The National Weather Service has 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.
Several inches of rain were expected in the coastal hills before a pause in the weather predicted Friday night, and with the ground already saturated, 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 into use as an emergency shelter, if needed.
After a respite earlier Thursday, heavy rain was predicted to hit the North Bay by Thursday evening, continuing through much of Friday and dropping 3-to-6 inches of rain on the region.
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 receive heaviest rainfall, National Weather Service Meteorologist Diana Henderson said.
But stream and creek flooding is possible as early as Friday morning because of saturated soils, the collection of runoff from Wednesday's drenching and the return of rain Thursday night, the weather service said.
The weather service stopped short of issuing a flash flood warning, which would suggest a more immediate and certain 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 go 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, or channel leaves and debris that could lend to street flooding.
Emergency officials also put residents on alert for the possibility of mud slides, fallen trees and downed power lines, particularly given the likelihood of gusting winds, in combination with saturated soils.
"Keep an eye on the stuff around you - the trees and power lines and other stuff," Henderson said. "And slow down on the freeway, normal common sense stuff... People tend to kind of forget common sense stuff whenever there's weather."
The Russian River was predicted to crest several feet shy of monitor stage in Guerneville and was not expected to present a flood risk this weekend, forecasters said.
(You can reach Staff Writer Mary Callahan at 521-5249 or firstname.lastname@example.org.)