Western states face powerful atmospheric river, potential bomb cyclone

A high-end atmospheric river will bring numerous weather threats to Western states this week, from heavy flooding rain and damaging gusty winds, to treacherous seas and prolific mountain snows.

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 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.

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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.

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The main risks

Currently near the heart of the expected threats, the National Weather Service office serving the Eureka, California, region is forecasting 2 to 4 inches of rain each day from Tuesday night through late week. Locations near the city could receive up to 12 inches.

A “prolonged period of moderate rainfall with an initial threat of small stream and urban flooding … and potentially even main stem river concerns,” are among the things the office is watching for.

Significant snow from a preceding storm continues in the Cascades on Monday. With another 10 to 15 inches expected above 2,000 feet, storm totals could reach 2 feet in some spots. Any break before the next round will be limited.

As the atmospheric river approaches, temperatures will warm and new snow will move higher into the mountains - which could mean melting snow will add to the flood risk.

Northern California’s Mount Shasta is forecast to pick up 6 feet of snow through Thursday, with more expected from there. Winds up to 115 mph are expected at the summit.

At lower elevations, a high-wind warning is in effect for coastal regions of Northern California on Tuesday night and early Wednesday. The Eureka-area Weather Service office is forecasting south-southeast winds of 30-40 mph with gusts of 70 mph, with the strongest winds likely at the immediate coast and higher elevations.

Downed trees and power lines, plus light structural damage and power outages, are a decent bet in warned areas.

Amid various coastal hazards, sneaker waves - so named because they are larger and less predictable than other waves occurring at the same time - are already a major risk in the run-up to the storm. The Weather Service suggests avoiding jetties and logs on beaches and keeping an eye on ocean conditions.

Storm warnings are also in place for Tuesday into Wednesday for offshore waters, where waves will climb toward or above 30 feet as the storm reaches peak intensity. Waves of 10 to 20 feet are possible right at the shore.

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What else to know about the looming storm

Weather models indicate the atmospheric river will be slung ashore by a bomb cyclone off the Pacific Northwest coast and just to the southwest of British Columbia.

Bomb cyclones are typically defined as cyclones that strengthen by 24 millibars in 24 hours, although the intensification rate shifts somewhat depending on latitude.

An overnight European model showed a pressure drop from 999 millibars to 944 millibars just 20 hours later. That’s an astounding 55 millibars of intensification - a bomb cyclone with enormous room to spare.

Analytics from the Center for Western Weather and Water Extremes at the University of California at San Diego suggest a zone from southwest Oregon to around the San Francisco Bay Area could face an atmospheric river rated at least Level 3 on a 1-to-5 scale.

Data also suggests a Level 4 could be reached in the maximum zone, which is currently expected around Northern California to southern Oregon. Some models even shown a Level 5 event.