[UPDATE, Oct. 9, 6:36 p.m.: With strong winds yet to kick up as of Wednesday evening, PG&E was able to restore power to 44,000 customers in the North Bay and Sierra foothills. The utility said it was still watching weather conditions for the East Bay and South Bay and hadn't yet decided the timing of shut-offs in those areas. Read the latest here.]

All was calm in the Bay Area Wednesday morning after sunrise, but forecasters warn strong winds will pick up gradually throughout the day, peaking overnight and bringing critical fire danger at a time of year when trees and grasses are dry and highly flammable.

The National Weather Service said the gusty conditions will likely mark the strongest offshore wind event since October 2017 when fierce gusts spread wildfire flames in the North Bay.

"It's hard to say right now if it's going to be as big or bigger than 2017," said Spencer Tangen, a forecaster with the NWS office in Monterey. "Once we come out of it, we can compare it better."

The National Weather Service has a Red Flag Warning in effect for the North Bay mountains and valleys and the East Bay hills and valleys Wednesday 5 a.m. to Thursday 5 p.m. A warning was also issued for the Santa Cruz Mountains Wednesday at 5 p.m. through Thursday at noon.

To mitigate wildfire risk, Pacific Gas & Electric is preemptively shutting down power to more than 250,000 customers in the Bay Area. As of 5:30 a.m., nearly 150,000 customers in the North Bay were in the dark.

"We have this strong offshore wind event and we haven't had any rain and that's why it's going to be a big impact," Tangen said. "This is when the fuels are still really dry. What's more, the relative humidities are going to be very low. The northeast offshore winds are going to bring in a dryer air mass."

Relative humidity levels today are expected to drop into the teens in the North and East Bay valleys and into the single digits in the hills. On average, these areas see relative humidity levels of 30 to 40 percent.

Tangen said winds were starting to increase Wednesday morning in the higher elevations of the North and East Bay hills, with wind speeds of 10 to 20 mph and isolated gusts up to 30 mph.

"The wind event is still on track from what we were expecting," Tangen said. "Winds will peak tonight."

For the higher elevations in the North and East Bay hills, overnight winds speeds are expected to average 20 to 30 mph with isolated gusts 45 to 55 mph.

"Some of the highest peaks in favored locations, like Mount St. Helena and Mount Diablo, will probably see gusts up to 65 mph and that could go even higher," Tangen said.

Winds will also kick up in East and North Bay valleys Wednesday afternoon and overnight speeds could average 20 to 30 mph with occasional gusts up to 40 mph.

"Often with these wind events, the lower elevations don't see the strong winds, but with this event we'll see the windy conditions down to the valley floors," Tangen. "Even for the bay shoreline, we have gusts around 3o to 35 mph for overnight tonight. Even in the city it's going to be pretty windy."

Fallen trees and power lines are expected with this event and the NWS has a wind advisory in effect for elevations above 1,000 feet in the North and East Bay hills Wednesday at 6 a.m. to Thursday at 3 p.m.

The winds are forecast to decrease late Thursday morning for most of the area.

The wind event is the result of a storm system known as an "inside slider" that dropped down from the Pacific Northwest into the Great Basin Tuesday night and generated ideal conditions for strong and gusty offshore winds across much of the San Francisco Bay Area. What's more, a dry and cold front spilling over the Sierra Nevada swept across the Bay Area last night, moving from the north to the south, kicking up northerly winds.

"That cold, dry air with the inside slider is causing very high pressure on the ground to form over the Great Basin, while low pressure is forming over the California Coast," says Tangen. "The difference in pressure will create the very gusty winds we're expecting to see through Thursday."

As chilly air continues to spill into the region on Wednesday, widespread 70s are likely. Temperatures are expected to climb back up starting on Thursday, and by Friday afternoon highs in inland valleys will be in the 80s.

FULL PG&E SHUTOFF COVERAGE:

- Map shows Bay Area neighborhoods that will be without power

- PG&E map down? Here's an interactive NorCal power shutoff map that actually works

- Essentials to buy for PG&E's planned power outage

- Here's what you should (and shouldn't) do during a power outage

- When will my power turn off?

- Cal, Bay Area schools announce canceled classes, closure

- UPDATE: 2 Bay Area tunnels expected to remain open

Amy Graff is a digital editor at SFGATE. Email: [email protected]