Because of those difficult conditions, Powers said crews are getting support from air tankers.
After the blaze broke out Saturday afternoon, officials from the Napa County Sheriff's Office ordered mandatory evacuations for Berryessa Estates, Snell Valley, the west side of Berryessa Road from Pope Creek Bridge to the Lake-Napa County line and Snell Valley Road.
Powers said roughly 200 residents are affected by the evacuation orders, which remain in effect.
The hot weather and low humidity that fueled the blaze on Saturday are expected to continue Sunday and into the week, with temperatures likely reaching into the low 90s. "Crews today again will be facing hot and dry conditions out there," Powers said.
There are 480 firefighting personnel battling the blaze with 56 engines, nine water tenders, three helicopters, 17 bulldozers and 10 air tankers.
No homes have been destroyed, but 180 structures are threatened. The cause of the fire is under investigation.