Why is there a heatwave in Southern California?
As of August 19, 2025, there’s a developing heat wave in Southern California driven by a high-pressure ridge over the Southwest.
The heat wave has been reinforced by the indirect effects of Hurricane Erin in the Atlantic. The storm’s energy altered atmospheric patterns, creating a persistent ridge that traps heat and draws humid, monsoonal air from Mexico (1). This combination is causing dangerously high temperatures and elevated wildfire risks across the region.
Which cities or areas are affected by the heatwave?
The heatwave is impacting a broad area, including Los Angeles County (downtown, San Fernando Valley, Woodland Hills), Santa Barbara, Ventura County, the San Gabriel Mountains, Antelope Valley, and desert regions including Twentynine Palms and Barstow.
An extreme heat watch issued by the National Weather Service extends up through much of the Central Valley as well (2).
How long will the heatwave last?
The heat is expected to peak on Thursday, August 21, with temperatures reaching up to 109°F in some areas.
While temperatures will start to gradually decrease on Monday, August 25, the extreme conditions are forecast to persist through at least Saturday, August 23, making this a multi-day event with prolonged exposure to hazardous heat (3).
Are there any alerts in place?
Authorities have issued several critical alerts to protect public safety (4).
- An Extreme Heat Watch covers most of Southern California (excluding immediate coastal areas) until Saturday evening, with temperatures ranging from 95°F to 110°F.
- Extreme Heat Warnings are specifically in effect for Twentynine Palms, Barstow, and the San Fernando Valley.
- A Fire Weather Watch has been declared for the mountains and foothills outside Los Angeles, beginning Thursday, August 21, and lasting through Saturday, August 23.
There is an increased risk of dry lightning and flash flooding in Ventura and Los Angeles counties, as well as the Antelope Valley, from Friday through Sunday.
In response to the heatwave, the City of Los Angeles has activated augmented cooling centers at several locations, including the Canoga Park Senior Citizen Center, North Hollywood Senior Citizen Center, Pecan Recreation Center, and Algin Sutton Recreation Center. These facilities are open from 10 a.m. to 9 p.m. from Wednesday, August 20, through Saturday, August 23, to provide relief for vulnerable populations. Mayor Karen Bass has urged residents to stay cool, hydrated, and informed, and to check on neighbors, especially those at higher risk of heat-related illness.
The Los Angeles Fire Department is also on standby to respond to any emergencies arising from the extreme conditions.
