An unprecedented heatwave has gripped southern California, boosting temperatures 15 to 20 degrees above normal for autumn.

Monday was a notably sweltering day for several cities.

Downtown Los Angeles saw a record high of 104F (40C), breaking the previous 5 October record of 101F (38.3) in 1953, according to the National Weather Service (NWS). About 30 miles south of Los Angeles, residents of Santa Ana experienced a record 106F (41.1C) day, up from the past daily high of 103F (39.4) in 1953, the agency also reported.

Extreme heat warnings were in place along the coast from Santa Barbara to San Diego through Thursday, as health authorities advise the region’s millions of residents to limit their outdoor activity and stay hydrated.

The NWS has attributed the blistering spell to a ridge of high pressure, when hot air is trapped over a region.

In San Diego and Los Angeles, the NWS has forecasted overnight temperatures hovering in the mid to upper 70s, which the agency noted provides only little relief.

The unusual spell of extreme heat has set records not only for its sweltering temperatures, but also its prolonged length.

This past week, Los Angeles recorded four consecutive days of 100F or more, marking the longest streak of triple-digit temperatures in October.

California’s coastal areas are expected to cool starting on Thursday with rain in the forecast for next week, according to the NWS. The state’s interior regions may also see cooler temperatures on Friday and Saturday.