That's expected to be just an opening act. The storm is expected to spread a daylong steady rain over the region on Wednesday, broken up by intermittent heavy downpours. For a finale, the weather system is forecast to unload a second wallop of atmospheric river-fed rain early Thursday.

Those forecast conditions, on top of a landscape saturated by recent storms, have led the National Weather Service to issue a flash flood watch for the entire Bay Area from late Tuesday through late Thursday morning. A high-wind warning is in effect for higher terrain -- elevations of 1,000 feet and up.

Brian Garcia, the warning coordination meteorologist for the National Weather Service's Bay Area office in Monterey, said the combination of rain, soaked soil and high winds portends trouble.

"We're going to put some decent gusts on top of the saturated soil -- we'll probably see a lot of trees down and power outages," Garcia said.

NOAA's California-Nevada River Forecast Center's Coastal forecast rain totals through Friday are ranging from nearly 9 inches in northern Sonoma County to 5 inches in the Santa Cruz Mountains. Much of the North Bay is expected to receive 3 to 5 inches.

Totals at lower elevations are projected to range from 2-plus inches in San Francisco and Oakland to between 1.5 inches and 2 inches along the bay shore and East Bay valleys.

That heavy rainfall is expected to cause a rapid rise on the Russian River, which is forecast to crest about 2 feet above flood stage late Thursday. Flows are expected to spike on the Napa River, too, with the river cresting just below flood stage at St. Helena and downtown Napa.

Most of the rest of California is in for a thrashing, too. Of special concern are conditions in the Sierra Nevada and its western foothills, which have received extremely heavy snowfall in the past two weeks.

The incoming storm is expected to cause snow levels to rise from 2,000 to 3,000 feet Tuesday night to 6,000 to 8,000 feet during the day Wednesday. That means that for some part of the storm, rain will fall on the snowpack. That, in turn, could enhance the runoff down streams and rivers toward reservoirs and valleys below.

Many of the reservoirs are near or at the level at which dam managers may be required to begin releasing water to maintain enough room to accommodate floodwaters and runoff later in the season.

Serious flooding is not forecast -- yet -- though the river forecast center outlook shows sharp rises on the Sacramento River with water cresting above flood stage at several locations. Water is expected to begin flowing through the Yolo Bypass, the huge flood bypass channel that runs north and west of Sacramento, sometime on Thursday.

Heavy rain is forecast throughout Southern California, too, with amounts ranging from 2 to 5 inches.

Forecasters were still adjusting forecasts as the storm began to brush the coast on Tuesday afternoon.

NWS Bay Area's Garcia said that's because weather models have a hard time predicting exactly where the most potent part of an atmospheric river will reach land.

"That kind of granular detail doesn't really make itself known until all the factors really start to come together," Garcia said. "And the final detail, the core of the highest amount of precipitable water, is very narrow. That little core -- we don't really know where it's going to set up -- until it really starts raining and pouring in a certain area."

But atmospheric scientists who have been watching models over the last week are fairly certain about the broad details.

The Center for Western Weather and Water Extremes at UC San Diego's Scripps Institution of Oceanography, which recently released a new category system for atmospheric river storms, issued a forecast advisory Tuesday afternoon rating the incoming system as Category 4 (out of five categories), a "strong" storm event with high hazard potential.

The center noted, though, that it's still uncertain how potent the second round of the storm, due Thursday, will be or how long it will last -- both key factors in trying to assess the system's impact in advance.