LOS ANGELES, July 11 (Xinhua) -- A broad heat dome expanding across the western and central United States is aggravating a destructive wildfire emergency while posing health risks and causing property damage.

Park visitors sit on a bench at Echo Park on July 08, 2026 in Los Angeles, California. (File photo: AFP)

The U.S. National Interagency Coordination Center (NICC) said in its Saturday report that the country was at National Preparedness Level 4, one level below the maximum, with 78 active large incidents covering about 2,961 square km, including Alaska. A total of 14,409 personnel were assigned to the incidents.

More than 1,000 structures were reported destroyed in the Aspen Acres fire in Colorado and the Cottonwood fire in Utah. A Saturday incident-team update put the Aspen Acres fire at 34 percent contained, while the NICC report listed Cottonwood at 70 percent contained.

Three federal wildland firefighters were killed and two others were injured in a June 27 burn-over while responding to the Knowles Fire in western Colorado, according to the U.S. Department of the Interior.

The NICC report said high temperatures, low humidity and gusty winds were increasing fire-weather concerns across parts of the western Great Basin, northeastern California, the northern Rockies and the inland Northwest. It forecast winds of 19 to 35 km per hour, gusts near 48 km per hour and afternoon relative humidity of 5 to 15 percent.

The National Weather Service office in Salt Lake City warned that temperatures could challenge records, forecasting a high about 41 degrees Celsius on Sunday. The Grand Junction office forecast lower-elevation highs of about 38 to 41 degrees Celsius across western Colorado and eastern Utah. The Bismarck office said air temperatures and apparent temperatures in parts of North Dakota could approach 43 degrees Celsius.

The Grand Forks office forecast major heat risk across the region, with extreme heat risk in some areas and overnight temperatures of about 24 to 26 degrees Celsius. The Weather Prediction Center said limited overnight cooling would add to the danger.

A 2025 multi-country study published in the journal Environment International found that the intensity and duration of hot nights contributed to heat-related mortality risk after accounting for daytime maximum temperatures and humidity.

In Colorado, state officials warned that high temperatures and wildfire smoke could create health risks.

Daniel Swain, a climate scientist with the University of California Agriculture and Natural Resources, wrote in a Friday Weather West analysis that the heat dome would likely bring record-breaking temperatures from the eastern Great Basin to the Upper Midwest.

Swain also identified a 20 to 30 percent chance of mostly dry thunderstorms in parts of northern California, adding that if those storms developed, lightning could produce new ignitions while gusty outflow winds could alter fire behavior.

The U.S. Department of Agriculture said in a Friday assessment that a heat wave was beginning across Montana and the Dakotas, with highs of about 32 to 38 degrees Celsius. It also said lingering drought and soil-moisture shortages were affecting some pastures, rangelands and summer crops.

The U.S. National Oceanic and Atmospheric Administration's Climate Prediction Center issued an El Nino advisory on Thursday. Federal forecasts and Swain's analysis identified the atmospheric ridge as the immediate meteorological driver.

The National Weather Service office in Salt Lake City said monsoonal moisture could increase next week, raising the risk of lightning and later flash flooding or debris flows on recent burn scars.