A potent Hawaiian-born storm is expected to drench Sonoma County this week, possibly triggering landslides and flooding as rain comes down at a half-inch or more per hour, dropping a total of 4 to 8 inches through Thursday.
Showers are forecast Tuesday, with heavy gusting winds and heavy rain — including a flash flood watch — starting at night and lasting through Thursday morning as the storm settles over the North Bay, San Francisco Bay Area and Central Coast, the National Weather Service said.
With the ground saturated from recent rains, weather officials warned that the incoming storm could lead to rock slides, landslides, debris flows and falling trees, with flooding from inundated storm drains and rapidly rising creeks, streams and rivers.
“We have another one of those pesky atmospheric rivers heading toward us,” Steve Anderson, senior meteorologist with the National Weather Service in Monterey, said Monday.
Anticipating heavy rain, dam operators released about 4 billion gallons of water in the past week from Lake Sonoma and Lake Mendocino, the North Bay’s primary sources of water for residents, farms and industry.
Showers are forecast Thursday morning, with partly sunny skies later in the day as the storm relents, but wet weather is expected through the weekend.
Temperatures will continue to be cooler than average, but not low enough to blanket North Bay hills with white as last week’s storm did.
“No snow from this one in our area,” Anderson said.
Warmer rain, however, will cause what is left of the snow on upper ridges to melt, adding to the runoff into the region’s waterways, he said.
“What’s clear is we’re going to get a lot of rain and a lot of wind,” Anderson said. “It’s likely to cause local creeks and streams to flood and flooded roadways” leaving mud and rocks on area roads.
North Bay dam operators did a quick turnabout Monday, after dumping water from Lake Sonoma and Lake Mendocino in advance of the storm. Releases from both reservoirs were dramatically cut back, also due to the prospect of storms pumping up the reservoirs and the Russian River.
The seemingly contradictory moves reflect the U.S. Army Corps of Engineers’ primary mission — flood control — at both federally operated and taxpayer-funded reservoirs.
“The best time to clear flood control space is good weather,” said Nick Malasavage, chief of operations and readiness at the Army Corps office in San Francisco.
So with Lake Sonoma northwest of Healdsburg at 255,669 acre feet and Lake Mendocino near Ukiah at 82,283 acre feet on Feb. 5, the dam gates were opened. That lowered Lake Sonoma about two feet and Lake Mendocino about three feet by Monday, creating storage space in both reservoirs, he said.
The releases from both reservoirs totaled about 12,000 acre feet. An acre-foot is 326,000 gallons, enough to supply a family of four for a year.
On Monday, the outflow from both reservoirs was scheduled to be cut back by about two-thirds to less than 500 cubic feet per second, Malasavage aid
“We don’t want to be releasing water when it starts raining,” he said.
Monday’s heavy gray skies seemed to portend what’s coming.
Light rain should move in Tuesday, with wind and rainfall increasing through the night.