On Wednesday, the state energy grid operator called for voluntary conservation of electricity from 4 to 9 p.m. to prevent the risk of a power outage as there was an expected high demand for air conditioning. No major power outages were reported after the alert ended, but the request was extended to 4 to 9 p.m. Thursday.

An excessive heat warning was issued by the National Weather Service across the desert into Nevada, and a heat advisory covered the Central Valley and through inland Southern California. In addition to the heat, the area has remained dry, and forecasters have warned of increased wildfire risk from dry lightning.

“The combination of possible dry lightning as well as strong winds with the dry fuels could lead to critical fire weather conditions,” forecasters said.

For more reporting from the Associated Press, see below.

A fire weather watch was issued for Thursday evening through Friday evening in much of the interior of Northern California due to a weather system that is expected to bring a chance of thunderstorms with lightning and erratic gusts.

Nearly 15,000 firefighters were making progress on 14 major wildfires and several smaller new fires, the California Department of Forestry and Fire Protection said. They include three of the state’s 20 largest fires on record.

In the northern Sierra Nevada and southern Cascades region, the second-largest fire in California history has scorched nearly 1,441 square miles (3,732 square kilometers). The Dixie Fire was 59 percent contained, but new evacuation orders were issued for part of Shasta County.

More than 1,280 structures have been destroyed, including 688 individual homes.

To the south in the Sierra near Lake Tahoe, the nearly 340-square-mile (880-square-kilometer) Caldor Fire remained 50 percent contained. Firefighters have had enough success against the state’s 15th-largest fire that residents of the city of South Lake Tahoe were allowed to return home last weekend. With inspections 95 percent completed, nearly 1,000 structures have been counted destroyed, including 776 single-family homes.

The four large casino hotels in Stateline, Nevada, began reopening Tuesday, including the Hard Rock Lake Tahoe, which is still being used as a command center for firefighters and emergency personnel. The Montbleu Resort, Casino and Spa resumed operations Wednesday.

Harrah’s Lake Tahoe owner Caesars Entertainment Inc. said a phased reopening would include guest bookings on Wednesday and table games on Thursday. The company’s Harveys Lake Tahoe casino is due to reopen for slots and table games on September 17 and to hotel guests September 18.

In the mountains of the far north coast, the state’s 18th-largest fire has ravaged more than 310 square miles (804 square kilometers) of the Shasta-Trinity National Forest. The Monument Fire was 45 percent contained but remained a threat to more than 10,500 structures.

California has experienced increasingly larger and deadlier wildfires in recent years as climate change has made the U.S. West much warmer and drier over the past 30 years. Scientists have said weather will continue to be more extreme and wildfires more frequent, destructive and unpredictable.