Yosemite Park sequoias under threat from wildfire amid western U.S. heat wave

High temperatures in the southwest and western U.S. were posing a challenge to firefighters in California and Utah, with Texas regulators warning of potential rolling blackouts on Monday.

In California, a heat wave was developing but winds were light as firefighters battled a wildfire that poses a threat to a grove of giant sequoias and a small community in Yosemite National Park.

The Washburn Fire on the western flank of the Sierra Nevada mountains had scorched about 5.8 square kilometres by Monday morning, an increase of about 120 hectares overnight, according to an incident update.

The fire was a threat to more than 500 mature sequoia treesin the park's Mariposa Grove and the nearby community of Wawona, which has been evacuated.

The area in the southern portion of Yosemite was closed to visitors but the rest of the national park remained open.

Grove protected since 1864

Mariposa Grove and Yosemite Valley have been protected since president Abraham Lincoln signed legislation in 1864.

A sprinkler system was set up within the grove to maintain moisture, and there were no reports of severe damage to any named trees, including the 3,000-year-old Grizzly Giant.

"Fortunately, the Mariposa Grove has a long history of prescribed burning and studies have shown that these efforts reduce the impacts of high-severity unwanted fire," a National Park Service statement said.

A heat advisory was issued for the Central Valley sprawling below the Sierra Nevadas, while up in the fire area a high of 31 C was forecast for Wawona.

The giant sequoias, native in only about 70 groves spread along the western slope of the Sierra Nevada, were once considered impervious to flames but have become increasingly vulnerable as wildfires, fuelled by a buildup of undergrowth from a century of fire suppression and the impact of drought exacerbated by climate change, have become more intense and destructive.

Lightning-sparked wildfires over the past two years have killed up to one-fifth of the estimated 75,000 large sequoias, which are the biggest trees by volume and a major draw for tourists.

2 major Utah wildfires

There was no obvious natural spark for the fire that broke out Thursday next to the park's Washburn Trail. Smoke was reported by visitors walking in the grove.

A fierce windstorm ripped through the grove more than a year ago and toppled 15 giant sequoias, along with countless other trees.

The downed trees, along with massive numbers of pines killed by bark beetles, provided ample fuel for the flames.

So far in 2022, over 35,000 wildfires have burned nearly 1.9 million hectares in the U.S., according to the National Interagency Fire Center, well above average for both wildfires and acres burned.

In Utah, smoke and ash emanating from a growing wildfire in rural Tooele County blew into Salt Lake City on Saturday. By Sunday afternoon, the Jacob City Fire had grown to 15.3 square kilometres, with zero containment, officials said.

Elsewhere in Utah, firefighters contending with heavy winds battled the 32.2-square-km Halfway Hill Fire in Filmore. Law enforcement on Saturday arrested four men who investigators said abandoned a campfire that ignited the blaze.

Texas near capacity for energy reserves

Meanwhile, the operator of Texas's power grid on Sunday called on state residents for the second time this year to conserve energy, warning of potential rolling blackouts amid predictions for record-high temperatures on Monday.

The state faces a "potential reserve capacity shortage with no market solution available," the Electric Reliability Council of Texas (ERCOT) said on its website, adding an energy emergency alert that advised of the potential for rolling blackouts.

ERCOT oversees power to more than 26 million customers.

Temperatures across the state hit records on Sunday, with 40.6 C recorded at Houston's Bush Intercontinental Airport, surpassing a record set in 1909, according to the U.S. National Weather Service (NWS).

High or dangerous heat levels are forecast for much of the state on Monday, with temperatures expected to exceed 37 C.

ERCOT asked residents to conserve electricity between 2 p.m. and 8 p.m., saying demand could reach 79,934 megawatts (MW) on Monday and 80,104 MW on Tuesday, not far off Monday's expected 80,200 MW of available reserves. One megawatt can power around 1,000 U.S. homes on a typical day, but only about 200 homes on a hot summer day in Texas.

Houston Mayor Sylvester Turner advised police and fire chiefs in the nation's fourth most populated city "to prepare in case the state's power grid fails during extreme heat."

The state's grid operator has called for more power from suppliers and asked large industrial consumers to reduce their energy use.