The weather service issued a heat advisory Tuesday — the first one issued since late May — for Thursday through late Friday covering eastern Sonoma County and most of Napa County outside the valley. On Thursday, the weather service extended it through Saturday night and issued a new heat advisory just for Thursday for the North Bay valleys.

Large portions of the Bay Area and the Central Coast were under a heat advisory Thursday morning, which has been extended for what could be the hottest days of the year.