North Bay forecast calls for another large rainstorm, starting Thursday

A whopper of a storm that could make last weekend look like a light sprinkle is headed toward the North Bay later this week, with the National Weather Service predicting that more than 9 inches of rain could fall on the coastal hills between Thursday morning and Monday night, and more than 6 inches on Santa Rosa.

The most intense rainfall Thursday and Friday is expected to bring the Russian River to flood stage at 32 feet in Guerneville by Saturday morning. It was expected to crest at that height at about 9 a.m.

The rain is expected to raise the Navarro River more than 10 feet above flood stage around 5 p.m. Friday.

The Army Corps of Engineers was prepping for the storm with flood releases from Lake Sonoma that started Tuesday and were to continue through this afternoon just to relieve a bit of the pressure in anticipation of more rain, according to Mike Dillabough, chief of operations and readiness for the Army Corps' San Francisco Division.

Runoff from the last storm was coming in faster than the outflow, even with the stepped up releases, he said.

The releases were to taper off in time for the arrival of heavy rain on Thursday, he said. Until Tuesday, no flood releases had been made from Lake Sonoma since at least 2011, Dillabough said.

The lake, which stood just above 252,000 acre feet of storage on Tuesday, rose on Monday into what's called its “flood pool” at 245,000 acre feet for the first time in years, the Army Corps said.

The lake has overall capacity for 381,000 acre feet, so there remained plenty of additional space. An acre foot is 325,851 gallons, or about the amount of water it would take to cover a football field one foot deep.

Lake Mendocino held more than 83,000 acre-feet on Tuesday, about 115 percent of its capacity for this time of year.

The rain was expected to arrive in the North Bay early Thursday and stall over the coast range for the day before spreading over the wider region Friday, bringing nearly 4 inches of rain to Santa Rosa by Friday night, the weather service said. Last weekend, Santa Rosa got just over 2 inches of rain between Friday and Sunday night.

“It's going to be impressive,” meteorologist Steve Anderson said. “The only question mark is when and where this front coming through on Thursday is going to stall. If it stalls over the North Bay for a long enough period of time, yeah, we're going to be seeing a lot of rain.”

You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.