Red flag warning expanded to entire North Bay region as PG&E plans power shut-offs

With strong winds in the forecast, the National Weather Service on Saturday expanded this weekend’s red flag warning to cover the entire North Bay, including all of Sonoma, Napa and Marin counties.

The warning will take effect at 9 p.m. Saturday and has been extended to 9 p.m. Monday. The alert originally had been issued mainly for higher elevations, including the North Bay hills, and was set to expire at 8 a.m. Monday.

All of Lake and Mendocino counties also are included in the warning.

Separately, PG&E widened the area of Northern California in which it plans to turn off electricity in the hopes of preventing its equipment from sparking wildfires, though only two homes in Sonoma County are expected to have their power turned off.

Red flag warnings are triggered when there’s a combination of strong winds, low relative humidity and warm temperatures, which can contribute to extreme fire behavior.

National Weather Service meteorologist Gerry Diaz said the agency expanded the alert to lower elevations because forecasters have a “higher confidence in those winds mixing down into the valleys.”

Diaz said the biggest risk for dangerous fire weather conditions remains in the North Bay hills, where wind gusts could reach up to 50 mph as the humidity drops to critically low levels. Additionally, fire crews in northern Sonoma County were preparing in the event the winds fan the largely contained Walbridge fire, which began in mid-August and has torched more than 55,000 acres.

“We are expecting those conditions to be around the same level of concern as we’ve seen (during red flag alerts) over the past few weeks,” Diaz said.

The alert came as PG&E announced on Saturday night that it would turn off electricity to 89,000 customers within its service area in 16 counties starting early Sunday.

The first wave will include 15,000 customers mostly around the Sierra foothills starting at 2 a.m. Sunday. Another 74,000 people will be affected at 4 p.m. Sunday. That group will include 55 customers in Lake County and 216 customers in Napa County along with the two Sonoma County customers.

Mark Quinlan, the incident commander for PG&E for the planned outage, said he had “a high degree of confidence” that the shut-offs would not expand on Sunday and could potentially shrink if weather conditions improve.

Scott Strenfel, a senior meteorologist for PG&E, said the main concern for the local region would be the North Bay mountains, where wind gusts could reach 50 mph.

Winds from the northeast could first hit the Napa hills Saturday after 9 p.m., according to National Weather Service forecasts. By Sunday morning, winds may reach North Bay valleys, with a second burst of gusts arriving Sunday night that could cause very dry conditions throughout the entire region, extending to the coast.

In addition to the red flag alert, the National Weather Service issued a heat advisory for most of the Bay Area starting 11 a.m. Sunday until 7 p.m. Monday. Temperatures in the hills and valleys are expected to reach the 90s to low 100s as offshore winds bring heat to the coast.

Fire officials said local crews are preparing for potential new starts and are patrolling the perimeter of the Walbridge fire burn area for embers and hot spots. The Santa Rosa Fire Department and other Sonoma County departments are together preparing 17 additional fire engines in anticipation of the red flag warning.

As of Saturday, Cal Fire reported containment on the Walbridge remained at 98%, with no growth beyond its existing 55,209-acre footprint.

Forecasters also expect winds at high altitudes to again bring smoke from the massive August Complex fire, which comprises 870,200 acres in six Northern California counties, to the North Bay.

In response, the Bay Area Air Quality Management District issued a Spare the Air alert for the Bay Area on Sunday. The agency recommends limiting outdoor activities during the hottest parts of the day when the alert is in effect.

While North Bay residents can expect hazy skies and diminished air quality, conditions are not anticipated to reach the unhealthy levels experienced earlier this month. The air district forecasts air quality to reach only “moderate” levels in the North Bay by Sunday.

You can reach Staff Writer Ethan Varian at ethan.varian@pressdemocrat.com or 707-521-5412. On Twitter @ethanvarian