Officials with the National Weather Service said dangerous fire weather conditions have developed in the North Bay and East Bay hills and the Diablo Range.
Weather officials this morning issued a Red Flag Warning for those areas because low humidity and gusty winds are expected.
The warning is in effect in the North Bay hills from 11 p.m. Tuesday until Wednesday morning. In the East Bay hills, the warning starts at 2 a.m. Tuesday and ends Wednesday morning.
North to northeast winds are expected to gust up to 60 mph in the North Bay hills and up to 50 mph in the East Bay hills.
Any fires that start will likely spread quickly, weather officials said.
The greatest threat is in the Napa County hills along the Yolo and Lake county line and areas around Mt. Tamalpais and Mt. Saint Helena.