The notice was issued as a rich stream of atmospheric moisture remained nearly stationary, pointing a steady stream of heavy precipitation into the North Bay counties. The deluge, which has continued with only brief breaks since early Monday, is expected to continue into early Wednesday morning.

"If you live in an evacuation area, we want you to leave now," Sonoma County Sheriff Mark Essick said at a 5:30 p.m. media briefing. "Please don't wait until 10 o'clock tomorrow night" — when the river is expected to crest 14 feet above flood stage at Guerneville — "please evacuate now."

The warning affects 25 communities in all, listed here.

The county has set up evacuation centers at the Sebastopol Center for the Arts and at the Sonoma County fairgrounds in Santa Rosa. Free shuttle buses began running from the Guerneville Veterans Memorial Building to the centers Tuesday afternoon.

The sheriff said 25 extra deputies and two boats were being deployed to the lower Russian River area to help people leave an area where many roads were already inundated by Tuesday afternoon.

Sheriff's Office spokeswoman Misti Harris said that despite Essick's plea, compliance with the evacuation order was "a mixed bag" Tuesday evening.

"We're getting mixed results from the public," Harris said. "Some people are flat-out refusing to leave."

The evacuation order was issued as the National Weather Service forecast the worst flooding along the river in a generation — exceeding a storm crest of 45 feet reached in 1997 but below catastrophic surges seen during February onslaughts in 1986 and 1995.

The service's California-Nevada River Forecast Center predicted the Russian River would exceed flood stage at Guerneville at 6 p.m. Tuesday and crest at 46.1 feet late Wednesday night before receding below flood level on Thursday night.

The flooding — which has closed dozens of roads across Sonoma County — is the result of the storm stalling and unleashing nearly nonstop downpours north of the Golden Gate.

That pummeling is expected to continue past midnight Tuesday before the storm shifts to the south and east. The National Weather Service's Weather Prediction Center, in College Park, Maryland, issued an advisory late Tuesday forecasting heavy rain across the entire region before the system passes. The center said rain rates as high as three-quarters of an inch per hour may occur in the Santa Cruz Mountains early Wednesday.

Daniel Swain, a climate scientist at UCLA, pointed to one factor above all for the soaking the North Bay has gotten from the second atmospheric river to come ashore in the North Bay this month.

"What's interesting about this one is that it's not an extreme atmospheric river from an intensity perspective," Swain said. "It's certainly wet, but if we had only a few hours of this in any one place it really wouldn't be any problem at all. But the main issue is that it's an atmospheric river of moderate intensity that's lasting for many, many hours in the same place, and the accumulation of water over that period is really quite substantial."

The rainiest locations in the Russian River watershed have received more than a foot of rain since the storm's onset early Monday, with the rain gauge at Venado, west of Healdsburg, recording a total of 16.16 inches through 7 p.m. Tuesday.

Similarly impressive amounts have been recorded throughout the area. Warm Springs Dam, on Dry Creek northwest of Healdsburg, has reported 7.94 inches during the storm. Santa Rosa has received 5.73 inches; Mount Veeder, west of the city of Napa, has recorded 10.12 inches; and Yountville, on the floor of the Napa Valley, has gotten 6.69 inches.

The heavy rainfall started rapid rises on the Napa River, which is forecast to reach flood stage at St. Helena and the city of Napa, though not to the potentially disastrous levels forecast for the Russian.

The river forecast center said the Napa would crest late tonight at St. Helena at about 3 feet above flood stage. In the city of Napa, the river is expected to reach its peak about dawn Wednesday.

The rising water led to some road closures across the Napa Valley on Tuesday, but even with the river exceeding flood levels, major damage is not anticipated.

"We have seen these river levels in Napa before and we generally know what to expect," said Rick Thomasser, flood control operations manager for Napa County.

The rainiest locations in the Russian River watershed have received more than a foot of rain since the storm's onset early Monday, with the rain gauge at Venado, west of Healdsburg, recording a total of 15.36 inches through 6 p.m. Tuesday.

Nine school districts in Sonoma County have canceled classes Wednesday due to the high water: Alexander Valley Union School District, Forestville Union School, Guerneville School District, Geyserville Unified School District, Harmony Union School District, Monte Rio School District, Montgomery Elementary School District, Oak Grove Union School District and El Molino High School and West County Charter Middle School, both in West Sonoma County Union High School District.

Update, 12:40 p.m. Tuesday: The deluge that has targeted Sonoma, Napa and Marin counties for more than a full day is now threatening to trigger a disaster along the lower reaches of the Russian River in western Sonoma County.

Sustained downpours have led to a sharp rise on the river, which is expected to begin inundating low-lying areas around Guerneville by early Tuesday evening. The river is forecast to crest at 45.9 feet — nearly 14 feet above flood stage — late Wednesday.

If that outlook from the California-Nevada River Forecast Center is accurate — and it's driven by the expectation of continued heavy rain north of the Golden Gate — much of Guerneville and the nearby communities of Rio Nido and Monte Rio will flood.