The National Weather Service has issued an Excessive Heat Warning for Sonoma County beginning Saturday, Sept. 5, at 11 a.m. through Monday, Sept. 7, at 9 p.m. A Fire Weather Watch also has been issued for the North Bay mountains for Monday evening at 10 p.m. through Wednesday at 9 a.m. The highest temperatures during this time are expected to occur during the early to mid-afternoon on Tuesday.