Heavy rain headed to Pacific Northwest this weekend and early next week

An atmospheric river is targeting the Pacific Northwest this weekend.

It will deliver heavy rain and relatively mild air, and probably will be followed by a powerful low-pressure system expected to slam into the region early next week.

Rain and high-elevation snow will be widespread Friday, from British Columbia in southwest Canada, through Western Washington and into Oregon. Other than some short lulls, precipitation will probably fall most of the time through at least Tuesday.

National Weather Service forecasts suggest that through the weekend, about 1 to 3 inches of rain will fall around the coast, including around 2 inches for Seattle. Mountainous areas expecting minimal snowfall are forecast to see 3 to 5 inches of rain. An additional widespread 1 to 3 inches is forecast Monday into Tuesday. The highest elevations farthest north could see several feet of snow through the same stretch.

Strong winds may also become a concern, particularly early next week, as the stronger low pressure swings by.

Much of the region needs the rain. Coastal zones of Washington and northern Oregon have tended to be quite dry this winter, with precipitation totals as low as 50 to 75% of normal encompassing a large part of the area as drought expands.

Atmospheric river, then strong low pressure

The first wave of enhanced precipitation arrives Friday and peaks over the weekend. The atmospheric river pointed at the Pacific Northwest – a fire hose of moisture from the tropics – will probably extend more than 3,000 miles Saturday.

During the weekend, “areas of heavy rain are expected,” according to the Weather Prediction Center. “Especially into the coastal ranges of Oregon and Washington as well as into the Cascades. Rain rates may reach up to a half-inch per hour at times.”

The Center for Western Weather and Water Extremes is forecasting a widespread Level 3 of 5 atmospheric river for coastal Washington and Oregon, with the potential for some locations to reach Level 4 of 5.

Given the tropical origins of the precipitation, snow levels will be higher than usual, meaning only the highest mountain peaks will see significant snowfall during much of the stormy stretch.

The heaviest accumulations, of potentially several feet, will be north of the core of the atmospheric river or focused on the northern Cascades northeast of Seattle.

High snow levels will also lead to more available runoff that can sweep into area creeks and rivers.

The weekend dousing will give way to an intense storm system that is developing on the atmospheric river’s tail. That system will ultimately produce stronger winds at its core and whip up larger waves, in addition to sending rounds of heavy rain to the region through early next week.

Tempered flood threats, with caveats

The Weather Service has a Level 1 of 4 “marginal risk” issued for the area beginning Saturday and lasting through at least Monday.

Since it has been on the dry side in the long term, and the area is not particularly susceptible to flooding, risks there are generally low.

Drier than normal conditions have dominated much of the coastal portion of Washington and northwest Oregon over the past several months.

Drought has been expanding in parts of the area, including the Cascades of Washington, according to the U.S. Drought Monitor.

Nonetheless, some flooding is anticipated.

The Weather Service office based in Seattle notes that the rain “will bring rises on many area rivers and could bring a handful closer to flood stage. Additionally, the Skokomish River is expected flood again by around Sunday.”

Over the entire period, weather modeling may be somewhat wetter than official forecasts. There is potential for 3 to 5 inches of rain near the coast, and as much as 6 to 10 inches in the mountains, before any lengthier break midweek.

Coastal flooding and wind potential

Several days of battering waves and periods of strong to potentially damaging winds are also likely.

Seas around 10 to 15 feet are forecast during the primary atmospheric river through the weekend.

The powerful low-pressure area passing the region early in the week should send higher swells in, if it is as strong as advertised. Waves up to 20-plus feet are possible Monday, leading to particularly dangerous conditions for boating or other waterborne activities.

“It warrants watching for the potential of storm force winds over the coastal waters and seas in excess of 20 feet,” the Weather Service office in Seattle wrote.

Strong winds on the order of 30 to 50 mph also add to the tumultuous conditions, especially near the coast Monday into Tuesday. Gusts up to 60 or 70 mph might lead to power outages, particularly in coastal and mountainous areas.