The National Weather Service [NWS] anticipates particularly high fire danger in the North Bay late Friday and all day Saturday, issuing a red flag warning for Napa and Sonoma counties.
The NWS weekend forecast for the Bay Area shows strong winds and dry conditions pairing up to create particularly hazardous conditions:
Dry northerly flow is forecast to develop over the North Bay during the day today with temperatures warming to above seasonal averages. Northeast winds will strengthen tonight into early Saturday morning with poor relative humidity recoveries followed by even warmer and drier conditions during the day Saturday.
The areas of greatest concern will be located in higher terrain of Napa County as well as far eastern Sonoma County. Any fire starts may spread rapidly in these conditions.
The warning notes that residents should avoid starting fires, mowing the lawn, setting off fireworks, driving on grass, or carelessly discarding cigarettes. (Although really you should avoid a lot of those no matter what.)
Cal Fire cautions residents to heed the the risks:
A Red Flag Warning is issued for weather events which may result in extreme fire behavior that will occur within 24 hours. [...] A Red Flag Warning is the highest alert. During these times extreme caution is urged by all residents, because a simple spark can cause a major wildfire.
The warning is in effect from Friday 11 p.m. to Saturday 11 p.m. High temperatures will exacerbate fire risks, with Napa heat forecast to reach as high as 94 degrees on Saturday.