North Coast braces for strong storm in wake of dry January(w/video)

A tropical storm bound for the North Coast is expected to bring an abundance of badly needed moisture to the region over the next several days, prompting a flash flood watch in Sonoma and neighboring counties, and dumping up to 10 inches of rain in the coastal hills, the National Weather Service said.

Overall, the system could deliver 5 to 7½ inches of rain to much of inland Sonoma and Mendocino counties by early Monday morning, the weather service said.

Areas of northern Mendocino and Humboldt counties could see up to 12 inches cumulatively during a series rainy periods lined up over the next few days, said Mel Nordquist, a meteorologist with the agency’s Eureka office.

“This is an atmospheric river event,” Nordquist said, using the term to describe a band of especially moist air coming out of the tropics. “The way I would describe this event is it will be in four episodes, or pulses.”

The first should arrive late Wednesday or early Thursday in the northernmost parts of California, working its way down toward Sonoma County around mid-day or early afternoon, weather service meteorologist Austin Cross said.

Rain is expected to continue off and on through Sunday, with Sonoma County getting the brunt of the storm, as far as the Bay Area goes, he said.

Forecasters said Friday should be the wettest day, with 2 to 3 inches of rain expected on the Santa Rosa Plain, as well as high, gusting winds, strongest on the coast.

A flash flood watch is in effect from 10 p.m. Thursday to 6 a.m. Saturday for Sonoma, Marin and Napa counties, with localized flooding expected in low-lying areas and roadways around Santa Rosa and Rohnert Park, the National Weather Service said. A similar watch was issued for Mendocino County and the Redwood Coast from Thursday evening to Friday night.

“The morning commute on Friday morning should be impacted for sure,” Cross said.

Healdsburg, which in December experienced substantial flooding in the central business district, made sandbags available to residents and shops Wednesday for those bracing for the prospect of more.

But while heavy, Friday’s rainfall should not be as substantial as the deluge that hit the area on a single day in December, closing schools countywide, flooding portions of Petaluma and sending the lower Russian River over its banks.

Twenty-four-hour rainfall totals during that Dec. 11 storm reached 4 and 5 inches in most cities, and more than 8 inches in the North Coast hills.

“Fortunately it looks like it’s going to be spread out a little bit longer than it was in December,” Sonoma County Emergency Services Manager Christopher Helgren said Wednesday. “That will be helpful and hopefully will minimize the localized flooding in the urban creeks and streams.”

The high winds expected late Thursday and Friday could topple trees and result in power outages, but Helgren said the complete lack of rain in January means the ground is dry and better able to anchor trees against the incoming storm.

“There may still be downed trees,” he said. “They’re saying 40-to-60 mph gusts, so there’ll probably be some” outages.

The storm system should bring the area’s season-to-date rainfall close to normal, despite the dismal January totals, at just 0.11 in Santa Rosa. Because of the December storm, however, season-to-date rainfall from Oct. 1 to Wednesday evening was 18.67 inches, compared to a 30-year average of 21.62 inches.

Rainfall totals by Monday should be above 100 percent of average, National Weather Service meteorologist Steve Anderson said.

Still, the storm is not not expected to pull the region out of drought, said Pam Jeane, assistant general manager of the Sonoma County Water Agency.

Water storage in Lakes Sonoma and Mendocino remains below average, and the area would need above-average rainfall through winter and into spring in order to recover, she said.

The incoming system also will be too warm to produce snow for the Sierra Nevada snowpack, which feeds the state water system, forecasters said,

“This is good for us,” Helgren said, “but for the state, it doesn’t help them.”

Staff Writer Derek Moore contributed to this story. You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.