(CBS SF) — The National Weather Service in San Francisco has issued a Red Flag Warning for gusty northerly winds and low relative humidity in the North Bay mountains and East Bay hills between 11 a.m. Friday and 11 p.m. Saturday.

The areas of greatest concern are located in the higher terrain of northern and eastern Napa County, the National Weather Service said.

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Fires in those areas will spread rapidly under the red flag conditions, and residents should avoid lawn work, driving on grass, tossing cigarettes, lighting campfires or fireworks especially on Friday and Saturday afternoons, the National Weather Service said.

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Northerly winds of 20 to 25 mph with gusts up to 35 mph are expected, and isolated highest terrain could see locally higher winds.

Minimum humidity readings of 10 to 20 percent will be common during the day, and overnight humidity recoveries will be poor and less than 30 percent.

The highest threat is in elevations above 1,000 feet especially in the extreme northern and eastern portions of the North Bay mountains, the National Weather Service said.

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