Lake Mendocino now can hold 22 billion gallons of water, most since its creation in 1958

Heavy rains this week left Lake Mendocino, the North Bay region's second-largest reservoir, with an extra 2 billion gallons of water that until now officials would have been obliged to release into the Russian River and eventually the Pacific Ocean.

With less than an inch of rain expected over the weekend and a subsequent long dry spell beginning next week, the Army Corps of Engineers plans to retain the water, enough for nearly 50,000 people for a year.

“We're in new territory,” said Nick Malasavage, chief of operations and readiness at the Army Corps office in San Francisco. “We're excited.”

Officials at Sonoma Water, the agency that delivers Russian River water to 600,000 Sonoma and Marin county residents, are equally pleased.

“Mother Nature is cooperating with us,” said Jay Jasperse, the agency's chief engineer.

They are happy because this winter - for the first time since the Coyote Valley Dam was built in 1958, creating Lake Mendocino near Ukiah - they can hold more than 68,400 acre feet (22 billion gallons) of water for future use by fish, farmers and residents.

Thanks to a $10 million program that blends high-tech weather forecasting with novel computer programming, the Army Corps has the latitude to retain an additional 11,650 acre feet of water, and Lake Mendocino has now impounded a little more than half that much.

If a drenching storm were on the near horizon, dam operators would be releasing water now to maintain capacity for flood control, the dam's primary mission. Malasavage said the Army Corps checks National Weather Service five-day forecasts twice a day, and the high-tech program - called Forecast Informed Reservoir Operation, or FIRO - provides a 15-day weather outlook.

On Friday, he said, the forecasts indicated “there is no need to release the water.”

Accuweather, a private forecasting service used by The Press Democrat, predicts that Monday will be the start of 12 straight dry days with temperatures in the low- to mid-60s.

The idea behind FIRO is to get through the rainy season with as much water as possible in Lake Mendocino.

The program sprang from the hard lesson learned in 2012 and 2013, when reservoir inflow from a major storm in December was released, per the rules. Unfortunately, the storm marked the start of a prolonged drought that dropped Lake Mendocino to a mere 25,000 acre feet a year later.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.