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Richard Vogel, Associated Press
A boy tries to catch a fish in Echo Park Lake near downtown Los Angeles on Friday, Aug. 14, 2015. The National Weather Service says Southern California's sharp warming trend will build into the weekend, with above-normal temperatures in many areas. The temperature hit 91 in downtown Los Angeles and topped 100 degrees in the San Fernando Valley and Riverside.

PHOENIX — Much of the West was smothered in a blanket of heat Saturday with triple-digit temperatures expected from Phoenix to Los Angeles.

Above-normal temperatures were expected through the weekend as a high-pressure system centered over New Mexico acted like a lid to block cooler air, leaving valleys, deserts and mountains high and very dry.

Authorities warned people not to leave small children or pets in cars, where temperatures can quickly soar. Los Angeles and other cities were keeping libraries and other facilities open late to serve as cooling shelters for those without air conditioning.

Phoenix broke a daily record Friday, reaching 117 degrees, and the expected Saturday high of 116 would top a 1992 record by 4 degrees, National Weather Service meteorologist Dan Leins said.

"Stay inside if you can," he said. "It's dangerous, regardless of how acclimated you are to the climate, because it can be deadly."

Mike Stephens took precautions for an early-morning run in Estrella Regional Park, a desert and mountain wilderness area in Phoenix. He carried 1 ½ liters of water and was careful not to overdo it.

"You have to know what your body can do," he said.

The National Weather Service issued warnings of excessive heat throughout Southern California into Saturday night, with some areas expected to see highs of 10 to 15 degrees above normal, said Scott Sukup, a weather service meteorologist in Oxnard.

A high of 95 was forecast for downtown Los Angeles, with the San Fernando Valley and other inland valleys ranging up to 108. Palm Springs could hit 117 and Death Valley's high could reach 120 to 125, according to the weather service.

Highs of 110 to 112 were projected for Las Vegas and much of the Mojave desert.

The Western heat wave began Thursday and was expected to continue through Sunday.

"There's been a big area of high pressure that was over New Mexico and it's been expanding westward," Sukup said.

The heat, coupled with low humidity, has increased the fire danger in California, where some two dozen major fires in recent weeks have destroyed thousands of acres of trees and brush left bone-dry by years of drought.

Winds were light "but it's so hot and dry right now that it's not going to take a lot of wind to spread fires," he said.

A brushfire that erupted Friday afternoon in forest foothills north and east of Los Angeles quickly grew to 4 square miles, torched several cabins and remained out of control Saturday. Firefighters worked in triple-digit heat and several were treated for dehydration or heat exhaustion.

Temperatures were expected to be cooler in California's coastal regions, with highs in the 80s.

About 1 million people were expected to hit 32 miles of Los Angeles County beaches this weekend, county lifeguard Capt. Kenichi Haskett said.

But they might have to share the water with some prickly guests.

Calm, warm waters have drawn the barb-tailed stingray to the surf line, where they bask in the shallows and burrow in the sand, Haskett said.

"We could have a couple of hundred at Santa Monica Bay," he said.

About two dozen people were stung Thursday and Friday, he said.

The venomous stings are painful but not lethal.

Stingrays aren't aggressive but will respond if stepped on. Haskett said people can avoid stings by doing the "stingray shuffle" — shuffling their feet through the sand when entering the water to give the stingrays warning.

Robert Jablon contributed to this story from Los Angeles.