The second front of a drenching three-bout storm made landfall Thursday and was expected to saturate the North Coast with 3 to 6 inches of rain overnight and through Friday.

The storm <NO1><NO>toppled trees across roadways and factored in more than a dozen vehicle crashes across Sonoma County on Thursday night. Most resulted in only minor injuries, a CHP spokesman said.

In advance of the soaking, the National Weather Service issued a flash flood watch for a large portion of Northern California, including the North Coast, saying small stream and creek flooding is possible Friday, and urban flooding likely in Petaluma and elsewhere with the arrival of a fresh storm.

More rain is expected to start Saturday. That third storm front could push the upper Russian River as much as 3 feet over flood stage in one spot of Mendocino County — Hopland — by Sunday afternoon, according to projections by the California-Nevada River Forecast Center. In Sonoma County, the lower river was not predicted to flood, but it could reach about 3 feet below flood stage by Monday.

Like the respite in rainfall late Wednesday and early Thursday, a pause in rain was predicted for tonight. But with the ground already saturated and runoff building, trouble was expected.

"We're very concerned actually," Petaluma Fire Chief Phil Sutsos said Thursday, shortly after arranging for the city community center to be put to use as an emergency shelter, if needed.

An even wetter storm was forecast for Saturday night, with total rainfall by Sunday exceeding a foot in the wettest locations — places like Cazadero, Venado and other coastal hill locations that typically see the heaviest rainfall, National Weather Service Meteorologist Diana Henderson said.

The weather service stopped short of issuing a flash flood warning, which would suggest a more immediate and likely breach of creek banks.

But "we're thinking — with all the water that has accumulated and how saturated the ground must be now — what we're expecting to come in is just going to raise the creeks and small tributaries, and we just want to give people a heads up if they live close to those things," Henderson said.

Downpours also may overwhelm culverts and storm drains, leading to street flooding.

Chris Helgren, Sonoma County's Emergency Services manager, said his agency was telling first responders to expect flooding around creeks and low-lying areas, including some roadways.

He advised drivers to exercise caution in the coming days when approaching flooded areas, repeating an oft-cited safety slogan for motorists during the wet season: "Turn around, don't drown."

<NO1><NO>The accidents Thursday night included collisions on Highway 101, a car into a ditch on Lakeville Highway and one into a power pole on Highway 116 near Guerneville Road. Several crashes led to road closures but no major injuries were reported as of 8:30 p.m.

About 400 households were without power in Sonoma County by 8:45 p.m. Thursday, most of them in Sebastopol, a PG&E spokesman said. Smaller outages were reported in Guerneville, Cazadero, Forestville and Healdsburg.

Emergency officials also issued alerts for the possibility of mud slides, fallen trees and downed power lines, particularly given the likelihood of gusting winds, in combination with saturated soils.

"It's going to be a significant storm," Helgren said.

One outlook for the upper Russian River showed it reaching as high as 25<TH>feet in Hopland by Sunday, a level that could cause widespread flooding of farmland, buildings and low-lying homes in Hopland, Ukiah and Talmage areas, according to the river forecast center.

The lower Russian River was predicted to reach monitor stage in Guerneville at about 29 feet Monday. Water managers were tracking the forecasts by the hour Thursday, and each time projected river levels were on the rise.

"I am pretty concerned at this point because it looks like they just keep changing their minds," said Pam Jeane, general manager of operations for the Sonoma County Water Agency. "Sunday is going to be a very interesting day for us."

You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan

@pressdemocrat.com. You can reach Staff Writer Brett Wilkison at 521-5295 or brett.wilkison@pressdemocrat.com.<NO>