Heat dome spreads across US, bringing dangerous days, little relief at night

An intense heat wave spread across parts of the western and central United States on Saturday, with triple-digit temperatures forecast from the Intermountain West into the Northern Plains and dangerous heat and humidity lingering in parts of the Southeast.

The National Weather Service's Weather Prediction Center said the heat would continue through much of the coming week as a strong upper-level ridge stretched from the western United States into the Midwest.

Several days of well-above-normal temperatures — including potentially record-breaking daytime highs and overnight lows — were expected to produce major to extreme heat risks in parts of the Intermountain West, Northern Plains, and Upper Midwest.

Sunday is expected to be hotter than Saturday across much of the north-central United States. Forecast highs in central and eastern Montana and North Dakota ranged from 100 to 110 degrees Fahrenheit (38 to 43 degrees Celsius), with higher local readings possible. Those temperatures would be about 8 to 14 C above normal for mid-July.

Forecasters said all-time temperature records could be challenged, including Salt Lake City's record of 41.6 C and Billings, Montana's record of 42.2 C. A cold front is expected to begin bringing cooler air into Montana late Sunday and Monday, but the heat is forecast to intensify farther east across the Northern Plains and Upper Midwest after Monday.

In Southern California, the National Weather Service office serving the Los Angeles area warned that temperatures could reach 35 to 41 C or higher across much of its forecast area during the coming week. Coastal communities are expected to remain cooler, while inland valleys, mountains, and deserts face substantially higher temperatures.

Forecasters in San Diego, California, said temperatures would begin climbing again on Tuesday and peak on Wednesday. The National Weather Service projected minor to moderate heat risk in inland valleys and mountains and moderate to major heat risk in the deserts.

The hot and dry conditions are also complicating firefighting efforts in Los Angeles County.

The Summit Fire, which began Friday afternoon near Llano, has burned 2,690 acres and was only 8 percent contained by Saturday evening, according to the California Department of Forestry and Fire Protection. Evacuation orders and warnings remained in effect.

The agency said temperatures near 38 C, single-digit humidity, and gusty winds along ridges were creating the potential for rapid fire growth and active fire behavior. The fire's cause remains under investigation.

Arizona, where extreme summer heat is common, also remained under heat alerts. An extreme heat warning covered the Phoenix metropolitan area and surrounding desert regions on Saturday.

Extreme heat warnings cover the Grand Canyon below 4,000 feet (1,219 meters) through Sunday and Marble Canyon, Glen Canyon, and Page during the weekend. Such conditions pose particular risks for hikers and other visitors in exposed terrain, where shade, drinking water, and medical help may be difficult to reach. The National Weather Service advises people in areas of extreme heat to stay hydrated, take frequent breaks, and limit outdoor activity when possible.

A central concern was that the heat would not always ease sufficiently after sunset. Warm nights can leave homes and other indoor spaces dangerously hot, particularly when air conditioning is unavailable, unaffordable, or not functioning.

The Centers for Disease Control and Prevention says older adults, young children, and people with chronic health conditions face elevated risks from extreme heat. People who work outdoors or in hot environments are also vulnerable because heat can accumulate in the body and lead to illness or injury.

This heat wave follows a deadly spell of extreme temperatures over the July Fourth weekend. Between July 2 and July 6, New Jersey health officials reported 29 suspected heat-related deaths.

The heat also presents a major test for Arizona, which has reported some progress in reducing its historically high death toll. Maricopa County recorded about 430 heat-related deaths in 2025, down from 608 in 2024 and a record 645 in 2023. Still, the county had confirmed 18 heat-related deaths this year, with 215 more under investigation, outpacing last year's heat-related deaths by this time, according to Maricopa County Department of Public Health.