4th winter storm now creeping across Puget Sound, slushy mess possible Monday

Shoveling snow in Bonney Lake (KOMO Photo)

This story is now archived. We're updating a new web winter storm story at this link>> https://komonews.com/news/local/more-snow-you-bet-snowstorm-no-4-rolls-into-western-washington

SEATTLE -- Seattle meteorologists have apparently met their match, as an incredibly complex winter storm situation is setting up to perhaps bring yet another blanket of snow to the region -- or maybe not. But one thing is for certain: Forecasters' hair may get a new blanket of gray.

Fresh off Sunday's evening's "What? We've got a *third* dose of snow?!?" that brought 2-5" of snow to the region, all eyes are wearily looking at Monday's fourth storm which has potential to bring heavy snows to a large swath of the region, but where and how much? Still a challenge even with the storm just a few hours away.

A mix-mash of Winter Storm Warnings and Snow Advisories were in effect across the region, dependent on where you were. But generally speaking, if you're in or north of Snohomish County, you've got a good chance of heavy snow Monday afternoon into Tuesday morning; King County and points south will see snow change to rain, with a quicker changeover the farther south you are, meaning less snow totals there.

Monday morning is a break -- the commute will be snow-free; just dealing with any lingering issues from Sunday's snow which left roads quite icy. The break is short lived as a much more potent storm -- perhaps the strongest one of the bunch so far -- is working its way into Western Washington.

MORE | School Closures | Seattle Snow Plow Map | Snohomish Co Snow Plow Map | Winter Storm Real Time Traffic Guide

But since it will have spent quite a bit of time over the Pacific Ocean, this storm 1) Has more moisture and 2) Has tapped into some warmer air. But there's all this (obviously) cold air in place and it's (obviously) cold enough to snow here now.

Relatively warmish storm, coldish air. The battle is on! The track of the storm will be ultra-critical to how the storm plays out, with multiple scenarios in play and perhaps wildly different impacts across the region. Then, add that in to our complex terrain, effects of being near water, mountain shadows, and varying elevation.

In fact, there are so many variables in play it just might be more complex than our calculations can adequately handle to get an accurate forecast ahead of time, meaning meteorologists will have to forecast in real-time as the storm unfolds. Unfortunately, it's made worse in that the uncertainty could mean a range of heavy snow, incredibly heavy snow, Snow to rain, or just a heavy rain day. And of course, 4 million people live in that zone of uncertainty. My hair is already turning gray(er) just writing this story! The good news is it appears freezing rain is less likely now as we look at the current atmospheric setup Monday morning.

Unlike the past two storms that have tracked offshore and to our south, keeping us on the cold, north wind side of the systems and everyone with snow, this storm is coming in like a more traditional rain storm track from the west off the Pacific. Being on the north side of the storm keeps you still on the snow-side as it keeps the winds from the chilly north. Being on the south side is the warmer side as you get milder marine winds from the southwest.

Thus, depending on where exactly this storm comes in, some areas -- perhaps many areas -- will see a transition to rain at some point. Heavy snow-to-slush-fest on several inches of snow? Eww.

The farther north you are, the more likely the snow falls longer and heavier before any changeover. Conversely, the farther south you are, the more likely of this transition to freezing rain and maybe even rain. Where does that transition occur? That is the million-dollar question.

Right now the northern extent of where the snow will likely change over to rain Monday evening/night is right over the heart of King County to maybe the southern edge of Snohomish County -- we're thinking Shoreline or Mountlake Terrace.

If we've nailed that line correctly, areas from Snohomish County north will likely see 4-8 inches of new snow, starting around midday Monday with heaviest snow in the evening into the night. There may be a late change to rain from south to north Tuesday morning but too late to stop the snowstorm. Winter Storm Warnings are in effect there.

From Pierce County southward, you'll see a few hours of snow with a change to rain. Obviously how long it takes to change over will affect how much snow you get, but 1-4" is the leading scenario before changing to rain in the afternoon or evening. Note that the snow will be heavy before the changeover so there could be snowy impacts to travel for a few hours.

In Seattle and King County, it's a taste of both -- more likely to get some snow up to 2-4" -- including Seattle, then changing over to rain in the late evening -- the National Weather Service is pegging around 7 p.m. for the changeover. That means a potentially snowy Monday evening commute then slush-mageddon after. But that should help relieve issues for Tuesday morning.

Now the Caveat...

So here's the deal: It's, what, 60 miles between Everett and Tacoma where 4 million people live? 60 miles is minuscule in terms of forecast model errors when you're talking tracking a storm that started in the Aleutian Islands. But here 60 miles makes all the difference. If there is a slight wobble in the storm track north or south, this forecast goes haywire. A wobble to the north, and the snow totals are way overestimated and we get a brief snow to slushy rain, even up into the North Sound. More worrisome, if it wobbles to the south, then much more of the metro area will get a heavy snow, not just Snohomish County.

We'll be watching in real time today (follow us on Twitter @ScottSKOMO and @komonews for live updates) but best advice: Plan for heavy snow Monday evening (including commute time) and Monday night with potential for several inches, but there is extremely high bust potential with this storm, especially the farther south you go, where you might get little to no snow at all. :/ It's also possible both will happen, with some areas getting 4-8" of snow while others get rain. Again better snow odds north; better rain odds south. Stinks to have a low confidence in such a potentially high impact storm but such is winter storm forecasting in the Northwest. We lucked out the first three snow storms had a more firm cold pattern to work with.

One place where the forecast IS easier? The mountains. Winter Storm Warnings in effect for 1-2 feet of snow in both the Cascades and Olympics from 4 p.m. Monday through 6 p.m. Tuesday.

So what happens Tuesday? Sunday night forecast model updates are trending toward a scenario where warm will eventually reach north toward the Canadian border Tuesday morning, gradually changing to rain in the North Interior too but by then we're tapering off and the snow damage is done.

We'll see a drying trend Tuesday night into Wednesday. This storm may have one silver lining in that mixing in the warmer air will push us out of the deep freeze pattern into more of a "it's just cold" pattern. More weather systems are still due in toward the end of the week, but now snow levels may occasionally push higher off the sea level floor, giving us more challenging forecasts where elevation and time of day have a say in who gets snow and who gets rain, as opposed to everyone gets snow. Easier on the commutes; harder on the forecasters' accuracy!

While we might start seeing the 40 degree mark toward the end of the week, 45 still remains elusive for the foreseeable future. Normal high is 50?!?

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