It’s Perfect Weather for Post Cards --and Fires : Heat: Visibility is up, and so is the mercury. On the downside, the near-record high fuels three blazes.

Near-record-high temperatures and breathtaking views from Santa Catalina Island to the mountains drew 200,000 people to the beach and jammed parks with picnickers Sunday, while firefighters and Coast Guard rescuers grappled with Mother Nature’s darker side.

Sizzling 90-degree heat caused by a Santa Ana condition was at least partly to blame for a 15-acre brush fire in Sylmar that started from spontaneous combustion, when an 8-foot-deep compost heap erupted in flames.

Meanwhile, about 200 acres of brushland were blackened in San Dimas and the adjacent Angeles National Forest.

Los Angeles County Fire Inspector John Lenihan said four 12-year-old boys “playing army and throwing smoke bombs” accidentally started the San Dimas fire, which was contained late Sunday by 150 firefighters from 15 engine companies.

Near Riverside, about 100 firefighters battled a blaze that had blackened more than 300 acres in the Pedley area, forcing evacuation of a wildlife reserve Sunday night, said Mike Wilson, a state Department of Forestry spokesman. The fire was reported about 10:30 a.m. at Bain Street and Limonite Avenue and spread along the Santa Ana River bottom in tall trees and thick brush, Wilson said. There was no estimate of when the blaze would be contained.

Calm seas and balmy weather created perfect scuba diving conditions, authorities said, but one diver narrowly escaped death Sunday after his seemingly lifeless body was pulled from the Pacific by fellow divers near Santa Cruz Island.

U.S. Coast Guard Lt. Peter Troedsson said the victim was revived by an Air Force surgeon and Coast Guard flight mechanic using cardiopulmonary resuscitation, or CPR. The victim, 48, whose name was not released, was airlifted to Los Robles Regional Medical Center in Thousand Oaks, where he underwent recompression treatment.

Despite those mishaps, the crystal-clear skies--seemingly absent of most smog--drew rave reviews and even inspired a bit of the poet in some.

“We can almost see the mist rising from Mauna Kea. . . . That’s the volcano on the big island of Hawaii,” said Lew Kuykendall, operations coordinator at Los Angeles International Airport. “This is the kind of perfect day when photographers come out to take the pictures for post cards. I’m thinking about taking the rest of the day off sick.”

“There’s unlimited visibility, no wind at all. You can see Catalina Island perfectly and it’s almost completely glassy out here,” said Bill Powers, senior lifeguard at Santa Monica Pier. “We have an awful lot of folks at the beach and in the water.”

Rick Dittmann, a meteorologist at WeatherData Inc., which provides forecasts for The Times, said a high pressure area was creating an offshore flow, forcing warm air from Nevada and the deserts into the Southland and out to sea.

That, combined with another high pressure system in the upper atmosphere, helped drive the Civic Center temperature to 90 degrees. Dittmann said the record for the date is 92, set in 1895.

It was warmer elsewhere, with Montebello and San Gabriel recording highs of 92; Santa Ana reached 91.

With a forecast of more perfect weather today--and only slightly cooler temperatures--Los Angeles was preparing to offer up a pleasant week for out-of-town visitors arriving for Thanksgiving.

“This weather will make everybody who doesn’t live here feel pretty bad,” said county Lifeguard Lt. Wally Millican at Hermosa Beach.

But the forecast also prompted warnings from fire officials, who said that hot temperatures, dry ground and parched brush made firefighting difficult Sunday and may spell even more trouble.

County Fire Capt. Jack Rumfola said brushy areas throughout the county “are substantially drier this year than they have been in the past”--and that campers and vacationers heading into outlying areas for the long holiday weekend should exercise special caution.