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Rain, gusting wind on their way for Sonoma County

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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.

“It really starts ramping up during the day Wednesday,” Anderson said. “The highest winds, heaviest rain should be midday through the afternoon hours.”

Winds from the south should bring gusts up to 40 mph in Sonoma County valleys and 50 mph along the coast Wednesday.

The slow‑moving storm could sit for hours over an area, bringing a deluge and possibly higher rain totals than forecast, Anderson said. The flash flood watch was issued for 10 p.m. Tuesday to 10 a.m. Thursday.

Sonoma County lowlands could get up to 4 inches of rain in all with as much as 8 inches in the mountains, with more rain expected this weekend.

As of Monday, Santa Rosa’s rainfall total was at about 21 inches, or about 91 percent of the average. This storm’s pouring rain ought to boost numbers close to 100 percent if not more, at least until the next dry spell. At this time last year, Santa Rosa had seen about 13 inches of rain.

“There’s another system coming down from the north, cooler. It’ll keep showers in our forecast and another one this weekend diving down, with colder air,” Anderson said.

The National Weather Service also issued a winter storm watch for the greater Lake Tahoe area with as much as 5 feet of snow possible above 8,000 feet and ridge gusts of 145 mph. The warning is for 10 p.m. Tuesday until 4 p.m. Thursday.

More than 11 feet of snow has fallen at Squaw Valley and Alpine ski resorts near north Lake Tahoe this month. This week’s storm is expected to bring about 4 feet of snow at about 8,000 feet, said Alex Spychalsky, public relations coordinator for the resorts.

“They just keep stacking up on top of each other,” Spychalsky said. “It’s lining up pretty perfectly” for the upcoming holiday weekend.