A strong high-pressure system over the nation's midsection built westward into California on Friday, baking inland areas of the southern counties in dry, triple-digit heat while raising wildfire danger in the parched region.

Day-to-day warming was expected to spread similar highs up the San Joaquin and Sacramento valleys during the weekend, forecasters said.

Readings of 100-plus degrees stretched from the interior of Los Angeles County eastward through the inland counties to the Colorado River, where the Interstate 40 town of Needles sizzled at 115 degrees by 1 p.m.

Overall, the day's temperatures were expected to be 10 to 15 degrees above normal for the season.

"The hot conditions will result in an increased risk of heat-related illnesses, especially for the homeless, the elderly, infants, outdoor workers and those participating in outdoor activities," the National Weather Service said.

Some Southern California beaches simmered in the 80s.

Forecasters issued warnings until midnight for fire danger across the mountains of Los Angeles and Ventura counties, and the Santa Clarita and Antelope valleys north of Los Angeles due to low humidity and gusty north to northwest winds.

Fire danger warnings were to extend until midnight Saturday on the south coast of Santa Barbara County. The area's infamous "sundowner" winds have often been involved in the dangerous spread of wildfires, sweeping down from the Santa Ynez Mountains to the populated south coast. Gusts in the 45 mph to 55 mph range were predicted.

The largest fire burning in Southern California was in northern San Diego County, where flames had scorched more than 3 square miles on the Marine Corps' sprawling Camp Pendleton. The fire was not threatening personnel or structures.

The National Weather Service has issued an excessive heat warning for Imperial and Riverside counties. Temperatures in those areas are expected to reach highs of 112 to 118 through midnight Saturday.

Try to avoid spending too much time out in the heat.

If you plan to hike or exercise outside, plan to do so early in the morning or late evening when it is cooler.

Consider wearing lightweight and loose fitting clothing.

Make sure to stay hydrated.

Keep children and pets out of parked cars in this heat as it can get to dangerous temperatures quickly.

Find a designated cool zone in San Diego here.

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