Atmospheric river to make landfall Thursday in Bay Area

North Bay homeowners who didn’t clean their rain gutters before the last series of atmospheric rivers hammered the region have about a day to take care of business before the next major storm arrives.

“We are anticipating an atmospheric river to create impacts across the Bay Area from Thursday into Friday,” said Roger Gass, a Monterey-based meteorologist for the National Weather Service.

Rain arriving late Thursday morning will increase in intensity “into the afternoon and early evening,” he said. That system, a plume of vapor delivering a vast amount of water from the subtropics, is expected to dump between two to 3.5 inches of rain in the valleys, and 3 to 6 inches in the mountains.

It will be accompanied by strong, gusty winds, with showers lingering into Saturday.

“We might see some trees and wires down again,” said Chris Godley, Sonoma County’s Director of Emergency Management. While the county is expecting “some localized challenges,” including traffic issues, “we are not anticipating what we saw in January,” he said.

The conveyor belt of storms that battered the state from late December through mid-January did at least $15 million in damage to the county’s roads, bridges, culverts, storm drains and utilities.

The National Weather Service has issued a flood watch for the Bay Area, from Thursday afternoon through Sunday, alerting residents to the possibility of “flooding caused by excessive rainfall.”

This storm will be characterized by slightly warmer moisture expected to accelerate melting of snow in the foothills of the Sierra Nevada mountains. While that could add to flooding concerns in the Sacramento Valley and Central Valley, said Gass, “it’s not going to have much of a direct impact for us here in the Bay Area.”

That said, given the saturation levels of the soil, due to wet winter the county has already seen, forecasters are expecting the Russian River to approach its “monitor stage” at Guerneville, which is 29 feet.

According to graph published by the California Nevada River Forecast Center, the Russian River is expected to crest at just over 29 feet in Guerneville around 2 a.m. Saturday morning.

Gass said Tuesday there was a 40% to 60% chance of the river reaching that level, “and a 25% to 40% chance of it exceeding flood state of 32 feet.”

A second storm, not quite as potent, is gathering on the heels of the first. “We are anticipating another system Monday into Tuesday,” said Gass.

“It looks like we’ll be staying in an active pattern through at least the middle of the month.”

Rainfall from the second system isn’t expected to be as heavy as it will be in the downpours arriving Thursday.

But if Russian River level is already high when the second storm arrives, said Godley, “that could cause some challenges.”

Nick Malasavage, the Army Corps of Engineers official who oversees operations at the Lake Sonoma and Lake Mendocino dams, said releases from those reservoirs will continue to stay “boring and inconsequential” in the days leading up to the storms.

“Lake Sonoma was so empty for so long, and there’s so much air space in there” — even after the deluges of December and January — “there’s plenty of space to accommodate the storms forecast over the next five to 10 days.”

Lake Mendocino has a little less room to spare, he added Tuesday afternoon, so releases from that dam “will uptick a little bit for another 24 to 48 hours, but then they’ll clamp down” once the storms arrive.

You can reach Staff Writer Austin Murphy at austin.murphy@pressdemocrat.com or on Twitter @ausmurph88.