The last day of the week-long heatwave is expected to be Friday, with temperatures expected to peak at around 80 to 82 degrees in San Francisco and up to 90 degrees in Sacramento. The heatwave was caused by an upper-level ridge in the mid-to-upper atmosphere, causing temperatures to reach between 84 and 94 degrees in some areas. The National Weather Service Meteorologist described the heat as "very hot temperatures" compared to those typically seen in August or September.