The "Antonio Fire" was reported just after 2 p.m. Tuesday in the area of Glendora Ridge Road and Mount Baldy Road.
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It started with a vehicle fire that spread to the brush, initially estimated at less than an acre. By 5 p.m. it had grown to 50 acres.
But the flames spread quickly amid hot, dry and windy conditions that are increasing the risk of fire all week throughout Southern California.
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Firefighters with LA County and the Angeles National Forest responded with ground crews and aircraft, reporting they are "making solid progress containing the fire with hose-lays."