Napa tied a heat record set in 1951 when the thermometer at Napa State Hospital hit 100 degrees on Wednesday, the National Weather Service said.
Most parts of the Napa Valley were hotter than this weather station, which isn't far from the breezes off San Pablo Bay.
The Weather Service predicted a high of 99 degrees at Napa State on Thursday, which would be short of the record of 102 degrees set in 1906. The rest of the valley will be in the low 100s, the Weather Service said.
Cooling is forecast to start on Friday, when Napa is expected to drop into the low 90s, but the Bay Area Air Quality Management District said the air will remain smoggy.
The district called a fourth consecutive Spare the Air alert for Friday, with people urged to not use their cars when possible.
Dramatically cooler weather is expected this weekend. Napa's high temperature is forecast to be close to 80 degrees, with Sunday in the mid-70s.