This article, first published Friday afternoon, was updated Saturday morning.

A parade of storms forming in the Gulf of Alaska is generating a relentless barrage of atmospheric rivers — strips of deep tropical moisture. The next and most powerful river is poised to blast the Pacific Northwest and California on Saturday night and continue into early next week.

Double-digit rainfall totals are possible in some spots through Tuesday, with more than four feet of snow expected in parts of the Sierra Nevada.

The copious precipitation totals are a dramatic turnaround from “exceptional” drought conditions recently gripping the West, with the rainfall set to make a dent in California’s years-long water deficit.

An atmospheric river soaked the zone from the Pacific Northwest to Central California on Thursday and Friday, while another — even more intense event — is expected to move ashore Saturday night and into Sunday. The Center for Western Weather and Water Extremes, based in La Jolla, Calif., tweeted that this second atmospheric river, rated a Category 5 on its 1-to-5 scale, will be the strongest to hit the San Francisco Bay area at this time of year since 2010.

Back-to-back “bomb cyclones,” ocean storms that intensify at breakneck speed, are responsible for driving these rivers in the sky into the coastline.

The copious amounts of rain and snow predicted in Northern and Central California should end the fire season there.

The National Weather Service issued flash flood warnings on Oct. 23 and cautioned heavy rains could cause mudslides in areas scarred by wildfires and drought. (National Oceanic and Atmospheric Administration (NOAA) via Storyful)

The precipitation may cause serious problems, however, with flood watches up for much of Northern California and the Central Valley. Rain falling on wildfire burn scars could cause debris flows and mudslides, too, and evacuation warnings have been issued in some vulnerable areas. High winds are a concern along the coast and in the mountains, where gusts could top 60 mph.

While some rain will reach Southern California early next week, it is not expected to be enough to ease the drought or wildfire potential there.

The rain so far

On Thursday and Friday, an initial atmospheric river generated bands of heavy rain that swept through the Pacific Northwest and Northern and Central California.

Nearly two inches fell in San Francisco, making this the wettest October there since 2016.

Even more substantial amounts fell at high elevations. Mount Tamalpais, a peak north of San Francisco, registered around nine inches between Wednesday and Friday. Up to five inches were reported in Shasta County, which surrounds Redding in the northern part of the state.

This week, Sacramento recorded its first measurable rain since March 19, ending a record 222-day streak without precipitation.

The same weather disturbance that brought the first round of heavy rain will generate a new storm over the Plains on Sunday, bringing a chance of severe weather and even a few tornadoes. It was generated by the first of two “bomb cyclones” offshore the Pacific Northwest.

Stronger system on the way

A second, even more intense bomb cyclone will drive the next atmospheric river onto the West Coast on Saturday night and into Sunday.

Although the cyclone itself will remain over the ocean, wind gusts up to 60 to 70 mph and choppy seas are expected along the coasts of northern California, Oregon, Washington and British Columbia. High-wind warnings are in effect for many coastal areas, and shoreline breakers along the coast in Northern California and the Pacific Northwest could reach 20 to 35 feet.

The offshore cyclone’s rate of intensification between Saturday and Sunday is predicted to be extreme, possibly attaining rare “double bomb” status. A storm is considered a bomb cyclone when its minimum air pressure decreases 24 millibars or more in 24 hours; this one could drop by nearly 50 millibars in that period.

The cyclone’s pressure could bottom out around 945 millibars, which is the typical minimum pressure of a Category 3 or 4 hurricane. Such pressures would rival the lowest observed off the Pacific Northwest coast since at least 1950.

Major precipitation totals

As the cyclone drags the atmospheric river onto the coastline Sunday, the result will be a fire hose of moisture, with origins north of Hawaii, aimed at Northern California. This atmospheric river is predicted to qualify as a Level 5 out of 5 in the San Francisco Bay area and a Level 3 or 4 in the Pacific Northwest and the remainder of Northern and Central California.

In most areas, the heaviest precipitation will occur between Sunday and Monday mornings, maybe lasting into Monday afternoon in Central California. A period of moderate rain may pass through Southern California on Monday night into early Tuesday.

Around the San Francisco Bay area, moisture levels are expected to be exceptionally high. Precipitable Water Indices (PWATs) could peak near 1.6 inches, challenging the all-time October record for Oakland. PWATs describe how much equivalent liquid is present in a column of atmosphere.

Atmospheric rivers carry the bulk of their moisture at the mid-levels, so the higher terrain of the Coastal Range and the Sierra Nevada will receive the greatest precipitation totals.

Precipitation totals could reach a foot by Monday night in the northern reaches of the Sierra Nevada, although above 10,000 feet it may fall primarily as snow; four feet or more of snow is possible where no rain falls.

Winter storm watches are in effect for most high-elevation areas of Central and Northern California from Sunday evening to Monday evening, mainly for elevations above 8,000 feet. In addition to multiple feet of snow, winds could gust over 60 mph and up to 100 mph along ridgelines.

“Avoiding travel in the mountains during this time would be a good idea,” wrote the National Weather Service in Reno, Nev.

Although some snow is predicted below 6,000 feet of elevation by Monday afternoon or evening, with some accumulation expected, rain is expected mostly below 8,000 feet.

Large areas of Central and Northern California are under flood watches.

Generally, 4 to 9 inches of rain are likely below the freeze line and along the coastal range; isolated amounts into the double digits are possible. The Central Valley will probably see 2 to 5 inches.

Five inches of rain is predicted in Tahoe City, Calif., which would be the second most during October in the past 12 years, the Weather Service in Reno wrote.

In the San Francisco Bay area, the heaviest rain is predicted for Sunday afternoon and evening, with 1 to 3 inches of rain expected with wind gusts up to 45 mph.

Debris flow concerns

California needs the rain, but any water that falls on burn scars could trigger mudslides and debris flows, which would threaten nearby communities.

“For this first major storm of the winter season, we’re primarily concerned about short-duration high-intensity rainfall,” Jason Kean, a research hydrologist with the U.S. Geological Survey who is assessing recent burned areas for debris-flow potential, wrote in an email.

Kean said he is most worried about the areas burned by the Dixie Fire and the Caldor Fire, although several other areas also have debris-flow potential.

The Weather Service in Hanford, Calif., for example, is warning of possible mud and debris flows for several fire areas in the southern Sierra, including the 2020 Creek Fire and the still-uncontained KNP Complex and Windy fires, which were ignited by lightning in September and have burned through sequoia groves.

The Alisal Fire zone near Santa Barbara, Calif., also is susceptible, although rainfall rates that would cause significant debris flows are not now expected there, according to the Weather Service office in Los Angeles.

“The highest risk is focused between the Dixie and Caldor burn areas where the heaviest rainfall is forecast,” wrote the Weather Service in Sacramento. “Nearly all of these burn areas have yet to be tested with such a magnitude of a storm, and the forecast rainfall is deeply concerning. Even if there were no burn areas to be concerned about, this is still far too much rainfall forecast in such little amount of time. For those living near and downstream of burn areas, have a plan in place to evacuate if necessary.”

Evacuation warnings were issued for areas that were burned in the 2020 CZU Lightning Complex Fire in Santa Cruz County.

The high-risk zones are places where fires burned at moderate or high severity on steep slopes; they do not encompass the entire fire footprint.

Wildfire burn scars typically have water-repellent soils, and bursts of rain can lead to runoff that picks up loose material and eventually becomes a debris flow, carrying ash, soil, trees, rocks and boulders rapidly downslope.

“It’s great news this system is bringing fire-season ending rainfall and will put a dent in the drought,” Kean said. “We just hope the rain doesn’t come all at once.”