San Francisco recorded its hottest day of the year on Tuesday and Phoenix set a record for the hottest October 1 on record, as the National Weather Service predicted record-breaking fall temperatures across the southwestern United States.

With temperatures reaching 38C or more in many places, officials and local media issued warnings that the heat posed “a significant threat to property or life.” Excessive heat warnings were in effect across the region, bringing with them an increased risk of wildfires, the possibility of widespread power outages in California and a rising number of heat-related deaths – a particularly deadly risk for the homeless and elderly.

In San Francisco, temperatures reached 93°F, while across the Bay Area several cities recorded temperatures “up to 25 degrees above normal” in October, the San Francisco Chronicle reported, and many Bay Area public schools responded by saying outdoor Athletics take off the heat.

In Arizona, Phoenix continued to break one heat record after another, with temperatures expected to surpass previous daily highs for October every day of the week. On Tuesday, the high temperature was around 113°F (45°C). So far in 2024, the city has recorded 67 days with temperatures above 110°F, compared to an average of about 21 in previous decades. At the start of summer, the city experienced 100 consecutive days of temperatures above 100°F.

The record heat is bringing with it a record number of heat-related deaths. More than 666 deaths in Phoenix this year have been confirmed as heat-related or are still being investigated as potentially heat-related, according to local health data.

Nearly half of Phoenix's heat-related deaths this year occurred among unhoused people, according to public health data. In extreme heat, sidewalks and asphalt can become so hot that serious burns can occur. But dozens of heat deaths in Phoenix were also recorded indoors, including in homes where air conditioners were broken or turned off, perhaps to save money.

While older people were at higher risk for heat deaths, about 40% of Phoenix victims were under 50, according to public health data.

In Las Vegas and the rest of Southern Nevada, officials said heat was a factor in the deaths of at least 342 people this year, the highest number ever recorded, the Las Vegas Review-Journal reported last week.