Why is there a heatwave in California?
As of August 11, 2025, a powerful high-pressure system, known as a heat dome, is fueling the U.S. Southwest heatwave, trapping intense heat and driving clear skies with record-shattering temperatures, intensified by persistent dry conditions (1).
This phenomenon is exacerbated by dry conditions and climatic patterns, contributing to record-breaking heat.
Which cities are impacted?
The heatwave affects multiple cities across the Southwest US, including:
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Albuquerque, New Mexico
- Dallas, Texas
- Fresno, California
- Las Vegas, Nevada
- Los Angeles, California
- Medford, Oregon
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Palm Springs, California
- Phoenix, Arizona
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Tucson, Arizona
How long will the heatwave last?
The heatwave began building around August 6, 2025, with peak temperatures occurring on August 7 and 8, 2025.
It is expected to continue through the weekend, with some areas experiencing elevated heat until Monday, August 11, 2025, before a gradual cooldown begins (2).
Overall, the duration of triple-digit temperatures in many affected areas spans approximately four to five days, marking the first significant heatwave of the summer in parts of Northern California (3).
An update indicates that while the extreme heat in the desert Southwest may lessen slightly over the weekend, widespread heat could return to other parts of the country later next week (4).
Are there any alerts in place?
Authorities have issued several alerts due to the heatwave and associated risks, focusing on public safety and prevention of heat-related illnesses.
Heat advisories, extreme heat warnings, and extreme heat watches have been issued for parts of California, Washington, Oregon, Idaho, and Nevada by the National Weather System (5).
Red flag warnings for critical fire weather conditions are active across five western states: Oregon, Utah, Colorado, Wyoming, and New Mexico, due to low humidity and high winds that could lead to rapid fire spread.
An elevated fire weather warning is also in place for Santa Barbara County, California, through Saturday, increasing the risk of wildfires in inland areas (6).
Additionally, an ozone advisory has been issued for Southern California, warning of unhealthy air quality expected in inland communities, particularly in the San Bernardino Mountains and Valley.
This poor air quality, driven by higher temperatures leading to ozone formation, is especially dangerous for sensitive groups like children, older adults, and those with respiratory issues (7).
