Update, 10:30 p.m.: Can't resist one last update for the evening to pass along the National Weather Service's summary of the remarkable continuation of Thursday's semi-mind-boggling storm. Here's a key passage from the NWS Bay Area forecast discussion tonight (with Teletype-style all caps retained for emphasis):

MODERATE TO HEAVY RAIN CONTINUES TO FALL ACROSS THE REGION WITH RAINFALL RATES ANYWHERE FROM A TENTH TO A HALF INCH PER HOUR. HEAVIER HIT AREAS IN THE SANTA CRUZ MOUNTAINS AND THE SANTA LUCIA RANGE SAW RAINFALL RATES FROM 1 TO NEARLY 3 INCHES PER HOUR EARLIER TODAY. FLORES CAMP LOCATED IN THE SANTA LUCIA MOUNTAINS REPORTED 2.84 INCHES IN ONE HOUR FOLLOWED BY ANOTHER 1.56 INCHES THE NEXT HOUR. THIS PROMPTED A FLASH FLOOD WARNING NEAR BIG SUR THIS AFTERNOON. CONTINUED MODERATE TO HEAVY RAIN PROMPTED THE ISSUANCE OF NUMEROUS FLASH FLOOD WARNINGS AND URBAN AND SMALL STREAM FLOOD ADVISORIES THROUGH THE EVENING HOURS.

Did you catch that? One spot in the Santa Lucias -- the mountain range rising high above the Big Sur coast -- had 2.84 inches in one hour and 1.56 inches the next. Arithmetic: 4.40 inches in two hours.

For comparison, San Francisco has recorded 3.21 inches of rain for the day through 10 p.m. If I'm reading the weather records right, that makes it one of the five rainiest December days in the city's weather history going back to 1849. And down there in the Santa Lucias, almost that much fell in just 60 minutes. It's a little challenging to imagine the intensity, or the result in terms of runoff.

Update, 5 p.m.: Today's storm has turned out to be everything weather forecasters and their computer models predicted, and more. The storm has brought widespread flooding to the Bay Area and blacked out more than 150,000 PG&E customers. And the weather system is showing some reluctance to move on, having stalled over the Central California coast, meaning moderate to heavy rain is continuing in much of the region.

The deluge has caused hundreds of incidents of street and highway flooding from Sonoma County down to Santa Cruz County.

One of the worst-hit locations was Healdsburg, on the Russian River about 65 miles north of San Francisco. Although the river has not exceeded flood stage, a downpour that began before dawn and lasted most of the morning led to street flooding in downtown and some other low-lying areas. For instance, this scene from a shopping center at the south end of downtown:

As the dousing continues -- weather radar shows moderate to heavy rain continuing to stream into the central Bay Area and South Bay -- the weather service has posted a flash flood watch for the entire Bay Area.

Jan Null, consulting meteorologist with Golden Gate Weather Services, said the storm's progress south through the region slowed to a crawl Thursday afternoon, as a "wave" -- a sort of secondary low-pressure center -- has formed along a cold front moving south across Central California.

"The wave has slowed it down, and now it's pretty much just sitting there," Null said. "The super-intense stuff has passed, but this is not going anywhere anytime soon."

In Sacramento, California's Office of Emergency Services is keeping an eye on the storm and flooding. Spokesman Kelly Huston said early Thursday that the agency is ready to handle even the most dangerous rescues.

"What we have been doing is planning on moving our resources for swift-water rescue teams into areas in which we think we'll have problems with flash-flooding," Huston said. "And those teams are specially prepared to basically respond immediately to anybody who gets caught in these creeks and we need to pull them out of it."

As to the power situation, PG&E has not released its overall estimate of Bay Area outages since late Thursday morning, when the utility reported about 150,000 homes and businesses had been without power. The biggest number of outages was in San Francisco, where about 100,000 customers lost electricity as the heart of the storm hit the city.

For a historical comparison, the big storm of January 2008 knocked out power to about 2 million customers. Another big storm in October 2010 put out the lights in about 1.2 million homes and businesses.

Update, 11:15 a.m.: National Weather Service flash-flood warnings still cover much of the region from Sonoma County down to Santa Cruz County, virtually all of the Bay Area: Sonoma, Napa, Marin, Contra Costa, Alameda, Santa Clara, Santa Cruz and Monterey counties.

The timing for the warnings:

• North Bay counties: through 12:15 p.m.
• Alameda and Contra Costa counties: through 12:15 p.m.
• Santa Clara, Santa Cruz and Monterey counties: through 1:45 p.m..
• San Francisco and northern San Mateo County: 9:30 a.m. (now expired)

It's possible the warnings will be extended.

Flooding has been reported throughout the region, but the North Bay appears to have been the hardest hit so far, with parts of downtown Healdsburg inundated. Sonoma State University has been closed for the day because of flooding on campus.

Meantime, PG&E reports the storm has knocked out power to about 150,000 Bay Area customers. The biggest and most disruptive outage is in San Francisco, where nearly 40,000 customers in a wide area from the Marina to the Financial District lost power about 7:20 a.m. About 50,000 other homes and businesses have been in the dark in scattered areas throughout the city.

The outage prompted BART and Muni to close their stations at Montgomery Street. BART has also closed the San Bruno station because of flooding.

Update, 7:40 a.m.: A major power outage is affecting large sections of San Francisco, from the Marina into parts of the Financial District. That's prompted the closure of both the BART and Muni stations at Montgomery Street.

Update, 7:20 a.m.: Extremely heavy rain is falling over southern Marin County and San Francisco, and weather radar is showing a swath of even more intense precipitation moving in from outside the Golden Gate. That's prompted the National Weather Service to issue a flash flood warning for Marin and southern Sonoma County.

The warning includes San Rafael, Novato, Petaluma and Rohnert Park and will be in place until 10:15 a.m.

Update, 6:50 a.m. Thursday: It's here.

So far, the storm has dumped more than 5 inches of rain in the hills of western Sonoma County through 6 a.m. Santa Rosa has gotten 2.79 inches. Just to the south, Petaluma has recorded 1.93 inches and Novato 2.05 inches. In the Napa Valley, where the Napa River is expected to crest near flood stage, St. Helena has gotten 3.15 inches. The rain totals are dramatically lower in the central Bay Area, with most totals a quarter of an inch or lower. But, as forecast, the heaviest rain has only just arrived over San Francisco and the East Bay.

As the deluge moves south, winds gusting over 70 mph have knocked down trees and power lines. PG&E is reporting scattered blackouts in the North Bay, East Bay and Silicon Valley -- with a total of about 20,000 customers affected before dawn.

The transportation impacts: 174 flights have been canceled at San Francisco International Airport this morning, about evenly split between arriving and departing flights. Morning ferry service from Oakland, Alameda and Vallejo to San Francisco has been suspended. High-wind warnings are in place for Bay Area bridges. On the plus side, the traffic volume on the Bay Bridge appears much lower than usual for the morning rush hour.

Update, 9:25 p.m.: Our much-anticipated storm continues to move in over the California coast. Parts of Northern California, including the watersheds that feed into the state's massive reservoirs on the upper Sacramento, Feather and Trinity rivers, are already getting heavy rain.

In the Bay Area, parts of Marin, Sonoma and Napa counties have also recorded significant rainfall. Novato, in northern Marin, has seen .79 inches, and Point Reyes Station, in west Marin, has gotten .71 inches since early afternoon. Mount Veeder, on the west side of the Napa Valley, has gotten .87 of an inch; Angwin, on Howell Mountain on the valley's east side, has gotten .88. Farther north, Venado -- a site in the Sonoma County hills west of Healdsburg -- has gotten 1.20.

Further south, we've seen only a spattering of rain so far.

Remember, forecasters say this evening's rain is just a precursor to a prolonged period of high winds and heavy rain beginning later tonight. A high wind warning takes effect at 10 p.m. for virtually all of Northern California. Winds have been picking up this evening, with 30 mph gusts already recorded on Mount Tamalpais and at Las Trampas Ridge in Contra Costa County. The forecast calls for sustained winds of up to 35 mph at lower elevations with gusts over 50 mph. Elevations above 1,500 feet could see sustained winds of 50 mph with gusts over 80 mph.

A period of extremely heavy rain is forecast to start around 4 a.m. in the North Bay and move south through the region, reaching the South Bay early in the afternoon. With 2 to 4 inches of rain expected in lowland locations and up to 8 inches in the mountains north and south of the bay, the NWS has also issued a flash flood watch covering the entire region.

It goes without saying, almost, that the storm will affect travel throughout the Bay Area. With the strongest part of the storm still to come Wednesday night, some flights arriving at San Francisco International Airport are already experiencing delays of more than two hours. If the heart of the storm arrives as forecast early Thursday, the morning commute will be seriously disrupted.

Responding to the dire-sounding forecast, schools in San Francisco, Oakland, Berkeley and Alameda will be closed Thursday, as well as several districts in the North Bay. San Francisco State University and City College of San Francisco will also be closed for the day. (See NBC Bay Area's comprehensive list of closures).

Update, 3 p.m.: We're going to have to check on the last time this happened: Public schools in San Francisco, Oakland and virtually all of Marin County have canceled classes Thursday because of the approaching storm.

The closures were announced Wednesday afternoon as heavy rain arrived over the lower Russian River watershed in Sonoma County.

"Yeah, it's pouring," a Sonoma County Sheriff's Office spokeswoman in Guerneville said at 3 p.m. "It's been going for about 45 minutes."

Forecasters say the heart of the windy, wet weather system is on track to roar into the North Bay Wednesday night and roll south through the Bay Area Thursday morning.

Acknowledging the disruption that school closures will cause for thousands of families, both Oakland and San Francisco officials said their primary concern was student and staff safety:

• San Francisco: "Closing schools is a serious decision," said SFUSD Superintendent Richard A. Carranza. "I did not arrive at this lightly. First and foremost, we don't want to risk having our students injured or seriously delayed transporting to and from school. In addition to student absences, the storm could result in large numbers of staff absences, which could then lead to inadequate supervision of our students. Furthermore, power outages could affect the district's ability to feed students school meals, among many other operational challenges."
• Oakland: "What the National Weather Service refers to as a 'powerful Pacific storm' is expected to bring torrential rains, damaging winds and flash floods, peaking during the morning commute hours on Thursday. Approximately 5 inches of rain are expected from Wednesday evening through Thursday with gusts reaching up to 39 mph. Since this poses a significant safety risk for our students and staff, we’re canceling school as a precaution for Thursday. Other districts, like the neighboring San Francisco Unified School District (SFUSD) and all Marin County public schools, have also taken the step."

Original post: In less than two weeks, much of California has gone from anxiety over drought to worry about the possibility of widespread flooding. How did that happen?

Well, the rain that arrived at the end of November and concluded with heavy rain last Wednesday is the first part of the answer. While those storms didn't break the drought -- we'll need to keep saying that for a while -- they did bring enough precipitation to soak soils throughout much of the region. That means more water will run off when the next major weather system arrives, and flooding could result.

And now, that major storm is on our doorstep. The National Weather Service says the system, driven by fierce jet stream winds and funneling moisture from the subtropical Pacific, will batter virtually all of Northern California with prolonged heavy rains and high winds. An exception to that statement: the Sierra Nevada above 6,000 feet elevation, where precipitation will be in the form of snow. Forecasters have posted blizzard warnings for the west slope of the mountains from Yosemite up to Mount Lassen.

The weather service has issued a full bouquet of other advisories, watches and warnings connected to the storm, including a flash flood watch, a high wind warning and a high surf advisory.

A chance of rain is forecast later today in much of Marin and Sonoma counties, around San Francisco and Oakland by 10 p.m., and early Thursday morning in San Jose. The main body of the storm is predicted to move through the region from early Thursday through late Thursday night. Periods of very heavy rain are forecast as a cold front moves through the Bay Area -- across the North Bay before dawn, the central Bay Area during the morning rush hour, and the South Bay during the early afternoon. Rainfall totals are expected to be in the 1.5- to 3.5-inch range in Bay Area cities and from 4 to 8 inches in mountains north and south of San Francisco Bay.

Areas throughout Northern California will see enough rain that rivers and streams will rise to flood stage around midday Thursday. The California-Nevada River Forecast Center says the Napa River will top flood stage at St. Helena and crest less than a foot below flood stage in downtown Napa. The Russian River is forecast to flood in Hopland, in southern Mendocino County, and in Sonoma County communities, including Guerneville. The rivers are predicted to rise and recede rapidly, dropping back below flood stage during the day on Friday.

But wait -- we haven't told you about the wind.

The NWS says the highest likelihood of damaging winds will arrive as a cold front passes through the North Bay before dawn Thursday, into the central Bay Area by 7 a.m., and south along the coast and into the Santa Cruz Mountains by early afternoon. Forecasters say urban locations could see gusts of 40 to 55 mph. Gusts could hit 60 mph along the coast and 80 mph on ridges above 1,000 feet in elevation.

Forecasters are comparing the potential impact of the imminent heavy weather to two relatively recent storms: one that struck Jan. 4, 2008, and another that hit Oct. 13, 2009.

Meteorologist Jan Null rates both those storms among the strongest to hit the Bay Area since the 1950s. What sorts of effects did they have?

The San Francisco Chronicle reported at the time that the January 2008 storm blew down "hundreds, if not thousands" of trees within the city limits. And, also according to the Chron, high winds and downed trees throughout the region knocked out power to as many as 2.1 million PG&E customers.

The October 2009 storm blacked out more than 1.2 million utility customers, the New York Times reported, adding this portrait of chaos by the bay:

Several major Bay Area roads, including Highway 101 and Interstate 580, were closed for much of the day by airborne construction materials and overturned vehicles, including five trucks that flipped on the Richmond-San Rafael Bridge, a major east-west thoroughfare spanning a northern finger of San Francisco Bay.

A downed tree on the tracks stopped BART rail service in the Mission District of San Francisco, sending evacuated passengers into the rain or onto buses. Morning ferry services across the bay were canceled as docked boats rocked like rubber ducks in a bath.

Scaffolding collapsed, breaking windows, taking down power lines and bringing electrically powered buses to a halt along at least one major San Francisco boulevard. People trying to make it to work dodged flying trash cans, orphaned umbrellas and dislocated newspaper vending machines.

Dozens of flights were canceled at the San Francisco airport, where winds topped 65 miles an hour at midmorning, making for even more flight delays than cancellations. Harrowing whitecaps from the bay lapped at the foot of the runways.