Firefighters got little help from Mother Nature on Saturday as much-needed rainfall from expected thunderstorms didn't materialize for a huge wildfire burning in the Southern California mountains near Palm Springs.

Fire officials were hopeful the storms, which can also bring wind, lightning and other volatile conditions, would douse some of the flames, but they said there hadn't been any significant rainfall.

Cooler temperatures overnight, however, helped firefighters make progress on the fire's northern and southern flanks in the San Jacinto Mountains, as personnel worked to spare nearby desert communities from damage.

The blaze, which grew to roughly 42 square miles, was 49 percent contained, officials said.

Thunderstorms present a major threat to progress this weekend. Combined with hot air on the ground, the unstable air could create a strong updraft that draws smoke high into the atmosphere, fire spokesman Capt. Mike Lindbery said.

If the smoke column rises too high, moisture at the top could freeze and the weight of the ice could cause the column to collapse, creating a powerful downdraft in all directions.

"We're very concerned because this is the condition in the past that has definitely caused big firestorms and the death of citizens and firefighters," Lindbery said.

Storm cells approached the area Friday afternoon but dissipated before reaching the fire zone. But the threat would remain through the weekend, and fire officials hoped it would be mild cloud cover and high humidity that could help in the firefight.

The blaze in the San Jacinto Mountains had expanded to roughly 42 square miles and was 49 percent contained, officials said.

Lt. Gov. Gavin Newsom, filling in for a vacationing Gov. Jerry Brown, declared a state of emergency for the area Friday night, freeing up more state funding and other resources to help with the protracted firefight that has already cost nearly $11 million.

Mandatory evacuations remained in place for a fourth day for about 6,000 people, and officials had advised another 700 to evacuate.

Some communities on the eastern edge of the fire were reopened to residents, but about 5,600 homes remained under potential threat.

The fire was less than two miles from Idyllwild on its western flank. It was a similar distance from Palm Springs below on the desert floor, where an enormous plume of smoke could be seen, but the blaze was showing little threat of moving toward the much larger city.

Popular campgrounds, hiking trails and a 30-mile section of the Pacific Crest Trail, which runs from the Mexican border to Canada, remained closed.

Nearly 3,400 firefighters, aided by nearly 30 aircraft, battled the fire, which stretched in elevation from 4,000 feet to 9,000 feet along the mountains.

Authorities said the fire was human-caused, but they wouldn't say whether it was accidental or intentional. There have been no reports of injuries.

The fire, which began Monday, has burned six homes and mobile homes, one cabin, and more than a dozen other buildings.