By Ted Buehner, Meteorologist & Red Cross Volunteer

Are you ready for a lot of rain this week? An atmospheric river will send a surge of rain into Western Washington Monday, sag south toward Northwest Oregon Tuesday, offering a brief break, and then pivot north again Tuesday night and Wednesday.

An atmospheric river is a stream of moisture from tropical or sub-tropical sources transported into mid-latitude storm systems, and produces copious amounts of rainfall with relatively high snow levels. This feature is actually quite common around the globe and helps produce the bulk of mid-latitude annual rainfall including the Pacific Northwest.

How Much Rain

Forecast rainfall amounts through Wednesday reflect how soggy this atmospheric river is. The Cascades will receive the greatest amount of rain, ranging from 6 to 12 inches of rain. The Olympics will also retain its rain forest reputation with 5 to 10 inches expected.

The coast and southwest interior are anticipated to receive 2 to 4 inches of rain through Wednesday, while areas from about Seattle northward will get the benefit of the Olympic Mountain rain shadow, reducing rain amounts to about a half to one and half inches of rain.

Atmospheric river events bring warm moist air, resulting in elevated mountain snow levels. Snow levels will ride a rollercoaster between 4500 and 7000 feet.

On Tuesday night and Wednesday as the stream of warm moisture pivots back north, there will be another surge of rain. With so little current mountain snow, the heavy rainfall amounts are quite likely to simply runoff, into rivers and streams.

Impacts – Flooding, Winds, King Tides, Landslides

The last time this kind of heavy rainfall amounts fell was back in November 2022. The most serious flooding was in the North Sound, primarily in Whatcom and Skagit Counties.

This atmospheric rain event will create more widespread flooding in Western Washington. Rivers from near the Canadian border to the Columbia River are anticipated to exceed flood stage starting either Monday night or Tuesday.

The most serious flooding is anticipated with rivers flowing out of the Cascades, including the Skagit, Stillaguamish, Skykomish, Snoqualmie, Tolt, Snohomish, Puyallup, Carbon, Nisqually, Chehalis, Skookumchuck, Newaukum, and Cowlitz rivers.

If you live in or just moved into a flood prone or low lying area, there are steps to take in advance ahead of the flood. Some resources include the Red Cross Flood Safety webpage, the Washington State Emergency Management Division flood preparedness webpage, FEMA’s flood readiness webpage, and your local county emergency management websites.

Those who commute through these low-lying flood prone areas may also find roads closed due to high water. Follow the detours. Remember, turn around, don't drown.

In addition, many other roadways may have standing water during the heavy rainfall. When driving, slow down, avoid hydroplaning, and leave plenty of distance behind the vehicle in front of you.

Winds

Winds can be another impactful factor this week. As each surge of rain with this system moves onshore, southerly winds are likely to pick up with gusts up to 30 to 45 mph at times. Local downed trees or limbs, and power outages are possible.

King Tides

This month's King Tides period peaked over this past weekend, yet high tides remain relatively high and minor tidal overflow flooding remains possible in low lying areas into Wednesday.

Landslide Threat

The calendar year has been a dry one. Through this past weekend, Seattle-Tacoma International Airport (SEA) was over 8 inches of rain behind for the year. This period of rainfall though will moisten up upper soil layers on steep slopes and make them heavy and prone to landslides over the dryer deeper soil layers.

What's Ahead

This atmospheric river is expected to taper off starting Wednesday into Thursday. Rain remains in the forecast heading into the coming weekend, but not fueled by sub-tropical moisture. Yet, it will take a period of time for rivers to recede back below flood stage.

The latest weather outlook as the calendar approaches the Christmas holiday continues to reflect warmer and wetter than average conditions.

As always during flood season, remain vigilant and prepared in case of significant flooding, not only for your home or business, but also for your commute if it takes you through river basins.