Western states face powerful atmospheric river, potential bomb cyclone

An atmospheric river will bring weather threats to Washington and other Western states this week, from heavy flooding rain and damaging gusty winds, to treacherous seas and mountain snowstorms.

As much as 15 to 20-plus inches of rain may fall in the higher elevations of coastal mountain ranges over the next week. A much larger swath, from the western Oregon-Washington border region through Northern California, is forecast to pick up at least 6 inches. In addition to flooding, there will be potential for landslides, especially in recent burn scars left by wildfires.

An atmospheric river is a relatively narrow band of tropical moisture that helps deliver warmth from the equatorial regions to the poles and is typically found near a low-pressure area. This one will be supersized by what’s known as a “bomb cyclone” – rapidly intensifying low pressure – as it quickly moves ashore by the middle of the week.

Strong wind gusts could lead to power outages, especially near the coast where wind-whipped waves will also be crashing into the shoreline.

Amid the fire hose of moisture, multiple feet of snow – perhaps enough to require a yardstick to measure – is expected to fall in the Cascade Mountains of Oregon and Washington, and especially southward to the Klamath Mountains of Northern California.

Eastern Washington will see its share of the promised drama as the bomb cyclone makes its way through the Cascades.

Residents and travelers should be ready for blizzard-like conditions on mountain passes in the coming days, said National Weather Service meteorologist Steven Van Horn, who works out of the Spokane office.

“It is a deep lobe that’s going to form off the coast,” Van Horn said. “It does have a lot of moisture with it and there’s going to be impacts in Eastern Washington, but the bigger impacts are going to be the Cascades.”

Snoqualmie and Stevens passes should have big snowfall. Areas above 2,500 feet elevation could see from 6 to 12 inches of snow from Tuesday evening into Wednesday.

Paired with gusty winds up to 40 to 50 miles per hour that could kick up existing snow creates a visibility of a quarter mile or less from Tuesday night to Wednesday morning.

“Obviously not pretty good conditions for traveling,” Van Horn said.

The lowland regions surrounding the Cascades: Wenatchee, the Waterville Plateau and into Okanogan County could see around 4-8 inches of snow, and up to foot in the Plateau area.

The weather service issued winter storm warnings for the Wenatchee area and north to Canada. East of Moses Lake and into the Idaho panhandle, the service issued a winter weather advisory.

Spokane’s experience through the bomb cyclone probably won’t reach Cascade levels of blowing snow, Van Horn said, though the city may see some snow and rain starting Tuesday night.

“For Spokane we are looking at some snow that’s going to be falling overnight Tuesday and continuing into the morning on Wednesday, it should be tapering off by the afternoon,” Van Horn said.

The service forecast up to 2 inches of snowfall overnight Tuesday in Spokane, though as the day warms into the 40s it’ll likely turn to light rain that will linger through Wednesday.

When will the storm hit?

A weaker storm that hit the Pacific Northwest over the weekend is shifting east on Monday, and related showery conditions will persist in much of the region through Tuesday.

By Tuesday afternoon, heavy precipitation and strong wind associated with the developing bomb cyclone and atmospheric river are expected to approach shore.

On Tuesday night, the focus of stormy conditions appears to be from Vancouver Island in Canada to around the Oregon-California border.

Heading into Wednesday, the atmospheric river will probably shift its focus slowly southward from southern Oregon into Northern California. At some point, it may come close to stalling or perhaps return north for a time as the large dip in the gyrating jet stream offshore also stalls out.

This could deliver days of torrential rainfall. For now, it seems that impact would be concentrated north of San Francisco, but the city will need to keep watch. The area also faces substantial rainfall by later in the week, either way.

Heading into the weekend, rain and high-elevation snow may shift toward central or Southern California as the jet stream dip moves east into the Pacific Northwest.

Spokesman-Review Reporter Elena Perry contributed to this report.