SACRAMENTO, the United States - Fueled by excessive heat and drought, several large wildfires raged across the western United States on Sunday, forcing thousands of people to evacuate their homes and worrying climate experts about more fires in the coming days.

In Northern California, a wildfire called the Boyles Fire erupted Sunday afternoon in the town of Clearlake, located in Lake County north of the Bay Area.

The fire has rapidly consumed 76 acres (about 0.3 square km) in just three hours, with 10 percent containment achieved so far, the California Department of Forestry and Fire Protection (CalFire) said.

The agency has declared the fire "an immediate threat to life", urging residents in affected areas to evacuate immediately. Approximately 30 structures are "involved" in the blaze, and around 4,000 residents in the town have been displaced from evacuation zones, said the agency in a fire update.

A live video from a local news channel showed the fire was burning through thick brush that had become highly flammable due to the hot weather and dry conditions, making it particularly challenging for firefighters to control.

In Southern California, the situation was even more dire. A wildfire called the Line Fire, which broke out on Thursday in San Bernardino County, has grown exponentially, quadrupling in size on Saturday.

This massive wildfire already injured three people and threatens thousands of homes and businesses. As of Sunday afternoon, the fire has scorched 17,459 acres (about 71 square km) with zero percent containment, forcing mandatory evacuation orders for over 11,000 people and putting more than 35,000 structures at risk.

Soaring temperatures exceeding 100 degrees Fahrenheit (about 38 degrees Celsius) and "critically dry" vegetation contributed to its rapid expansion toward the San Bernardino Mountains.

California Governor Gavin Newsom declared a state of emergency in San Bernardino County Saturday. The area was under a red flag warning for wildfires on Sunday.

More than 600 firefighters have been dispatched to contain the Line Fire, but steep terrain and limited access have impaired their ability to reach some areas.

"Thunderstorm outflow winds are causing very erratic fire behavior," while "smoke is limiting aircraft access to parts of the fire," said CalFire.

"Hot and dry conditions mixed with thunderstorms are expected to challenge firefighters for the next few days," said the agency.

In Idaho, the Lava Fire, first reported on Sept. 2, has burned 2,493 acres (about 10 square km) as of Sunday. This lightning-caused fire was burning through heavy timber, brush, and grass, prompting evacuation orders for nearby residents, the US Forest Service said.

Climate scientists are particularly concerned about the formation of pyrocumulonimbus, or fire-generated storms. In photos posted on social media Sunday, pyrocumulonimbus clouds were clearly visible on top of the Lava Fire.

These large plumes of smoke, generated during massive fires, can create their own weather systems. The intense heat from the fires rises into the air, and if enough moisture is present, it can generate thunderstorms.

Above-normal temperatures will continue across the western United States into the High Plains, with highs from Arizona to the Central Valley from 105 to 110 degrees (about 40-43 degrees Celsius), with 95 to 100 degrees (about 35-38 degrees Celsius) expected in the Inland Northwest, according to the National Interagency Fire Center's update on Sunday.