Western US faces powerful atmospheric river, possible bomb cyclone

A major atmospheric river is set to impact Western states this week, bringing a range of severe weather threats, including intense flooding rain, damaging winds, treacherous seas, and heavy mountain snow.

As much as 15- to 20-plus inches of rain may fall in the higher elevations of coastal mountain ranges over the next week, News.Az reports, citing US media.

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 an elevated potential for landslides, especially in recent burn scars left by the summer’s 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 before meandering for several days.

Widespread strong wind gusts may also lead to power outages in the region, with a focus nearest 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.

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.

News.Az