SAN ANSELMO, CA - DECEMBER 10:  Staysea Colteaux (L) and Chris Furbee (R) set up sandbags and seal a vent in front of Dogville on December 10, 2014 in San Anselmo, California. The San Francisco Bay Area is bracing for a severe storm that is expected to bring high winds and heavy rain that could topple trees and cause widespread flooding. Urban areas could see up to 6 inches of rain in less than 24 hours.  (Photo by Justin Sullivan/Getty Images)
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Story highlights

NEW: Service on San Francisco's Muni Cable Car lines is suspended

Business owners in Northern California are prepping with sandbags; schools close

An "atmospheric river" is pumping moisture into the drought-stricken West Coast

Such "rivers" can branch off from bands of tropical moisture and be 250 to 400 miles wide

Affected by the storm? Send your photos to CNN iReport, but please stay safe.

(CNN) —  

It hasn’t rained much in California in recent years. But when it does rain, it apparently pours, as the state’s residents will find out Thursday.

From Crescent City to San Diego, plus many points inland, the Golden State is expected to be inundated with precipitation and hit with potentially perilous winds in spots. Forecasters expect some higher elevations north of San Francisco to get 10 inches of rain, a gushing fit for a tropical island, let alone a state that is almost entirely experiencing exceptional drought.

Given all the problems this drought has caused – draining reservoirs, threatening farmers, driving up food prices – most any wetness is welcome.

But when it comes down so much, so fast, it can be dangerous.

Thursday’s rain comes on top of what fell the previous evening, when parts of Northern California got half an inch per hour. And it’s going to stick around for a while, leading to flash flood watches through Friday evening in some parts of the state.

And the fact that this all comes as part of a powerful storm brings another set of problems: Parts of the Sierra Nevada, for instance, are under a blizzard warning amid forecasts of heavy snow in the mountain chain, coupled with winds that could gust as strong as 80 mph.

In fact, the majority of California and much of neighboring Oregon are under high wind warnings. Sustained winds of 15 mph and stronger, paired with gusts up to 70 mph, could knock down tree limbs, delay air travel and rock some of California’s signature bridges.

An ‘atmospheric river’

Much of this moisture, at least along the California coast, comes from an “atmospheric river,” a band of heavily moist air that split off from a larger such band at the tropics.

It’s as if a river in the sky broke its banks, sending a new tributary 250 to 400 miles wide northeastward to California. It will soak the state Thursday, and when it hits the mountains in its east, it will fall as snow – lots of it.

Such atmospheric river drenchings in California are rare but also normal and necessary, the National Weather Service said. They’re how the parched state gets up to 50% of its annual rainfall.

The current “river” has been around since late November and already soaked the state last week.

School canceled in parts of Bay Area

Still, the latest onslaught could be the worst yet. And residents and officials are taking precautions.

Several Bay Area districts closed schools on Thursday, some even on Friday.

Commuters and those just trying to get around the area were affected as well, with the San Francisco Municipal Transportation Agency announcing on its website the cancellation of ferry services and the suspension of service on the Muni Cable Car lines. Shuttle buses are running instead along the cable car routes “until further notice,” the agency said.

And even though the town creek in San Anselmo, a community about 20 miles north of San Francisco, was at a trickle Wednesday, local officials rushed to make sure people had sandbags and flood gates in case it breaks its banks.

In last Wednesday’s storm, it took San Anselmo barber Dan Bridges and his father hours on inundated roads to get to their barber shop.

“If it takes us three hours to get here and three hours to get home, we might as well stay home,” Bridges told CNN affiliate KNTV.

That’s why, this time, Bridges doesn’t plan to take any chances – and may even close for the day.

Many people in Northern California were bringing lawn items inside ahead of the storm. But not everything.

“I’m just trying to bring some stuff down that might blow away. We are not taking down the Christmas decorations though, leaving those up unless it gets real bad,” Ryan Higgins of Sunnyvale said.

Battering fronts

The series of fronts and the jet stream won’t just be battering places like San Francisco. They will also affect areas farther down the California coast in Big Sur and all the way up the West Coast in northern Washington state.

Folks in Oregon were expecting high winds Thursday, but in Seaside, 80 miles northwest of Portland, it was already blustery Wednesday.

“I was waiting for trees just to go blowing right by the beach,” Denise Abramson told CNN affiliate KATU. “It was really, really windy. Really bad. And the waves were probably …15 feet high coming in. The swells are coming in really, really fast.”

In fact, forecasters are warning that gales could push waves up over 20 feet high along the West Coast.

Fort Bragg in California will get hit from two sides, with up to 5 inches of rainfall by late Thursday and breakers up to 24 feet pounding the shores, the weather service said.

Mountain blizzard

Far from the shore, in Sacramento, there will be no respite from the storms and the wind, with many inches of rain and 80 mph gusts in the forecast.

And just a few miles farther inland, at frosty high elevations, the downpours will transform into a blizzard, decking the peaks of the Sierra Nevada in snow 6 inches to 2 feet deep.

The same whipping winds will blow snow into a blinding whiteout blur. Stay home, the National Weather Service warned residents.

“If you must travel, have a winter survival kit with you,” it urged. “If you get stranded, stay with your vehicle.”

The weather service warned that snow may trigger the closing of mountain passes for hours.

-

CNN’s Steve Almasy, Dave Alsup, Greg Botelho and Michael Martinez contributed to this report.