FastCast—Friday, Nov. 4 to Saturday, Nov. 5:
A significant storm will impact the Pacific Northwest from late Thursday through Saturday. The main impacts of this storm will be strong winds, heavy rain, river flooding, and mountain snow. Winds will increase late Thursday night and into Friday morning, with a potential lull around sunrise. Peak winds for the lowlands will be from late morning to around 9 PM, with gusts up to 45-50 mph, strongest near the water. A significant westerly wind surge is expected for areas from Port Angeles to Everett after dark. Gusts in this area will reach 50-60 mph. Tree damage and power outages are expected across Western Washington, as this will be the strongest windstorm of the season. Eastern Washington will have winds gusting 50-65 mph, strongest Friday afternoon to early Saturday morning. It will also be quite rainy, with an atmospheric river forecast for late Thursday through early Saturday. In areas directly east of the Olympic Mountains (including Seattle), there will be a rain shadow, with those areas receiving 0.2-0.5 inches of rain. Outside the rain shadow, expect 1-3 inches in the lowlands, 3-6 inches on the coast, and a whopping 6-30 inches in the mountains. This will be especially problematic with snow levels topping out near 7,500 feet. River flooding is expected for most rivers flowing off the Cascades and W/SW Olympics. On Saturday, winds will decrease to 25-40 mph area-wide, with strong Convergence Zone bands possible as widespread rain becomes showers. Colder air will move into Western Washington, and snow levels will crash from 7,500 feet to around 2,000 feet, bringing 4-12 inches of snow to all the passes. There’s a lot of details with this system, so keep reading below.
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Continue reading the full blog below!
A significant and likely impactful storm is ahead for the Pacific Northwest. There are a lot of things to cover, but we’ll start with the winds.
Below is the HRRR forecast showing wind gusts at 2 PM Friday.
This forecast shows peak winds of 40-50 mph for the lowlands and 50-55 mph on the coast on Friday afternoon.
If we fast forward to 9 PM Friday, we can see the very strong westerly wind surge down the Strait of Juan de Fuca and very strong winds in Eastern Washington.
By Friday night, winds along the Strait all the way inland to Everett and Lake Stevens will be gusting 50-60 mph, with isolated higher gusts along Admiralty Inlet. Eastern Washington’s winds will be increasing, with 50-60 mph gusts there.
That’s a lot of wind, but we have only looked at one forecast. To get the best prediction, it is important to check multiple forecasts to ensure accuracy.
The next forecast is the NAM model, which shows gusts at 1 PM Friday.
The NAM shows stronger winds for the interior lowlands, peaking at 50-55 mph. This forecast also brings gusts of 50-60 mph to the coast and an earlier start to 50-60 mph gusts in Eastern Washington.
Let’s fast forward to 10 PM Friday.
Here, we see the NAM agreeing with the HRRR, showing a westerly wind surge from the Strait to Everett, with gusts of 50-60 mph. The NAM shows stronger winds for Eastern Washington, with gusts of 55-65 mph (isolated gusts up to 70 mph in this forecast).
This will be the strongest windstorm by far this season, for both Western and Eastern Washington. Expect tree damage and power outages across the region.
Now for the rain…as we have been talking about recently, a strong atmospheric river will be a major part of this system. Below is the UW forecast for 12 PM Friday.
This is showing integrated water vapor transport (IVT), another way of saying the amount of water vapor in the atmosphere. Notice the current of very moist air aimed directly at Western Washington. This will cause the heavy rain and eventual flooding.
Below is the HRRR forecast, showing total rain through Saturday evening.
This forecast shows a clear rain shadow over Seattle and the Kitsap Peninsula, as well as a large part of Eastern Washington. Rain-shadowed areas will get 0.1-0.4 inches of rain. Outside the rain shadow, the lowlands will get 1-3 inches, the coast will get 3-6 inches, and the mountains will get 6-20 inches. Non-rain shadowed areas in Eastern Washington will get 0.4-1.25 inches.
Let’s compare this with the NAM forecast (below), also showing total rain through Saturday evening.
The NAM shows a larger rain shadow over the Central Sound, with 0.1-0.4 inches in the rain shadow. Outside the rain shadow, the lowlands will get 1-3 inches, the coast will get 3-6 inches, and the mountains could pick up a staggering 6-24 inches.
This is a great time to remember that wind + rain = clogged storm drains. Be ready for standing water and urban flooding due to clogged drains, and be cautious around streams and creeks as well.
The huge amounts of rain that will fall in the mountains, plus very high snow levels of 7,500 feet will bring river flooding around the area. Below is the river flooding outlook from the NWS Advanced Hydrologic Prediction Service.
Here’s how to read this: blue = no flood categories, green = normal, yellow = action stage (bankfull), orange = minor flood stage, and red = moderate flood stage. The Skokomish, Chehalis, Puyallup, White, Carbon, Green, Nisqually, Tolt, Snoqualmie, Snohomish, Skykomish, Stillaguamish, Skagit, Nooksack, and Cowlitz Rivers will be at bankfull or flood stage. The highest flooding is expected on the Skokomish (near Hood Canal) and Snoqualmie Rivers.
Additionally, there is a threat of flash flooding due to burn scars. The excessive rainfall/flash flood threat forecast from the NWS Weather Prediction Center is below.
This shows a marginal to slight risk of excessive rainfall and flash flooding for most of Western Oregon, all of the Cascades & Coast, and parts of Western Washington.
One final note: snow is expected in the Cascades and parts of Eastern Washington overnight Thursday, but will change to rain by early Friday morning. Snow will begin again early Saturday morning as snow levels drop, but more on that tomorrow night.
There are a lot of weather impacts coming up. The best way to stay updated is to click on the Twitter logo on the right side of the blog (it will open in this current window). I will have another blog post Friday night, so stay tuned.
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