Weather Blog

Armchair snow forecasting: Tuesday edition

Armchair snow forecasting: Tuesday edition
Note: An Updated Version has been posted. You can find the latest forecast at the main weather blog site.

With the weather becoming a larger and larger headline as some arctic air looms this weekend, I figure I'll devote the blog this week to giving daily forecast updates, and allow you the reader to put in your own forecasting notes and questions using the commenting features below. You can also e-mail me directly anytime at scotts@komonews.com and I can post the Q and A in the commenting field.

In case you haven't heard, or just popped back into town from some place like Arizona where 60 degrees is cold for December, we finally have our first cold weather snap looming -- and if the general theme of the long range forecasting models are correct, this could be one of the longest cold snaps we've had in a few years.

The fun appears to begin Friday. A big ridge of high pressure out in the Pacific Ocean is expected to drift to the west. That will push the jet stream way up north into Alaska, then down the backside of the ridge along the B.C. coast and into the Pacific Northwest. (Normally, that ridge is farther east, which shoves the arctic air farther east into the Rockies and Midwest).

That shift will allow a storm system to drop out of Alaska and the Yukon and move south along the B.C. coast and into Washington. From here, let's break it into time line -- something that will make it easier to edit as we go forward and the forecast inevitably shifts.

Friday Morning through Afternoon

This will be largely a mountain snow event, with rain in the lowlands. Expect rain to begin around sunrise and it'll be a cool, raw, rainy, blustery day. Temperatures are still expected to be in the 40s during the day, so this isn't a lowland snow concern just yet, although the mountains could get a foot of snow or so. We'll also have to watch the track of the low as it might be a little windy here too, especially north, although nothing that would make wind the headline.

Friday Evening and Night

As the storm passes, the cold Yukon air will rush in behind. This will be close as to whether enough cold air gets here to make it snow in the lowlands while there is still some residual moisture left from the storm. I'd say at this point, snow levels will probably only make it down to about 500-700 feet. However, there are a few events in play that could make for snow down to the surface in a few spots.

First and foremost is a potential Convergence Zone Friday evening and through the night. This could be enough of a trigger to bring snow to the ground in the North Seattle to Everett corridor and points east (especially since a good chunk of that area is 300-500 feet). Other places that could see some sea level-ish snow would be the Olympic Peninsula south of Highway 101, where you get some upsloping off the Olympics. This will all depend on the wind pattern in the wake of the storm and the storm's track -- whether we'll get a northwest, north, or northeast wind. So this is a heads up.

Again, I think the main Seattle area would still be rain at this point as general snow levels will be over 500 feet without other help to make it lower, but we'll have to watch any Convergence Zones. Lows will be generally in the mid 30s, with a few spots near low 30s.

Saturday

Cooler air keeps pushing into the region, likely now dropping the snow levels near sea level. Only problem is, we're running out of moisture as that storm pushes away and takes its snow-generating toys with it. Still, I'd expect a few showers around Saturday, and they could be snow or a mix, with snow a better chance with a bit of elevation. Nothing looks too heavy and I'd only expect light accumulations.

One wild card in this scenario is a secondary trough of low pressure dropping by. Right now, forecast models have it going by off the coast to our west -- too far west to bring much of any additional moisture, but if it ends up coming through closer, it could bring a greater chance of snow showers, or in addition, could help make the air unstable and we could get some "pop-up" snow showers, akin to those pop-up rain showers we get in the spring. High temperatures are only expected in the mid 30s and breezy north winds will make it feel colder.

Saturday Night -- Clearing Out, Gettin' Cold

As skies clear Saturday night and cold air keeps pouring in, lows will drop well below freezing -- with mid 20s in the Seattle area and teens expected in the outlying areas.

Sunday -- Here Comes The Really Cold

On Sunday, we're sunny, but so much for the sun's warmth. What we call an "arctic high" will be dropping out of the Yukon and settling into British Columbia. This will bring a re-enforcing shot of cold air to the region, courtesy of the Fraser River Valley.

The Canadian Cascades will block most of the arctic air from just spilling into Western Washington, but there is a hole in the dam -- the Fraser River Valley, which extends from central B.C. into Whatcom County. This "leak" will allow that arctic air to flow into Western Washington. Those that live at the mouth of the valley -- Bellingham, Lynden, Ferndale, Sumas -- know this ritual well. It's like standing at the end of an icy blow drier.

So for Sunday, expect sunny skies, but very cold weather. For most of Western Washington, highs will only be in the low 30s with a stiff north breeze of 15-30 mph (especially in the Puget Sound area) making wind chills even lower. But up in Whatcom County, I expect winds to be stronger -- not sure how strong yet, but probably 30-40 mph as that arctic air races out of the Fraser. Highs there will likely stay in the 20s, and wind chills will be much lower.

Sunday Night -- Cue Jack Frost

With the cold air in place and mostly clear skies, this could be the coldest night we've had in a while -- I haven't looked but I'll bet it might be colder than anything we saw last year. Lows in Seattle will likely drop to the low 20s, with teens widespread outside the city and perhaps even low-mid teens in the really cold spots like Olympia and in Bellingham.

Next Week

The long range models keep the same cold pattern as Sunday into Monday, with highs Monday from 28-32 and lows 15-22. After that, details have diverged enough that it's hard to give any specifics. But overall, the general theme is to keep us locked in this cold pattern for several days as the large scale weather pattern remains in place. Right now, it appears it could remain colder than normal, with days much colder than normal, through Christmas.

As for any lowland snow events beyond this weekend, you can see it appears we'll have the right set up for it, we just need some moisture. The general look to the pattern is pretty dry while we're cold. There are have been a few model runs that show some potential snow mid to late next week, but it's been so inconsistent that it's hard to peg any confidence so in that sense, stay tuned.

And bundle up! I'll keep this blog updated as we go.