuck” in the central Sound as it battles with winds moving north up the Sound. This “convergence zone” could produce some spots of heavier snow with a few inches of accumulation, mainly in spots above 500 feet in elevation away from bodies of water. Though if precipitation is quite heavy, that can quickly “drag” the snow level down to sea level. This is just something we’ll have to monitor on radar as it unfolds. Given the warm ground, there’s not likely to be any significant widespread travel impacts nearer Puget Sound unless the precipitation rate is very heavy. However, if a burst of heavy snow occurs, things could get slushy and slippery for a time on roads, with rather rapid melting after snow ends. We’ll be watching the densely-populated central Sound closely Monday afternoon and evening.

SOUTH SOUND: It’s looking less likely to get any snow accumulation in the South Sound except above 500 feet away from the water as the cold air will be the latest to arrive here and moisture will quickly be on the way out. However, in the unlikely event the cold air is faster to move south that expected or moisture holds on a little longer into late Monday afternoon or Monday evening, there could be some minor accumulations, especially on grassy surfaces for a time.

As mentioned before, the ground is warm so there will be continued melting, and even despite the “colder air” moving in, it’s not going to be getting to freezing or much below in most lowland areas Monday all the way into Tuesday. Melting will continue just about nonstop even if there are some accumulations for a time.

Above 1,000 feet elevation and into the Cascade passes, there will be more snow with 2-6 inches of snow in spots that could stick around through Monday night.

Obviously, topography and small-scale features can cause changes to the eventual outcome. Such is snow forecasting around Puget Sound! It appears as of this writing Sunday night that the vast majority of folks who see snow falling from the sky on Monday won’t have any roadway impacts and there won’t be refreezing into Tuesday morning. That’s good news from a travel standpoint.

So while it might really look like a snow globe outside for a time Monday, whatever falls won’t be sticking around very long.

Cox Media Group