Weak Atmospheric River Bringing Rain & Gusty Winds

 FastCast—Thursday, Oct. 27 to Sunday, Oct. 30:

The next system, which will be a weak atmospheric river, will arrive in Western Washington on Thursday. The main impact on Thursday will be gusty winds ahead of the system. A Wind Advisory is in effect for the coast and areas from Everett northward, with NWS Seattle expecting gusts up to 50 mph for most of the day Thursday, but peaking in the afternoon. The rest of the region will have gusts of 30-35 mph, with some spots (mainly exposed areas or areas near the water) reaching up to 40 mph. Rain from the weak atmospheric river will move from north to south late Thursday night, continuing into Friday morning. Expect 0.3-0.7 inches of rain in the lowlands, with 0.75-1 inch from Skagit County northward. The coast and mountains will receive 1.5-3 inches. Snow levels will rise to around 6,000 feet with this warmer system, and lowland highs on Thursday will be in the upper 50s to low 60s. Conditions will be calm and overcast from midday Friday through Saturday evening, with highs in the mid 50s and lows in the upper 40s. Another atmospheric river is poised to arrive late Saturday night, lasting into Monday morning. While details are still uncertain, this looks to be a stronger atmospheric river, that may bring 1-2 inches of rain to the lowlands and 3-5 inches to the coast and mountains, along with gusty winds and high snow levels. Stay tuned!

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Continue reading the full blog below!

A weak atmospheric river (essentially a slow-moving cold front in this event) will impact Western Washington with gusty winds and rain on Thursday and Friday. The UW forecast below shows the weak atmospheric river aimed at Western Washington at 8 PM Thursday.

The atmospheric river will bring another round of rain to the area, but it will also bring gusty winds. This is because the atmospheric river will be slowly pushed south by a cold front, and the pressure gradient ahead of that cold front will produce gusty winds around the region. We’ll take a look at the winds first.

Below is the forecast for peak winds from the European model.

This forecast shows Thursday’s gusts topping out around 30-35 mph from Everett southward, with 40-50 mph gusts possible from Everett northward and on the coast. The European model shows peak winds on Thursday afternoon.

The NAM forecast for 10 AM Thursday shows a similar story.

The NAM shows similar gusts as the European, but differs with an earlier time for peak winds, generally midmorning to early afternoon.

Usually we don’t look at 3 different forecasts for one event, but something is showing up in the HRRR forecast that is worth noting.

The HRRR forecast above (3 PM Thursday) shows a narrow area of strong wind gusts due to a meso-low. In this instance, a meso-low is a small area of low pressure that forms on the leeward (wind-sheltered) side of the Olympic Mountains. The HRRR shows this meso-low bringing a narrow area of winds gusting up to 50-55 mph (black rectangle), stretching roughly from the Hood Canal Bridge to Shoreline.

Regardless of small weather features like a meso-low, Thursday’s winds will be enough to cause isolated tree damage and spotty power outages in the South & Central Sound, with a higher threat of tree damage and power outages from Everett northward and on the coast. Be prepared for these threats.

Now for the rain…let’s take a look at the European model forecast for rain through 5 AM Friday.

This forecast shows 0.5-0.8 inches of rain in the lowlands from Everett southward to Portland. It gives the North Sound 0.75-1.25 inches, and the mountains and coast get 1-3 inches.

Now, let’s compare this to the NAM forecast through 5 AM Friday.

The NAM gives lesser amounts to the lowlands, showing 0.3-0.6 inches, with even less south of Olympia. The NAM gives the North Sound 0.5-1.25 inches and the mountains/coast get 0.5-3 inches. 

This atmospheric river has the potential to cause flash flooding near area burn scars, especially the Bolt Creek Fire burn scar along US-2 from Index to Skykomish. With the combination of higher snow levels and heavy rain, be prepared for sudden flash flooding or debris flows near burn scars.

Finally, the mountain snow outlook. Atmospheric rivers have a subtle subtropical element, meaning that they bring warmer temperatures and higher snow levels. Forecasts show that snow levels in the Cascades will rise to 5,500-6,000 feet by Thursday night. The NAM forecast below (through 5 AM Friday) shows little snowfall in the mountains.

Due to high snow levels, most snow accumulation with this system will be confined to elevations over 5,500-6,000 feet. A trace to 2 inches of snow is possible at the higher passes before changing over to rain.

A potentially stronger atmospheric river is ahead for Western Washington, with rain possible from late Saturday night through Monday morning, along with gusty winds. Then, we may transition to an overall colder pattern to begin November, so stay tuned!