NWS Forecast Office San Francisco, CA

A Red Flag Warning has been issued for the North Bay Valleys valid beginning 11 PM PDT on Monday October 19th through 08 AM Wednesday October 21st. Winds will become more north-northeasterly and are forecast to increase over the highest peaks and ridges to 10 to 25 mph, with gusts generally up to 30-35 mph, with maximum gusts up to 40 mph possible at the highest peaks. When the Fire Weather Watch was upgraded to this warning, it was decided to include all of the North Bay Mountains, although the strongest wind gusts are expected to be over Napa and Northeast Sonoma Counties. These offshore winds will provide critical fire conditions with dry humidity values through the event, and even drier conditions overnight Tuesday and into Wednesday with poorer humidity recovery expected due to less marine layer influence.