Saturday Storm: Heavy Rain, Strong Winds, Chance of Thunderstorms

FastCast--Saturday, Nov. 4 to Wednesday, Nov. 8:

The next storm has arrived in Western Washington, with an embedded atmospheric river bringing heavy rain at times, with widespread gusty winds likely as well. Expect 0.9-1.5" of rain in the lowlands through Sunday morning, with less (0.3-0.5") in the traditional rain shadowed area. Overnight from Friday into Saturday, winds will peak on the coast, with gusts of 40-50 mph from Westport northward and 50-60 mph from Westport southward. During the day on Saturday, expect gusts of 40-50 mph on Whidbey, Camano, and the San Juan Islands, plus along Admiralty Inlet and in western Skagit and Whatcom Counties. The remainder of the lowlands will likely gust 30-40 mph, strongest near the water. Additionally, there is a region-wide chance of thunderstorms from mid-morning to late afternoon on Saturday, with the highest chance on the coast. Heavy rain could cause areas of ponding/standing water, localized urban flooding, and low visibility. Winds could cause isolated power outages and some tree damage. Remember to stay indoors if you hear thunder! Keep reading below for a full update on the storm!

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Continue reading the full blog below!

A multi-faceted storm is currently impacting Western Washington. Main impacts will be heavy rain, strong winds, and a chance of thunderstorms. Let's start with the rain forecast.

Below is the European model forecast showing the atmospheric river aimed at the Northwest.

This atmospheric river will bring heavy rain to the Northwest through Sunday morning (lasts longer the further south you are).

Now, let's take a look at the European model rain forecast through early Sunday morning.

In the lowlands, expect 0.9-1.5" of rain with this storm, except less in the traditional rain shadow areas NE of the Olympic Peninsula. On the coast, expect 1-2", with 1-1.5" in the Willamette Valley and 1.5-3" in the mountains. Eastern Washington will get 0.5-0.8" on the east slopes of the Cascades, 0.2-0.4" in the central part, and 0.6-1.2" for the eastern third of Eastern WA.

Let's compare this to the GFS (American) model forecast, also through early Sunday morning.

The GFS forecast is very similar to the European, showing 1-1.5" in the lowlands, 1.5-3" on the coast and mountains, and a clear rain shadow NE of the Olympics. Two noticeable differences are slightly less rain in the Willamette Valley (0.8-1"), and less for Central WA (0.05-0.2"). 

Let's take a look at 2 high-resolution forecasts, starting with the NAM forecast.

The NAM forecast shows 0.5-1.3" of rain in the lowlands, 0.9-1.3" in the Willamette Valley, and 1.5-2.5" on the coast. The mountains will get 4-8", and Eastern Washington will get 0.1-0.4", except 0.4-0.8" near the Idaho border.

Finally, we'll look at the UW WRF high-resolution forecast for rain in Western Washington through Sunday morning.

This high-resolution forecast shows 0.75-1.5" of rain in the lowlands, with the most from Tacoma to Monroe and in the foothills. The rain shadow is prevalent from Port Angeles to Everett, where 0.05-0.2" are possible. The coast will get 1-2.5", and the mountains will get 2-5".

Now, let's take a look at the high-resolution wind forecasts from the NAM and HRRR models, starting with the HRRR, showing peak gusts.

This forecast is by far the strongest in terms of region-wide winds, showing gusts of 40-45 mph for the entirety of the lowlands, with the coast hitting 45-55 mph. This scenario will bring more tree damage and potentially more power outages, but is somewhat less likely.

Let's compare this to the NAM forecast, seen below showing gusts at 8 AM Saturday.

Around 8 AM, the NAM forecasts peak wind gusts of 30-40 mph for the lowlands, up to 45 mph around Island, San Juan, Skagit, and Whatcom Counties. The coast will gust up to 40-50 mph according to this forecast.

Finally, there is a region-wide chance of thunderstorms on Saturday, mainly from mid-morning through late afternoon. Below is the NAM high-resolution forecast for instability on Saturday afternoon.

This forecast shows instability for most of the lowlands and the coast, with the highest chance of thunderstorms likely on the coast, spreading into the lowlands in the afternoon. Remember, if you hear thunder, go indoors and wait at least 30 minutes to go outside again.

Stay tuned for another update on Saturday night, as the next system is ahead.