Rising humidity came to the aid of about 1,000 firefighters battling flames in rough brushland near Castaic Lake on Tuesday, but Angeles National Forest officials were worried about predicted thunderstorms that could bring lightning and more erratic winds.
After charring an estimated 12,000 acres in the inaccessible Ruby Canyon area, the blaze was about 25% contained with full containment expected by 6 p.m. Thursday, said forest information officer Tom Horner.
"The very high humidity has helped to get the fire to lie down quite a bit," Horner said, "so now we're directly attacking it, right at the fire edge."
Los Angeles County fire Capt. Garry Oversby said the humidity was increased by both a flow of tropical moisture moving up from Baja California and by on onshore flow of marine air to the north.
The Mexico system, however, was bringing isolated thundershowers and lightning activity northward. Those storms threatened to move into the fire area by Tuesday evening and continue this morning, the National Weather Service said.
"There's going to be lightning all over the place, mainly in the mountains," said weather service forecaster Richard Hickey.
As the showers and thundershowers moved into the San Bernardino Mountains and the Riverside area Tuesday afternoon, the state Department of Forestry reported at 3 p.m. that there had been 840 ground strikes of lightning during the past 11 hours in San Diego, Imperial and San Bernardino counties.
No injuries were reported, however.
The situation, explained Matt Sullivan of the Earth Environment Service, a private forecasting firm in San Francisco, was created by a "really moist" upper level low-pressure system centered more or less above San Diego and drawing a counterclockwise flow of winds from the deserts to pump moisture up from Baja California.
San Diego, with showers and thundershowers much of the day, recorded .31 of an inch of rain by 4 p.m. Tuesday.
The muggy, tropical weather pushed the temperature up to 99 degrees at the Los Angeles Civic Center on Tuesday, the last day of summer. Relative humidity downtown ranged from 90% to 65%. There was no rain there by late in the day, but a 20% chance of showers was forecast for the entire Los Angeles area during the night.
In the meantime, ground crews were fighting the perimeter of the Castaic Lake area fire, rather than setting backfires. The latter technique, Oversby pointed out, made the latter technique difficult.
The hottest section of the fire remained to the northeast, where 10 helicopters and eight air tankers supported the firefighters until nightfall.
Four firefighters had been treated for minor injuries and returned to duty, Horner said.
The fire erupted Sunday morning and was driven by light Santa Ana winds. The cause was under investigation. Although winds blew it away from the community of Lake Hughes, the blaze burned a U.S. Forest Service lookout tower at Warm Springs.
No other structures were reported burned.
While Southern California firefighters awaited the possible onslaught of lightning and the probable renewal of winds, other blazes remained out of control in the Klamath National Forest at the north end of the state.
The total acreage burned there since the fires erupted during late August was estimated at 194,000 acres--almost a third of the statewide total burned in three weeks.
In the Shasta-Trinity National Forest, all fires had been contained except for the 12,400-acre North Complex wilderness blaze north of Weaverville. Containment was not expected until next week, said a fire information officer.
Times staff writer Rick Serrano in San Diego contributed to this article.