Drenching rain on the way turns focus to Sonoma County burn scars

A drenching, late-season rainstorm expected to batter the North Coast over the next several days could cause localized flooding and is expected to present the stiffest test so far to burn scars left by October's wildfires.

An atmospheric river funneling subtropical moisture from Hawaii is set to begin pelting Sonoma County with rain late Thursday, dumping up to 3 inches of rainfall across Santa Rosa and southern Sonoma County by Saturday, up to 6 inches in the county's western and northern regions, and even higher levels in coastal hills, according to the National Weather Service.

“It does look like a lot,” Steve Anderson of the National Weather Service said Wednesday.

The ground is dry enough to soak in much of the expected precipitation, depending how slowly the storm moves across the region, he said. Rainfall this season is only about 63 percent of normal, and even after the upcoming storm will be well below seasonal average of about 33 inches.

But emergency officials plan to keep close watch on local creeks, especially in and around the burn zones, as well as the scar areas themselves once the heaviest rainfall hits Friday afternoon and night, Anderson said.

They'll get help from a new system of rainfall and stream gauges installed around the areas that were scorched last fall by the devastating Tubbs and Nuns fires, which raised the risk of flash-flood and debris flows for the next several years.

Twenty-two gauges - 12 rain-only and 10 measuring rainfall and stream flow - are now collecting real-time data that can be consulted by emergency officials and the National Weather Service to determine if and when any public alerts or advisories may be necessary. The public can find the data, along with maps and other helpful online links at https://sonoma.onerain.com.

High-intensity fires can leave the ground incapable of absorbing moisture and devoid of the plantlife that can help slow and suck up runoff.

Thousands of Sonoma County households are considered at risk of flash floods, mudslides and debris flows that could result, though hydrologists and other experts are hopeful the coming rainstorm won't be the one to cause trouble.

“This is the first atmospheric river that is really going to test the burn scars,” said Brian Garcia, warning coordination meteorologist with the National Weather Service in the Bay Area.

The key factor for post-fire risk is the intensity of rainfall. As a rule of thumb, county and weather service officials say that a 15-minute downpour at a rate of about one inch an hour is enough to raise streams, loosen soils and unleash trouble, though most guidelines for risk assessment are based on historic observations in Southern California, where soil types, geological formations, vegetation and other factors may not translate.

County personnel will be checking at-risk areas during the rainstorm, said Barry Dugan, program specialist with the Sonoma county Water Agency.

It's critical, however, that the public stay alert, especially those close to the burn areas. A post-fire hazard assessment map is available at https://bit.ly/2Gxsgbq.

Residents are urged to sign up for county notifications through SoCoAlert.com as well as the Nixle system used by the law enforcement agencies.

They also should keep their cellphones turned on and settings enabled to accept wireless emergency alerts, Garcia said.

After an initial mailing in January to about 3,000 households alerting them to potential post-fire risks, a second mailing has just gone out, this one to about 10,000 households who live within the fire zones or downstream of them. And just in time, Dugan said.

“This really is the kind of storm that we've been worried about since the fire,” he said, “and it's very unusual for it to come this late. But it's here, and we're hoping that everybody who lives in the burn area and downstream are taking necessary precautions.”

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.