Why is there a heatwave in the Southwest?
The heatwave is driven by a strong ridge of high pressure over the eastern Pacific and the West Coast. High pressure causes air to sink and warm as it descends. The ridge is forecast to stay in place for an unusually long time, which is why the heat could last well into October.
Light offshore winds are adding to the heat. These winds blow from the inland deserts toward the ocean, pushing hot, dry air all the way to the coast. They also limit the marine layer of clouds and fog that normally keeps coastal areas mild in autumn.
Unusually warm ocean water is also playing a role. Sea surface temperatures along the Southern California and Baja California coastlines are near record highs. This weakens the cooling effect the ocean usually provides and keeps nighttime temperatures high. The heat also follows an exceptionally hot summer, with August 2026 ranking as the warmest on record in several Southern California counties (1).
Which cities or areas are affected by the heatwave?
The heatwave is affecting large parts of the Southwest, including the following cities and areas:
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Los Angeles
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Ventura
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Santa Barbara
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Orange County
- Inland Empire
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San Diego
- Central Coast
- San Joaquin Valley
- Sacramento Valley
- San Francisco Bay Area
Triple-digit highs are expected to be widespread in the Inland Empire and inland valleys, while coastal areas will see highs in the upper 80s and low 90s.
Beyond California, Las Vegas, Reno, and Phoenix will also see warmer-than-usual weather, and heat alerts could expand to Nevada and Arizona if temperatures continue to rise (2).
How long will the heatwave last?
The heatwave began on Friday, October 2, and is expected to last through at least the end of next week. The first peak is expected over the weekend of October 3 and 4, when highs could reach up to 102°F in parts of Southern California.
The heat is not expected to ease after the weekend. According to the National Weather Service, offshore winds and persistent high pressure will continue through the week (3).
Some relief may arrive late next week, with gradual cooling expected to begin around Friday, October 9. An upper-level trough bringing tropical moisture could lower temperatures further over the weekend of October 10 and 11.
Are there any alerts in place?
The National Weather Service has issued an extreme heat warning for the coast and valleys of Los Angeles, Ventura, Santa Barbara, Orange, San Diego, Riverside, and San Bernardino counties.
The warning runs from 10 A.M. Friday, October 2, to 8 P.M. Thursday, October 8, and may be extended if the heat continues. Highs are expected to range from the mid 80s at the coast to 105°F inland, with overnight lows staying in the 70s.
Shorter heat advisories are also in effect for the Central Coast, the San Joaquin Valley, and the San Francisco Bay Area from October 3 to 4, and for the Sacramento Valley from October 3 to 5.
Gusty winds combined with low humidity raise the risk of wildfires, and smoke from any new fires could quickly worsen local air quality. Residents should find out where their local cooling centers are and be ready for possible power outages (4).
