An atmospheric river, or fire hose of moisture aimed toward the coastline, will batter parts of the West Coast, including California, this week, bringing flooding rain in the lowlands, strong winds and six or more feet of mountain snow. The storm is already lurking just off the coastline in the northeast Pacific, with impacts set to begin by early Tuesday.

High wind watches are in effect for northern California and southern Oregon, including in and around the Shasta Valley, where gusts will hit 65 mph.

“Damaging winds could blow down trees and power lines,” warned the National Weather Service in Medford, Ore. “Widespread power outages are possible. Travel could be difficult, especially for high profile vehicles.”

Flooding is possible around California between Wednesday and Friday, with 5 to 7 inches of rain or more in the higher elevations. That could lead to the “potential for moderate to rapid rises along area rivers, streams, and creeks across the region,” the National Weather Service wrote.

Advertisement

Moreover, special sensors installed by the U.S. Geological Survey indicate that the top layers of soils are reaching saturation because of recent heavy rains. That means “widespread shallow landslides are likely,” the Weather Service has warned.

The precipitation will wind down into Friday, but more could be on the way into the start of early next week. NOAA’s Climate Prediction Center said an active weather pattern will continue on the West Coast and in the Desert Southwest, with odds of above-average precipitation looking likely over the next three months.

The pattern

On Monday afternoon, a large-scale, comma-shaped system was off the coast of the Pacific Northwest. That tail of that low-pressure system was a cold front, ahead of which a conveyor belt of deep, robust moisture was sweeping northeast from the tropics. A secondary low will drag more moisture northeast as well.

That tongue of moisture will lap at Washington and Oregon beginning Tuesday. Then the atmospheric river will work down the coast Wednesday into Thursday, bringing impacts to Northern California around the start of the workday Wednesday.

The Center for Western Weather and Water Extremes anticipates this atmospheric river will correspond to a level 4 on their 1 through 5 scale, with 5 being the most significant. That ranking corresponds to its IVT, or integrated vapor transport — how juiced-up the atmosphere will be. While it’s still early, and timing/forecast specifics are tough to iron out, the confidence in a significant storm system is growing.

Advertisement

That’s why the system will have the potential to dump considerable precipitation totals, especially in the mountains, since the high terrain helps focus moisture. (Atmospheric rivers carry the bulk of their moisture at the mid levels.)

Potential impacts

Heavy rainfall

Rainfall impacts will be limited in central/northern Oregon and Washington, where 1 to 2 inches of rain is anticipated in the coastal plain. The rainfall totals will be considerably greater in southern Oregon and central/Northern California, where the moisture fire hose will linger for longer.

In the coastal range of mountains in southern Oregon, 3 to 4 inches of rain is probable. In California, there will be two jackpot zones:

• In the coastal range, 4 to 6 inches of rain, and locally more, is probable. The greatest amounts will be realized north of the Bay Area. South of San Francisco and all the way to the Mexican border, coastal locations will see closer to 2 inches of rain, plus or minus.
• In the Sierra Nevada, where locations below the snow line will see up to 5 to 7 inches or more.
• The National Weather Service in San Francisco wrote that there is a 99 percent chance that the “higher elevations of the North Bay and other coastal ranges will see at least 3 inches in 24 hours.” It’s also almost a guarantee that the Sonoma Coast range, Santa Cruz Mountains and Big Sur coast see at least 4 inches in 24 hours.

The heavy rains will cause swift rises on area creeks and rivers, as well as some urban and small stream flooding.

Advertisement

“The creeks that are most likely to see rapid rises are located within Sonoma County such as Mark West, Green Valley, and Laguna De Santa Rosa,” the Weather Service warned. “However, rapid rises could also occur along the San Francisquito Creek and San Lorenzo River.”

A more pressing concern will be landslides, however.

“In terms of impacts, soils are near saturation and well primed from the previous storm cycle,” the Weather Service in San Francisco wrote. “The most likely impacts look to be stream and urban flooding and numerous shallow landslides along steep slopes.”

The snow line will be around 6,000 to 7,000 feet Wednesday, but will drop to between 4,500 and 5,500 feet into Thursday as cold air accompanies the core of low pressure. Above that level, 1 to 3 feet of snow is possible, with 5 feet or more in the highest mountain summits.

Advertisement

Thunderstorms

That cold air at high altitudes will also destabilize the lower atmosphere, causing pockets of air to rise. That could even make for some thunderstorms Thursday in the lower elevations, with small hail and/or an isolated funnel cloud.

Strong winds gusting to 40 mph or greater are likely up and down the entire West Coast, with 50 mph gusts from the Canadian border to Northern California at the shoreline. A few gusts over 60 mph are possible in coastal Oregon.

Looking ahead

In the medium range, another atmospheric river is likely over the weekend into early next week. Thereafter, the Climate Prediction Center is projecting above-average odds of continued heavy rainfall over the next two weeks, one month and three months.

In other words, a wet spring is anticipated — partly thanks to an active Pacific jet stream stemming from El Niño. More atmospheric rivers are projected to come ashore in the weeks ahead.

ncG1vNJzZmivp6x7uK3SoaCnn6Sku7G70q1lnKedZMSmrdOhnKtnYmV%2FdXuPamZrcV%2BWwa670qmfnqqZmHqztdWeqWaen6eypK3SrWScmZyes7C%2BzaKYZqeimrSwuoywmKygmaO0tbvNaA%3D%3D