Arctic Air Arrives: First Mountain Snowstorm, Cold Temperatures, and Heavy Rain

Special Note: This is the 600th blog post on The_Weatherman2 Weather Blog!

 FastCast--Tuesday, Oct. 24 to Friday, Oct. 27:

A major weather change is ahead for the Pacific Northwest. Arctic air will move into the region through this week, with a strong frontal system bringing the first snowstorm of the season to the Cascades and Olympics, plus heavy rain to the lowlands. The frontal system will be followed by a weaker system from late Thursday to early Friday. Then, temperatures will drop significantly, with sub-freezing lows expected this weekend. On Tuesday, expect rain in the lowlands, mainly from mid-morning onward, with 0.5-1.3" in the lowlands, except 1.3-1.75" from Everett northward, including normally rain-shadowed areas like Whidbey Island. Rain will be heavy at times on Tuesday evening. As the system moves through, snow levels will plummet to 2,000 feet. Snoqualmie, Stevens, and White Passes will receive 6-12" of snow, with up to 18" at Stevens Pass. Travel will be most challenging from Tuesday evening to midday Wednesday, when the heaviest snow will be falling. Lingering showers will be scattered around the area on Wednesday, and another chance of light rain/light mountain snow exists from late Thursday to Friday morning. Temperatures will be noticeably colder. Through Friday, expect lowland highs in the upper 40s to low 50s, with lows in the mid 30s to low 40s. Sunnier and colder conditions are expected this weekend, so stay tuned. Keep reading below for a full update on the incoming storm and weather change!

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Continue reading the full blog below!

Since the last blog post, forecasts have come into very good agreement on the arrival of Arctic air into the Pacific Northwest, bringing the first mountain snowstorm and significantly colder temperatures to the entire region, plus heavy rain on Tuesday. Let's dive into the forecast!

First, we will look at the snow forecast for the mountains, since that is the highest-impact part of this event. Below is the European model forecast for snow through late Friday night. 

The European model forecast shows 10-18" of snow at the passes (less will actually accumulate since the ground is still warm due to this being the first snow of the season). The Olympics will get 8-12", and the Blue Mountains will get 4-10". This forecast also shows a chance of 1-3" of snow for Eastern Washington, mainly from I-90 northward.

Next, let's take a look at the NAM high-resolution snow forecast through late Wednesday.

The NAM forecast shows 6-12" at the passes, except up to 18" at Stevens Pass. This forecast agrees with 8-12" for the Olympics and 4-8" for the Blue Mountains. This forecast shows more widespread snow for Eastern Washington, with 2-4" for Wenatchee, the northern Palouse, and the area from Wenatchee to Spokane. Other areas of Eastern Washington, mainly from I-90 northward and along the Idaho border, could get a dusting to 1".

Finally, let's take a look at the ultra-high resolution UW WRF forecast, showing total snow through Wednesday evening.

The UW WRF forecast is very similar to the NAM, showing 6-12" at the passes, 8-12" in the Olympics, and 4-8" in the Blue Mountains. This forecast agrees with enhanced snow around Wenatchee (up to 4-6" in this scenario), along with 1-3" of snow from Banks Lake east to Spokane, mainly from I-90 northward.

Snow won't be the only precipitation falling in this storm. Below 2,000 feet in Western Washington, there will be a decent round of rain, heaviest from Tuesday afternoon to late Tuesday night. Let's take a look at the European model forecast below, showing total rain through Wednesday evening.

This forecast shows 0.7-1" for the lowlands, except 1-1.8" from Everett to Bellingham, including the San Juans and Whidbey Island. The coast will get 1-1.3", and the Willamette Valley will get 0.9-1.3". Note: Remember that this is a rain forecast, and thus all rain in the mountains will fall as snow.

Let's compare this to the GFS model forecast, also showing rain through Wednesday evening.

The GFS forecast shows 0.7-1.5" for the lowlands, focusing the heaviest rain a bit south of the European model, showing it from Tacoma to Oak Harbor, with up to 1.8" in the typically rain-shadowed areas of Port Townsend and Sequim. This forecast shows 1-1.5" on the coast and 0.4-0.7" in the Willamette Valley.

Now, let's take a look at temperatures. High temperatures will be noticeably colder over the next few days. We'll start below with the European model forecast for highs on Tuesday.

Expect highs in Western Washington in the upper 40s to low 50s, Eastern Washington in the mid 40s to mid 50s, and mountain towns in the upper 30s to low 40s.

Next, Wednesday's highs from the European model, seen below.

Western Washington will reach the mid 40s to low 50s, while Eastern Washington, mainly north of the Tri-Cities, will only reach the mid 30s to mid 40s. The Tri-Cities and Walla Walla will reach the low to mid 50s. Mountain towns will stay in the low to mid 30s.

Below are highs on Thursday, from the European model.

The lowlands will remain in the upper 40s to low 50s on Wednesday, with Eastern Washington in the mid 30s to low 40s north of I-90 and in the mid 40s to low 50s south of I-90. Mountain towns remain in the low to mid 30s.

Now, let's take a look at expected low temperatures, from the European model, starting with lows on Wednesday morning.

Wednesday morning's lows will drop to the mid 30s to low 40s in Western Washington, the low 30s to mid 40s in Eastern Washington, and the upper 20s to low 30s in the mountains.

Next, Thursday morning's lows from the European model.

Thursday morning will be colder, with lows in Western Washington dropping to the low to mid 30s, coldest in outlying areas. Eastern Washington will drop to the low 30s, except the mid to upper 30s in the Columbia Basin. Mountain towns will drop to the low to mid 20s.

We'll end this blog with a preview of the coldest temperatures for Western Washington, likely Saturday morning. Skies will clear late Friday, bringing clear and cold nights this weekend under a strong high pressure ridge (the Arctic high). Below is the European model forecast for Saturday morning's lows.

On Saturday morning, expect lows in Western Washington to drop to the upper 20s to low 30s, with the first frost likely for the entire region. Eastern Washington will be quite cold, with lows in the low to mid 20s, except in the upper 20s to low 30s from the Tri-Cities to Wenatchee. Mountain towns will drop to the upper teens to low 20s.

Lots of weather changes are ahead, and it'll feel more like winter than fall by this weekend! Stay tuned as we continue to monitor the latest forecast!