Active Period Ahead: Heavy Rain, Windstorm, and Coastal Flooding Possible

 FastCast—Monday, Dec. 26 to Wednesday, Dec. 27:

An active period is ahead for the Pacific Northwest, with heavy rain, a potentially strong windstorm, and river/coastal flooding possible. A fast-moving atmospheric river will bring 1-2 inches of rain to the lowlands through Tuesday night. This will bring 2-3 inches on the coast and 3-7 inches in the mountains. Two systems will bring a potential for windy conditions. A system Monday morning will bring gusts of 25-35 mph to the lowlands and 40-50 mph to the coast and North Sound. Then, a much stronger system will approach the Washington Coast on Tuesday. The placement of this system will determine if the Puget Sound area and Willamette Valley will get strong winds. It’s certain that the Oregon Coast will have gusts of 60-70 mph. Stay tuned for more about this storm. With king tides occurring, coastal flooding is possible on the coast and around Puget Sound on Monday and Tuesday mornings.

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Continue reading the full blog below!

A stormy period of weather is ahead for the Pacific Northwest, with heavy rain, winds, and coastal flooding expected. 

Below is the UW forecast showing the amount of moisture in the atmosphere at 4 AM Monday.

This system is typical of an atmospheric river, but it will be on the fast-moving side, so  it doesn’t officially qualify as an atmospheric river. 

However, a significant amount of moisture will remain around the region at 1 AM Tuesday.

This will bring a significant atmospheric river to the Southern Oregon Coast and Northern California, while still bringing heavy rain to the Puget Sound area. 

So…how much rain will fall? Let’s take a look at the forecasts, starting with the European model through 10 PM Tuesday.

The European model shows 1.5-2.5 inches for the lowlands, with 3-5 inches in the mountains and on the coast, 2.5-3 inches in the Willamette Valley, and 0.5-1.25 inches in Eastern Washington.

This matches up well with the forecasts from the NAM high-resolution forecast, seen below through 11 PM Tuesday.

The NAM shows a bit less rain for the inland areas, with 1-2 inches for the Puget Sound area and Willamette Valley, 0.25-1.25 inches in Eastern Washington, 3-5 inches on the coast, and 5-10 inches in the mountains, increasing the potential for river flooding.

There is also a significant potential for a region-wide windstorm on Tuesday. First, a system offshore will bring breezy conditions to the lowlands and windy conditions to the coast on Monday, as seen below in the European model forecast for Monday morning’s peak wind gusts.

Expect 25-35 mph gusts around the lowlands, with gusts 40-50 mph from Everett northward and on the coast. 

A significantly stronger windstorm is possible on Tuesday, as seen in the UW forecast for 7 AM Tuesday.

A strong storm is offshore of the Washington/Oregon border, moving northward. Note: lines on the map are lines of constant pressure, how meteorologists measure the strength of winds and pressure. On Tuesday morning, this storm will be bringing 60-70 mph gusts to the Oregon Coast and potential coastal flooding to the entire region due to the combination of king tides and very low pressure. 

By 7 PM Tuesday, the storm has moved inland over Southern Vancouver Island.

Notice the very strong pressure gradient (closer lines) across Western Washington. This could bring significant winds to the Puget Sound area. 

However, as of Sunday night, it’s too uncertain to tell exactly what the impacts from this storm will be, so stay tuned to the next blog!