Why is there a heatwave in the Western USA?
As of August 2, 2026, a strong ridge of high pressure is trapping hot air close to the ground across the Western United States. This weather pattern, known as a heat dome, extends from Arizona to Montana.
The high pressure also limits cooling sea breezes, while tropical moisture is contributing to high humidity and warm overnight temperatures in Southern California (1).
The most intense heat is centered over California, Arizona, Nevada, and Utah, although heat alerts extend north toward Montana and the Canadian border. Around 45 million people were under heat alerts on August 2.
Which cities or areas are affected by the heatwave?
Dangerous temperatures are affecting cities, deserts, valleys, and national parks throughout the Western U.S.
Phoenix reached 116 degrees Fahrenheit on August 1, while Las Vegas reached 115 degrees. El Centro approached 118 degrees, and Great Falls and Helena exceeded daily temperature records (2).
The heatwave is affecting the following key regions and locations:
- Arizona: Phoenix, Tucson, Scottsdale, Mesa, Tempe, Chandler, Yuma, Sedona, Page, and lower elevations of the Grand Canyon
- California: Los Angeles, the San Fernando Valley, the Inland Empire, Palm Springs, El Centro, Imperial Valley, Coachella Valley, Mojave Desert, Death Valley National Park, and Joshua Tree National Park
- Nevada: Las Vegas, Henderson, Laughlin, Pahrump, and areas near Lake Mead and Lake Mohave
- Utah: Salt Lake City, Provo, Ogden, St. George, Moab, Zion National Park, Arches National Park, Canyonlands National Park, Capitol Reef National Park, and Lake Powell
- Montana: Great Falls, Helena, Billings, Lewistown, Glasgow, and communities across eastern Montana
Hot, dry conditions are also increasing wildfire danger across the Pacific Northwest, including eastern Washington and Oregon (3).
How long will the heatwave last?
The heatwave was underway by July 31 and intensified across much of the Western U.S. during August 1 and 2. Conditions are expected to ease in many parts of Southern California and Nevada by Sunday night or Monday, August 3.
Extreme heat warnings remain in effect through Tuesday, August 4, for parts of Arizona, southeastern California, and the Grand Canyon (4).
A follow-on heat advisory is scheduled from Tuesday, August 4, through Thursday, August 6, across the Rogue, Illinois, Applegate, Scott, and Klamath River valleys in southern Oregon and northern California. Temperatures there could range from 105 and 113 degrees Fahrenheit (5).
The duration will therefore vary by location. The most intense phase will last about four to five days in many areas, while dangerous heat could persist for approximately one week in locations covered by later advisories (6).
Are there any alerts in place?
Extreme Heat Warnings are in effect across parts of California, Arizona, Nevada, Utah, and Montana. Authorities advise residents to remain in air-conditioned locations, drink plenty of water, limit strenuous outdoor activity, and never leave children or pets inside unattended vehicles.
Cooling centers are open in Los Angeles, Phoenix, and Las Vegas. Residents of the Phoenix metropolitan area can call 2-1-1 for information about free cooling centers, transportation, and water (7).
Heat, low humidity, and strong winds have also created critical wildfire conditions. Washington declared a state of emergency, and authorities banned most outdoor and agricultural burning through the end of September. A rare Particularly Dangerous Situation Red Flag Warning was issued for eastern Washington on August 1 (8).
