By GRACE TOOHEY Los Angeles Times/TNS
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LOS ANGELES — Another bout of dangerous, triple-digit heat is headed for much of California’s interior this weekend, creating new concerns for firefighters battling wildfires across the state, including two that have surpassed 15,000 acres.

“Friday will kick off a fairly dramatic warm-up as a large and hot upper-high over the middle of the (country) expands westward,” the National Weather Service wrote in its Wednesday morning forecast. The upper-level ridge of high pressure — known colloquially as a heat dome — will most directly affect the San Joaquin Valley and Southern California deserts, but almost all of the Golden State can expect elevated temperatures this weekend.

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The Los Angeles area will remain on the western edge of the system as it moves across the Southwest before expanding north and west — creating significant “compressional heating” below the ridge of high pressure, said Joe Sirard, a National Weather Service meteorologist in Oxnard. Sirard said “heat dome” isn’t a precise meteorological term, but describes the process that pushes temperatures up as air sinks below the upper ridge.

In Southern California, Saturday is expected to be the warmest day, when highs in Los Angeles County valleys will reach into the 90s while the deserts and low mountains will peak in the mid-100s.

As of Wednesday morning, the Post fire was 39% contained and appeared to be holding at 15,690 acres, according to fire officials. After days of treacherous red-flag conditions — gusty winds and low humidity — firefighters were able to make “significant progress” Tuesday night, according to a Wednesday morning update.

In northern Colusa County, the Sites fire had grown to 15,565 acres by Wednesday morning, almost catching up in size to the Post fire, which recently became the state’s largest wildfire of the year. The Sites fire was only 5% contained, according to the California Department of Forestry and Fire Protection.

Fire officials there were hoping to make significant progress Wednesday and today — during a reprieve between strong winds that were subsiding and high temperatures on the horizon.