California and parts of the West Coast have been dealing with extreme drought, but a waterlogged weather pattern is on the way. A series of atmospheric rivers, carrying a tremendous amount of moisture from the Pacific Ocean toward the coastline, will drench parts of the moisture-starved Golden State into early next week.

Parts of the Sierra Nevada will see more than five feet of snow, with up to 10 inches of rainfall likely in the lower elevations. Mudslides, debris flows and pockets of flash flooding are possible, the rains in some places arriving after more than half a year without a drop of water.

High winds are also anticipated, with some coastal and high-elevation areas seeing gusts to 60 mph.

The atmospheric rivers, spurred by exceptionally intense storms lurking offshore of the Pacific Northwest, will bring turbulent conditions to the region into at least next week. The first and weakest of the three moved ashore Wednesday. The next two, predicted Thursday into Friday and this weekend into early next week, will be progressively stronger.

While the blasts of heavy rain and snow will lead to disruption, they should make a welcome dent in the West Coast’s precipitation deficit, portending encouraging news for water resources across the West. California Gov. Gavin Newsom (D) recently expanded a drought emergency to encompass the entire state.

Most areas of central and northern California need at least six to 12 inches of rain or the equivalent amount of snow for the severe drought conditions to ease. The predicted precipitation will cut into that deficit while ending the fire season.

More modest amounts of rain are predicted in Southern California early next week, not nearly enough to significantly ease its drought conditions or reduce the wildfire threat.

What’s driving this storminess

The ocean storms thrusting the atmospheric rivers into coastline are both “bomb cyclones,” or mid-latitude systems whose air pressures plummet rapidly, signaling rapid intensification.

On Thursday morning, the first of the bomb cyclones had seen its pressure drop to nearly 950 millibars, which is typical for a Category 3 hurricane. The second bomb cyclone predicted Sunday could see its pressure drop to 940 millibars, characteristic of many Category 4s.

“Today’s cyclone west of Oregon is the strongest for October since at least 1950, and this weekend’s storm may be even stronger,” tweeted Richard James, a meteorologist who runs the World Climate Service, a private forecasting firm.

The intensity of the atmospheric rivers generated by these storms is predicted to reach top-tier levels. Atmospheric rivers are narrow strips of rich moisture with origins in the deep tropics. Such features that impact the Pacific Coast are often referred to as the “pineapple express,” since they have connections as far away as Hawaii.

On a scale of 1 to 5 for atmospheric river strength, coastal-central California and southwest Oregon are predicted to see Level 5 conditions, driven by the event Sunday into Monday. Parts of the Washington state and Oregon coastline are expected to see Level 4 conditions.

The rating is assigned based on forecast integrated vapor transport — or how much moisture the river is moving through a given volume of air. Each atmospheric river is like a conveyor belt. Atmospheric rivers transport moisture most efficiently at the mid-levels of the atmosphere, which is why the heaviest snow totals often occur above 5,000 feet.

How the atmospheric rivers will evolve

The first and weakest of the three atmospheric rivers came ashore Wednesday in the Pacific Northwest, bringing light-to-moderate rainfall Thursday morning in parts of central and northern California. Shasta Dam near Redding recorded 1.28 inches. Sacramento International Airport has seen only 0.03 inches.

The second atmospheric river is predicted to land in the Pacific Northwest on Thursday afternoon and sweep down into northern and central California by Friday morning. The National Weather Service in Portland, Oregon, is predicting a “burst of intense rain,” while winds along the coast could reach 40 to 60 mph Thursday afternoon and evening.

A flash flood watch has been posted in northeast California. “An intense band of precipitation will move through late Thursday evening through Friday morning,” the Weather Service in Sacramento wrote. “Periods of moderate to heavy rain are expected, especially Friday morning. These could bring potential ash and debris flows over recent burn scar areas in Shasta, Tehama, Butte and Plumas counties. Potential rainfall rates will be over a half inch per hour this evening.”

This second atmospheric river will dissipate by Friday evening before the third atmospheric river takes shape Saturday and drives ashore Sunday and Monday, which is when the heaviest rain and Sierra snows will fall. That atmospheric river will begin to relent late Monday, though indications point to yet another taking shape toward the midweek.

Forecast rain and snow amounts

A general one to four inches of rain can be expected with the round of storminess through Friday, with totals likely to double and then some by Monday. Up to 10 inches of rain can’t be ruled out on the western, or windward, side of the Sierra Nevada, with four to seven feet of snow for the highest peaks. Rainfall of three to five inches is possible in the Bay Area.

Through Sunday, snow levels will be high, generally above pass levels, but are expected to dip to 5,500 to 6,500 feet by Monday. At the highest elevations, above 9,000 feet, computer models show a probable high of at least 18 inches and up to five feet.

Growing concerns for mudslides and debris flows

Much of the rainfall will come down in short bursts, which could be problematic in areas that burned during recent wildfire seasons. That can trigger flooding and mudslides on recently burned areas, sending fast-moving rivers of ash, rock and burned debris through stream channels. These “debris flows” are a threat to those living downslope of wildfires, particularly in steep terrain. Mudslides are also a concern; the National Weather Service deems the threat as “moderate.”

Burn scars have a high probability of debris flows when rain rates exceed a quarter inch in 15 minutes, according to post-fire assessments by the U.S. Geological Survey. The National Weather Service in Sacramento expects a brief periods of heavy rain late Thursday into Friday morning and has issued a flash flood watch for the Dixie Fire and the 2020 North Complex fire.

“There is enough potential there that we wanted to highlight the threat,” said Benjamin Bartos, an incident meteorologist on the Dixie Fire.

However, the bigger concern is the much stronger system that will move in later this weekend.

“The Sunday-into-Monday weather system has a higher potential of more major impacts to the fire,” he said.

Intense wildfires consume vegetation and alter soil properties that make it likely that water will scour the surface rather than infiltrate into the ground. Flood and mudslide risks are greatest immediately following a wildfire but can remain elevated for two to five years as vegetation on the burn scar grows back.

This early-season series of storms is arriving to very recently burned ground.

On Jan. 9, 2018, a damaging and deadly debris flow occurred on the Thomas Fire in Montecito, Calif., near Santa Barbara, during the first rainstorm of season.