A strong atmospheric river-fueled storm arrived in California, with heavy rain drenching many regions and flooding in some hard-hit areas.

The atmospheric river is being driven by a rapidly intensifying low-pressure system over the Pacific Ocean, which has met the meteorological criteria for a “bomb cyclone.” Unlike the bomb cyclone that made landfall over San Francisco in March 2023, this week’s storm will spin up hundreds of miles away from California.

Humboldt and Siskiyou counties were pounded by wind and snow, enough to force the closure of Interstate 5 near California’s border with Oregon. In the Bay Area, the heaviest rain fell in Sonoma and Napa counties.

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Rain finally made its way south of San Francisco, albeit in the form of some light showers. Oakland and Milbrae picked up about one-tenth of an inch of rain, but south of there no measurable precipitation was recorded.

It's been a completely different story across the North Bay. Santa Rosa recorded almost 6 inches of rain on Wednesday. The hills around the Petaluma, Sonoma and Napa valleys received anywhere from 3 to 7 inches of rain in just 24 hours.

Overnight, the rain won't completely go away, but its coverage will become a bit more scattered. Expect bursts of moderate to heavy rain to continue across the North Bay, especially in areas with any kind of elevation. For the rest of the Bay Area, showers will be intermittent overnight and not much additional accumulation is expected.

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But we are far from the conclusion of this atmospheric river event. Things get recharged on Thursday as a new low pressure system forms and many of the same locations that were hit hard today with rain and snow will see more heavy accumulations on Thursday.

A stalled front associated with a strong atmospheric river is bringing heavy rain and strong winds to the North Bay, creating low visibility and challenging flight conditions, according to a Wednesday afternoon update from the National Weather Service. At San Francisco International Airport, light rain and gusty southeast winds are expected through Thursday. Monterey Bay will experience mostly clear skies with occasional showers and gusty winds, particularly in the Salinas Valley. On the water, strong winds and rough seas up to 20 feet will move southward, with gale-force winds expected to persist into Friday and Saturday.

U.S. Sens. Alex Padilla, D-Calif., and Lisa Murkowski, R-Alaska, have proposed a bipartisan bill to boost atmospheric river forecasting, enhancing flood risk reduction and emergency readiness. The Improving Atmospheric River Forecasts Act mandates the National Oceanic and Atmospheric Administration to form a specialized forecasting program. Atmospheric rivers, responsible for most Western U.S. floods, may increase due to climate change, potentially costing billions in damage by 2090. The bill ensures every West Coast state, including Alaska, has an atmospheric observatory, advancing storm forecasts, improving public safety and facilitating more effective water management. “For the past several years, California communities have witnessed firsthand the ongoing threat of destructive flooding caused by intense and frequent atmospheric river storms,” said Padilla. Additionally, the legislation supports technological advancements and data integration to enhance forecast accuracy and communication about flood hazards.

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Heavy, wet snow is expected to blanket the Cascades and far Northern California, with snowfall rates of 2 to 3 inches per hour and wind gusts reaching 65 mph, creating whiteout and “blizzard conditions” that will make travel nearly impossible at pass levels, according to a late Wednesday update from the National Weather Service. As Pacific moisture continues to flow into the region, snow levels will rise, shifting the heaviest snowfall to higher elevations and bringing rain to lower elevations. “Persons should delay all travel if possible,” the weather service said in a winter storm warning. “If travel is absolutely necessary, drive with extreme caution and be prepared for sudden changes in visibility.”

Downpours caused creek flooding in Sonoma County on Wednesday afternoon. Colgan Creek near Sebastapol briefly reached moderate flood stage after waters rose 8 feet over the course of the day. Santa Rosa Creek at Willowside Road near Santa Rosa reached minor flood stage.

Wednesday brought moderate and strong rains at times to the North Bay. “Even if you have a light rain for a long enough time, you would see those kinds of rises,” said Rachel Kennedy, a meteorologist with the National Weather Service.

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A few Northern California rivers are expected to reach flood stage later this week. With their smaller surface area, creeks generally respond more quickly than larger, deeper rivers, Kennedy explained. The weather service also received reports of flooded roadways.

Several schools in Sonoma County will be closed Thursday because of severe weather, the county Office of Education said. They are:

Fort Ross Elementary

Kashia Elementary (also closed Friday, Nov. 22)

Monte Rio Union Elementary

Montgomery Elementary

Information and updates about closures can be found online at www.scoe.org/.

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The National Weather Service in San Francisco issued a high surf advisory Wednesday afternoon, in effect until 6 a.m. Friday, for San Francisco, the North Bay, Point Reyes National Seashore, the Peninsula, Southern Monterey Bay and Big Sur. Large waves ranging from 14 to 19 feet along southwest-facing beaches and 19 to 22 feet along exposed west-facing shores will create dangerous conditions for swimming and surfing, as well as localized beach erosion. The agency said sudden waves can sweep across beaches without warning, pulling people into the ocean from rocks, jetties or the shore. “Inexperienced swimmers should remain out of the water due to dangerous surf conditions,” the advisory said.

A fallen tree is blocking the southbound Highway 1 lane just south of the Bodega Highway near Bodega Bay, according to the CHP. The tree was reported at around 4:22 p.m. and no one was injured. Crews were on the way to clear the area.

A landslide occurred at 2:46 p.m. today, about two miles east-northeast of Larkfield-Wikiup in unincorporated Sonoma County, the National Weather Service reported. The slide blocked the westbound lanes of Mark West Springs Road near Redwood Hill Road. Local law enforcement is on the scene.

Due to this week’s atmospheric river storms and the inspection and clearing required afterward, Fairfax-Bolinas Road in Marin County will be closed from Meadow Club to Highway 1 until Dec. 1, the Marin County Public Works Department said Wednesday. The closure affects all traffic except active emergency vehicles.

The National Weather Service has issued an urgent small craft advisory for Monterey Bay, effective until 3 a.m. Friday. Mariners in the region should expect south winds ranging from 15 to 25 knots, with gusts reaching as high as 40 knots. These conditions pose significant hazards to small craft navigation. “Inexperienced mariners, especially those operating smaller vessels, should avoid navigating in hazardous conditions,” the advisory said.

In addition to rain and snow, the atmospheric-river-fueled storm brought windy weather to Northern California on Wednesday. Bay Area gusts topped 70 mph on some mountain peaks in Marin County, said National Weather Service meteorologist Brayden Murdock. Generally, there haven’t been too many gusts over 50 mph, with gusts at lower elevations mostly around 20 to 35 mph, Murdock added.

The weather service has received reports of downed trees and reports of unsecured objects in roads. A wind advisory remains in effect through 6 a.m. Thursday, with strong winds still possible, especially in the North Bay. “Areas that have already been seeing winds are going to continue seeing winds,” Murdock said. “And then as we go deeper into Thursday and Friday, we’ll start to see the stronger winds push further off towards the south.”

— Jack Lee

The atmospheric river had bifurcated into two branches as of Wednesday afternoon, with one stream focused over the North Bay and stretching back into the Sacramento Valley and the other stream focused over Oregon. Just having the atmospheric river pointed in your direction does not necessarily mean it results in heavy rain or snow. Another mechanism is needed to “activate” the rain potential. Up until now, that mechanism has been the bomb cyclone spinning off the Pacific coast, but as we transition into Wednesday evening and Thursday, we will be keeping an eye on small “waves” of instability embedded within these atmospheric rivers as they ride along the moisture boundary with an impact on where the atmospheric river settles and how much precipitation gets squeezed out.

At midafternoon Wednesday a bit of a lull occurred in the steady rain across the North Bay. It was expected to last for a few hours before moderate to heavy rain reloads and moves back in sometime after 5 p.m. The downpours could even spread down into San Francisco and areas to the south that have not yet seen much rain so far, and persist through much of the evening.

The North Bay has been drenched, with a widespread 2 to 5 inches of rain extending back into Sonoma and Napa valleys, where a flood advisory has been issued as creeks and streams start to overflow. Santa Rosa has recorded up to 3.3 inches of rain.

The biggest rain totals continue to be in the Marin County hills, where 5 to 7 inches of rain has fallen in some spots. South of the Golden Gate, there has been little rain to speak of. Totals are generally ranging from 0.01 to 0.15 inches of rain in San Francisco and Oakland, with no measurable rain from about San Mateo/San Leandro and stretching to the south through the Santa Clara Valley.

Caltrans reports that chain controls are currently in effect on I-80 in the Sierra Nevada. “Chains required on all vehicles except four-wheel/all-wheel drive vehicles with snow tires. Eastbound Cisco Grove to Truckee, Westbound Truckee to Eagle Lakes. Minimum restrictions for trucks,” the agency said in an advisory.

The atmospheric-river-fueled storm bringing rain and snow across Northern California on Wednesday is expected to linger for days, bringing continued risk for flash floods.

The latest update from the Weather Prediction Center calls for a high risk for excessive rainfall Thursday in northwestern California, including Eureka. This category, shown in purple in the map below, means widespread flash floods are expected.

This risk decreases Friday, though there’s still a slight risk for excessive rainfall in Eureka, Redding and Santa Rosa, shown in yellow below.

— Jack Lee

The National Weather Service issued a flood advisory for Sonoma County until 8 p.m. Wednesday due to excessive rainfall. The areas affected include northern and central parts of the county, specifically the Sonoma coastal ranges and the 101 corridor, including Santa Rosa. The agency said Santa Rosa Creek and Colgan Creek are experiencing rises due to the heavy downpour. Residents in impacted locations, including Santa Rosa, Rohnert Park and Windsor, are advised to avoid flooded roads. “Turn around, don’t drown when encountering flooded roads. Most flood deaths occur in vehicles,” the weather service noted. It added, “In hilly terrain, there are hundreds of low water crossings which are potentially dangerous in heavy rain. Do not attempt to cross flooded roads. Find an alternate route.”

On Tuesday night, Golden Gate Ferry canceled all ferry service to and from Tiburon save for three trips that were able to run before 8:40 a.m. Wednesday while the water was calmer. None of the ferry operator’s other routes — including Sausalito and Larkspur — were affected, a spokesperson told the Chronicle. The operator was monitoring wind forecasts and was “hopeful” that it would be able to restore Tiburon service starting Thursday morning. As of Wednesday afternoon, Golden Gate Ferry did not anticipate any other service changes for the rest of the week, according to the spokesperson.

— Megan Munce

Here are the latest active weather warnings for counties north of the Bay Area:

Wind advisories have been issued for coastal and interior Mendocino County. Farther inland, wind advisories are in effect for Lake and Napa counties and the Sacramento Valley. While gusts in these areas may not reach the same extreme levels, the sustained winds could still disrupt commutes and cause isolated power outages.

A winter storm warning is in place until Wednesday at 10 p.m. for the following regions: Shasta Lake Area / Northern Shasta County; Burney Basin / Eastern Shasta County; Mountains Southwestern Shasta County to Western Colusa County; Western Plumas County/Lassen Park; West Slope Northern Sierra Nevada. Snowfall in these areas could complicate travel over higher passes and mountain roads.

Rain and snow falling in the Klamath Basin, along the California-Oregon border, is so far thought to be a boon for the recent dam-removal project on the Klamath River.

Officials at the Klamath River Renewal Corporation, which was responsible for demolishing four dams and restoring the natural flow of the river, say the increase in basin runoff could wash sediment into the waterway, increasing turbidity, but not enough to be a problem.

Hopefully, the sediment, they say, will wash downstream to parts of the river that have been starved of sand and gravel. Meanwhile, the cold, abundant water filling the channel will help salmon migrate upstream, to areas where the dams had previously blocked fish passage.

Over 7,700 customers are without power near Truckee, including areas around Donner Lake and Tahoe Donner, according to the Truckee Donner Public Utility District.

The agency reported the outages just before noon and warned that repairs could be delayed due to an active public safety outage in Reno. “This is a winter storm outage,” the utility noted.

As of Wednesday morning, San Francisco International Airport reported 47 cancellations and 203 delays, as severe weather conditions disrupted travel at one of the region’s busiest airports. The storm, which brought heavy rain, high winds and limited visibility, disrupted operations across several major airlines, including United, American Airlines, Delta, Southwest, Alaska, Frontier and Breeze Airways. With flights either grounded or diverted, passengers faced longer-than-usual travel delays.

Airport officials said that with the storm system expected to linger, travelers should prepare for more disruptions.

Nearly 4,200 Santa Clara County households and businesses were without power Wednesday afternoon, representing the greatest concentration of mostly storm-related blackouts in the Bay Area, Pacific Gas and Electric Co. officials said.

PG&E crews in Humboldt County reduced the number of customers without power to 3,758, down from 5,094 Wednesday morning.

Falling branches and trees, as well as other debris thrown into power lines by stormy winds, were the most common cause of power problems, PG&E spokesperson Tamar Sarkissian said.

The utility company had extra crews on hand to respond quickly and get the lights back on.

Other Bay Area counties with significant numbers of power outages Wednesday morning included 569 customers in Sonoma and 516 in Contra Costa.

Search outages by address with the Chronicle’s PG&E outage map.

— Julie Johnson

The Weather Prediction Center called for a moderate risk of excessive rainfall Wednesday across parts of northwestern California, including in Ukiah.

This area is shown in red in the map and indicates where flash floods are likely. Parts of San Francisco face marginal risk for excessive rainfall, meaning isolated flash floods are possible.

The heaviest rain remains concentrated over Marin, Sonoma, and Napa counties, where 3 to 4 inches have already been recorded across the Marin coastal terrain. Notably, a remarkable 5.44 inches has fallen near Austin Creek State Recreation Area. Across the North Bay, rainfall totals range from 2 to 3 inches in both the valleys and the hills of Sonoma and Napa, slightly exceeding earlier forecasts.

Winds have been howling as well, with gusts ranging from 25 to 45 mph across much of the region. Elevated locations above 1,500 feet in the North Bay have reported gusts exceeding 55 mph, adding to the storm’s intensity and contributing to difficult travel conditions and localized tree damage.

New model guidance raises concerns about a plume of moderate to heavy rain lingering over Marin, Sonoma, and Napa counties through Friday. Higher elevations could see upwards of 10-15 inches of rain, while valleys and cities like Santa Rosa may accumulate between 5-10 inches.

Meanwhile, rainfall across the Peninsula, South Bay, and East Bay remains much lighter, with most areas measuring less than 0.25 inches so far. The Santa Clara Valley and areas south to Santa Cruz have seen little to no rain. However, moderate rain is expected to develop this afternoon as the atmospheric river shifts slightly southward.

Robert Honeyman has experienced his share of snowstorms after 23 years of living in Mount Shasta. But the 57-year-old mountain man was blown away by Wednesday’s blizzard conditions.

“This is a winner,” Honeyman said. “This will be talked about for a while. Even locals are going, ‘Oh yeah! ’”

With his beard covered in an icy glaze, Honeyman helped stranded travels dig out their buried vehicle in Weed, where he stayed for the night while his truck was in the shop.

“We’ve gotten it pretty bad before, but what’s making this one so dangerous is not necessarily the snowfall amounts, it’s just a long duration storm,” Honeyman said.

— Anthony Edwards

Wednesday’s storm has led to full closures on Interstate 5 in Northern California. Southbound I-5 near Edgewood in Siskiyou County is completely shut down with no estimated reopening time, according to Caltrans.

Read more here about closures to I-5.

Remarkable imagery from NOAA satellites showed the formation of the bomb cyclone bringing rain and whipping winds to Northern California.

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The California Nevada River Forecast Center expects a few rivers to surpass flood levels as up to 15 inches of rain falls in some parts of Northern California over the next three days.

The Eel River is forecast to reach moderate flood stage at Fernbridge (Humboldt County) and the Russian River is expected to reach minor flood stage at Hopland (Mendocino County).

— Jack Lee

Heavy precipitation is expected across Northern California over the coming days. Look up your rainfall forecast in this interactive map and find out more about the timeline of impacts in the latest Chronicle forecast.

— Jack Lee

A map from Iowa State University provides preliminary data on which Bay Area roads could be flooded on Wednesday morning. According to the map, flooded roads were reported in Sonoma, Santa Clara, Marin, Alameda, San Francisco and San Mateo counties.

See the map of the Bay Area’s flooded roadways here.

The heaviest rain remains focused from San Francisco northward to Point Arena, extending inland across the Sacramento Valley from Redding to Yuba City. The North Bay valleys and the higher terrain in Sonoma and Napa counties are seeing some of the most intense rainfall. Areas around Lake Sonoma have recorded the highest totals so far, with over 1.5 inches falling in just the last three hours. Healdsburg, Santa Rosa, and Rohnert Park have each measured more than an inch of rain in that same period.

South of the Golden Gate Bridge, rainfall has been lighter so far, with the southern edge of this atmospheric river holding steady just north of the city. Over the next few hours, the axis of heavy rain is expected to drift slightly south, bringing steady rain to the Peninsula and South Bay by the afternoon.

Ezequiel Herrera pulled into Hensel’s Materials about 10 a.m., looking for roof shingles.

He was doing a favor for his brother, who awoke at 3 a.m. to go to work, only to find the heavy winds blowing shingles off the roof of his garage.

The storm hadn’t damaged Herrera home, he said, but he had lost power for several hours Tuesday night.

Now, he and his brother were going to have to reroof the garage — just a year after installing a new roof.

“I don’t know about this weather,” he said, as drops began to fall.

“People don’t put roofs on in this weather unless it’s an emergency,” Hensel’s Materials owner Ken Canepa said, moments after Herrera walked out.

— St. John Barned-Smith

A winter storm warning remains in effect until 10 p.m. Wednesday for several regions in Northern California, including the Shasta Lake area, Burney Basin and the Northern Sierra Nevada.

Heavy snow was expected, with total accumulations ranging from 10 to 20 inches above 4,500 feet and up to 4 feet in higher elevations. Snow may also accumulate as low as 3,000 feet in Shasta County.

Winds gusting up to 55 mph could create hazardous conditions, with snow-covered roads and possible tree damage, according to the National Weather Service. Travel is likely to be difficult or impossible, especially during evening commutes, with chain controls expected on many roads. Snow levels will gradually rise throughout the day, reaching 5,000 to 6,000 feet over the Sierra.

In Humboldt County, officials had been monitoring severe wind that hit Tuesday night.

“The impacts were less severe than anticipated,” Emergency Services Director Ryan Derby said. Some homes had lost power, with about 6,300 Pacific Gas & Electric Co. still without electricity Wednesday morning.

The power utility “seems like it’s staying on top of things fairly well,” Derby said.

Though the winds had not been as severe as anticipated, officials were also concerned about significant rain expected to fall across the county over the next three days — along with another bout of wind later in the week that he worried could topple trees sitting in waterlogged soil.

Multiple rivers were at flood stage, Derby said, and could overflow their banks — including the Mad River in Arcata and the Eel River. Derby said he was also monitoring the Van Dusen, Elk and Mattole Rivers.

“We’re about as well prepared for this thing as we could be at this point,” he said.

— St. John Barned-Smith

Approximately 6,336 customers remained without power on Wednesday morning, according to the Pacific Gas and Electric Company’s power outage map.

The cities and areas seeing the most outages were Unincorporated Humboldt with 5,094 customers without power, Eureka 491 customers, Arcata 442, Trinidad 267, Bayside 41 and Blue Lake 1.

According to PG&E, about 10% of customers in Humboldt County were without power. Restoration times were not available on PG&E’s website.

More than 500,000 customers were without power in Washington state as a major storm swept across the Pacific Northwest on Wednesday morning, according to PowerOutage.us.

The areas most affected by the outages were Ferry, King and Snohomish counties, according to the power outage map.

About 1.32 inches of rain fell at Sonoma County Airport since midnight, with a half an inch between 8 a.m. and 9 a.m. Light rain pushed south into San Francisco, which should continue off and on throughout the day.

The Golden Gate Bridge was expected to be the cutoff between light showers and steady moderate to heavy rain. However, if the atmospheric river shifts 10 miles to the south, San Francisco could see more rainfall. San Jose could remain completely dry Wednesday.

— Anthony Edwards

The atmospheric river-fueled storm has begun impacting the Bay Area, with moderate rain now falling in San Francisco. Check live storm patterns on our real-time map.

Rainfall amounts in San Francisco, Oakland and the interior East Bay are forecast to be between 0.5 and 1.5 inches, with slightly higher totals in the East Bay hills. In the South Bay, including San Jose and Santa Cruz, rainfall will generally range from 0.25 to 0.5 inches, except in the Santa Cruz Mountains, where 1 to 2 inches are expected.

The National Weather Service has issued a flood watch for the North Bay, including Sonoma, Napa and Marin counties, effective through 4 a.m. Saturday due to the potential for heavy rainfall. The warning alerts residents to the possibility of flooding along creeks, streams, rivers and urban areas, with hazardous water ponding on roadways and low-lying regions. Authorities caution that conditions could lead to dangerous travel disruptions, including flooded low-water crossings and rising water levels in local water bodies.

“It’s raining and it’s going to continue,” said National Weather Service meteorologist Cindy Palmer. “We have issued an urban and small stream flood advisory for Santa Rosa proper. The ground is saturated and we’re starting to see runoff. Needless to say at this point of time we are starting to see rises on rivers. As far as wind gusts, we have seen quite a few gusts of 40 to 50 mph in the coastal range of Sonoma County.”

A flood watch has been issued for areas in the North Bay, including Marin, Sonoma and Napa counties, where rainfall could exacerbate existing risks of creek and river overflow.

Wind advisories have been issued for coastal areas of San Francisco, Marin and Sonoma counties, which are expected to face southerly gusts reaching up to 45 mph, potentially causing power outages and downed trees and branches. The advisories are in place until Thursday morning, with the highest winds expected Wednesday.

On Tuesday evening, the storm system brought misting rain and bitter, cold winds to the Avenue of the Giants and communities across Humboldt County.

The state’s office of emergency services stationed more than a dozen fire trucks and other rescue vehicles across the county, on top of stations' regular deployment, officials said.

In the southern part of the county, severe winds threatened to bring down trees, blocking roads and wrecking power lines, said Shelter Cove Fire Chief Nick Pape.

“We’re concerned about public safety, keeping the roads open,” he said.

In and around Eureka, all volunteer fire stations were fully staffed, said Talia Flores, spokesperson for the Humboldt Bay Fire Department. Also staffed is a water rescue team, in case any residents get caught in high water, as happened during winter rains earlier this year.

Like Pape, she said first responders would be available to respond to downed trees and power lines as well as more typical 911 calls such as vehicle crashes or medical emergencies.

She asked locals to stay inside if possible, aware of high water, and have generators ready, in case of power outages.

County officials and those in local municipalities had provided thousands of sand bags, sand and gravel.

In Eureka, the looming storm made Hensel’s Materials owner Ken Canepa recall one of the worst deluges the region has ever endured, the Christmas Flood of 1964. That storm brought 15 inches of rain in 24 hours, resulting in 47 deaths across California, Idaho and Oregon and property damages at what today would be valued at nearly $4 billion.

“This isn’t going to be bad as that,” said his business partner, Ken Canepa.

Still, the business had already sold 2,000 to 3,000 sandbags along with other anti-flooding tools.

In Arcata, Melinda Bacon ducked into Kebab Cafe to pick up a gyro. She and head cook Drea Taylor chatted briefly, wondering about how bad the storm would be.

The restaurant was closing early, and Octavia Perez recalled the summer of 2019, after fires left residents stranded without power, searching for gas.

She’d stocked up on blankets, found a lantern and headlamps.

“I live in a pretty drafty apartment,” she said, with a laugh.

Without power, the restaurant would stay closed on Wednesday and Thursday, they said, and another employee would bring a generator to power the refrigerator. But “if we lose power we keep the doors closed,” Taylor said.

Bacon, who moved to the area in 2006, had charged the batteries of her trailer, and had two chainsaws and generators ready, in case high winds took out power. She and her cattle dog, Ranger, would be fine, she said.

“I’m probably better prepped than most,” she said, with a laugh.

A safety inspector with the U.S. Coast Guard, she was thinking more about the high winds and the high seas.

“Nobody’s going out in this,” she said.

— St John Barned-Smith

The system bottomed out and reached its peak strength on Tuesday evening, dropping to a minimum central pressure of 942 millibars, similar to some weak hurricanes. (Normal air pressure, or the force exerted by the weight of the atmosphere, is about 1,013 millibars.) This would make it a “triple bombogenesis,” or a system that is at least three times the threshold to qualify as a bomb cyclone, and the strongest storm on record in this part of the Eastern Pacific.

A buoy offshore from Vancouver recorded a 101 mile wind gust on Tuesday night.

Bomb cyclones form when a clash of very cold and moist air masses forces the moist air to rise, generating clouds and precipitation. This rising air then causes a sharp drop in air pressure at the center of the system, the beginning of a powerful, counterclockwise-spinning storm.

Powerful winds and heavy snow resulted in power outages and store closures throughout Weed on Wednesday morning. Travelers took refuge in a Comfort Inn lobby off Interstate 5, where the power was out and blowing snow pushed through the main entry doorway.

Pacific Power reported nearly 1,500 customers without power in Weed at 7 a.m. Wednesday. Interstate 5 was closed in both directions, from 10 miles north of Redding to 21 miles south of Yreka.

7:06 a.m. | Atmospheric river takes aim at North Bay

The frontal boundary associated with the atmospheric river dropped toward the North Bay overnight, with precipitation extending just south of Petaluma at 6 a.m. Rainfall totals have exceeded an inch in the higher elevations of Sonoma County. Some mountainous locations in Humboldt County have measured more than 2.5 inches of rain in the past 24 hours.

Bay Area impacts are not forecast to be equally distributed Wednesday. The High Resolution Rapid Refresh model predicts an additional 2.5 inches of rain at Sonoma County Airport by midnight, but less than 0.1 inches at San Francisco International Airport, just 65 miles south. If the moisture plume and frontal boundary move just 10 to 20 miles south of current predictions, San Francisco and Oakland could receive considerable precipitation Wednesday.

— Anthony Edwards

Thousands of Pacific Gas & Electric Co. customers were without power Wednesday morning as a storm fueled by an atmospheric river barrelled down on Northern California.

The Chronicle’s PG&E outage map showed snaking clusters of customers without power in Humboldt County, beginning in Eureka and extending north to Arcata, McKinleyville, Trinidad and Orick.

PG&E said Tuesday that the utility company had “mobilized crews and resources in key regions to respond to potential storm-related damage and power outages.”

While there were pockets of power outages in the Bay Area Wednesday morning, it wasn’t immediately clear if they were storm-related.

In Washington state, a woman was killed on Tuesday night when a tree fell on a homeless encampment. Two were also injured when a tree fell on a trailer.

The National Weather Service has issued flood watches for the North Bay, North Coast and Sacramento Valley starting Tuesday night. About 1.4 million Californians live in places that face a high chance of flooding each year, according to the California Department of Water Resources. Climate change is driving more extreme storm activity, making it essential for communities to understand their flood risk. Mitigation measures, such as building floodgates and retrofitting homes, not only can save lives, but also prevent property loss, according to a 2019 report by the National Institute of Building Sciences.

Go here to see our interactive map of which California neighborhoods are especially vulnerable to flood impacts year after year, according to data from First Street.

On Tuesday, rain and snow fell over Shasta and Siskiyou counties in the system’s warm sector, then conditions rapidly deteriorated Tuesday evening along Interstate 5 near Mount Shasta as the cold front hit.

According to CalTrans, Interstate 5 closed to southbound traffic 21 miles south of Yreka in Siskiyou County on Tuesday evening, and chains were required on all vehicles except 4-wheel-drive vehicles with snow tires along several stretches of the highway.

Snowfall totals could range from 1 to 3 feet from Tuesday night through Wednesday afternoon in Weed and the city of Mount Shasta, with extreme winds gusting up to 70 mph. The National Weather Service has issued a high wind warning and winter storm warning, urging against travel and due to “whiteout” and “life-threatening” conditions.

“The hazardous conditions could impact the Monday evening and Tuesday morning commutes,” the weather service wrote in its winter storm warning. “Very strong winds could cause extensive damage to trees and power lines. Roads, and especially bridges and overpasses, will likely become slick and hazardous.”

Temperatures were expected to be close to freezing in Mount Shasta, resulting in a heavy, wet snow.

Farther south, the system’s warm sector was expected to push the snow line above 9,000 feet in the Sierra Nevada by Wednesday afternoon. A few inches of snow was forecast in the Tahoe Basin on Tuesday night, but a gradual transition to rain was expected Wednesday.

The lower confidence factor in all of this is the snow levels,” the Reno weather service office wrote in an online forecast. “With the initial wave pushing in on Wednesday, the snow levels will start quite low (near 5,000 feet), but then are projected to rapidly climb (near 9,000 feet) late Wednesday with the arrival of the mild, subtropical moisture associated with the atmospheric river.”

Snow levels were predicted to crash again Friday, with 1 to 2 feet of snow likely above 7,000 feet in the Sierra this weekend.

— Anthony Edwards

Chronicle meteorologist Greg Porter surveyed the satellite imagery of bomb cyclone and then broke down the elements of this storm system that are combining to create “a feast of weather unlike anything we’ve seen so far this season.”

See the images here.

Atmospheric rivers are bands of moisture that can flow for thousands of miles from the tropical Pacific, near Hawaii, all the way to the West Coast. Even though you can’t see them, these rivers in the sky transport colossal amounts of water around the planet.

When that moisture falls as rain, it can have devastating consequences. Scientists believe that a series of atmospheric rivers caused the Great Flood of 1862, a megaflood that left 6,000 square miles of California’s Central Valley underwater. Atmospheric rivers cause about $1.1 billion in flood damage annually across the western United States.

But atmospheric rivers also provide important benefits.