(CBS SF) – A Red Flag Warning has been issued for much of the North Bay and East Bay, along with the Santa Cruz Mountains starting Wednesday, as the National Weather Service warns of “critical fire weather conditions” due to rising temperatures, dry conditions and offshore winds.

The warning is in effect from 5 a.m. Wednesday through 11 a.m. Friday and covers the North Bay and East Bay interior valleys, along with the higher elevations.

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“Winds will begin to increase early Wednesday morning, especially over Napa County, but also farther south into the Diablo Range. Significantly stronger and gustier winds will develop and become more widespread in the hills Wednesday evening and peak overnight Wednesday night into early Thursday morning, then diminish during the day on Thursday,” the Weather Service said in a statement.

Officials expect north to northeast winds from 15 to 30 mph, gusting to 35 to 45 mph in the mountains and ridges, with maximum gusts up to 55 mph possible. Meanwhile, those in the valleys should expect winds ranging from 10 to 20 mph, with gusts around 30 mph.

Winds could pick up again Thursday night, but winds are not expected to be as strong, the weather service said.

Along with offshore winds, the weather service said an extremely dry air mass will move in from the north and northeast, sending humidity levels down into the teens and possibly single digits. Meanwhile, temperatures later this week will also be above average for this time of the year, with highs inland well into the 90s on Thursday and Friday.

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With fire danger expected to be high, PG&E announced that about 50,000 customers in 21 counties could be impacted by an upcoming Public Safety Power Shutoff (PSPS) starting on Wednesday.

Bay Area counties that could see power shutoffs include Alameda, Contra Costa, Napa, San Mateo, Santa Clara, Solano and Sonoma. Approximately 21,000 customers will be impacted in those counties.

“The highest probability areas for this PSPS are the Northern Sierra Nevada foothills; the mid and higher elevations in the Sierra generally north of Yosemite; the North Bay mountains near Mt. St. Helena; small pockets in the East Bay near Mt. Diablo; the Oakland Hills east of Piedmont; the elevated terrain east of Milpitas around the Calaveras Reservoir; and portions of the Santa Cruz and Big Sur mountains,” the utility said in a statement.

PG&E is promising regular updates for the customers affected during the PSPS event. Additional information is available on the utility’s pge.com/psps website.

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