After multiple weeks without significant precipitation on the West Coast, an atmospheric river disrupted the spell of dry weather with rain and snow in Washington, Oregon and Northern California on Friday.

A second system, also pulling in a stream of moisture, will arrive on the West Coast most likely on Monday, before the first one has exited. It is expected to stall over Northern California and bring continued rain and mountain snow to the region into next week.

“These systems are right on each other’s tails, so in some places you might not get a break,” Julie Kalansky, deputy director at the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography at the University of California, San Diego, said of the storms’ impact on Northern California.

Coastal areas of Oregon and Washington could receive three to five inches of rain from late Thursday into Saturday, while the coastal mountain ranges of Northern California are expected to be drenched with seven to 15 inches of rain over the next week, according to the National Weather Service. The northern Sierra Nevada could pick up over four feet of snow in the next three days.

The first system pushed into the Pacific Northwest on Thursday night, and drifted south on Friday along the coast into Northern California, where it will linger through the weekend as the second system arrives.

The second storm “is forecast to stall over California, wobbling up and down along the coastline,” said Roger Gass, a meteorologist with the Weather Service office in Monterey, Calif.Atmospheric rivers are ribbons of moisture carried by powerful winds. These storms have the potential to unleash deluges of rain when they reach land and especially when they push up and over mountains. But their paths are narrow, and it can be difficult to pinpoint exactly where they will line up and release the heaviest rainfall.

This early in the forecast, meteorologists can confirm that a wet weather pattern will persist on the West Coast into next week, but they cautioned that the storms’ tracks could shift and the details of the forecast could change.

There’s a slight chance for excessive rainfall that could lead to flooding in portions of the Northern California coastal mountains, the Bay Area, the Sacramento Valley and the northern and central Sierra Nevada from Saturday morning into Tuesday, according to the Weather Prediction Center.

The risk also stretches inland across the Sacramento Valley and into the Sierra Nevada. Streams are predicted to swell in southwestern Oregon and Northern and Central California, and the greatest potential for flooding is expected in Northern California.

“One of the more impressive things about this storm is how long it’s going to last in Northern California,” said Chad Hecht, a meteorologist at the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography.

Here’s how things may unfold
Friday: After beginning to affect the Pacific Northwest on Thursday, the system spreads south into Northern California, while rain and snow continue to fall over Washington and Oregon. The system starts pushing into the Sierra Nevada late Friday.

Saturday: The system exits Washington early in the day and Oregon by later in the afternoon or evening, while Northern California continues to get hit with heavy rain and snow.

Sunday: A chance for rain and snow continues mainly in Northern and Central California.

Monday through Wednesday: A second pulse of moisture or atmospheric river brings more heavy precipitation to Northern and Central California and a chance for rain and snow to Washington and Oregon.

While Washington, Oregon and Northern California had wet starts to the rainy season late last year, these areas have not received significant rainfall for multiple weeks. Seattle has recorded about 1.2 inch of rain since Jan. 1, compared with five inches for a typical January.

With the two storms, Seattle is expected to pick up as much as an inch and a half of rain, with some of the precipitation potentially falling as snow. Kayla Mazurkiewicz, a forecaster with the Weather Service in Seattle, said that these storms aren’t big rainmakers for Seattle, but that “it’s a change since we haven’t had much rain in two weeks.”

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San Francisco has not received any rain in more than three weeks and has had little rain in all of January, recording less than a quarter of an inch since the start of the month. In a typical January, the city gets about 4.5 inches. The dry spell is coming to an end though, with San Francisco predicted to record over four to six inches in the next seven days. Just to the north, the mountains of the North Bay could receive more than eight inches, with valley locations recording about half as much as higher elevations.

The Bay Area got soaked by an atmospheric river in November, and Brian Garcia, a meteorologist with the Weather Service office in Monterey, Calif., said this system was not expected to be as nearly as wet.

“That storm was something special,” Mr. Garcia said. “We picked up about a foot of rain in Santa Rosa in 48 hours. That corresponds to pushing a 1,000-year event.”

Southern California has had one of its driest starts to winter ever, according to records going back more than 150 years. The dry conditions helped fuel multiple devastating wildfires across Los Angeles County this month.

While the storm systems are expected to be focused over Northern California, the second one could bring up to an inch of rain to Southern California and Los Angeles County next week.

“Some forecasts have the rain tapering off before the storm gets down there, and others show it lingering and bringing some rain, but not a ton of precipitation,” Mr. Hecht said. “That’s not to say things can’t change.”