'Critical fire conditions' continue to hamper firefighting as blazes across California grow

Crews battling wildfires across California on June 18, including the Post fire in Los Angeles County, face another day of gusty winds and low humidity that will push flames into tinder-dry fuel, weather forecasters warned.

"Dangerous fire-weather conditions are expected to continue through Tuesday," the National Weather Service wrote in a red flag warning for the Interstate 5 Corridor, the San Gabriel Mountains and Ventura County mountains. Northern winds there were expected at 20 to 30 mph with gusts up to 55 mph, and relative humidity is expected to be at 5% to 10%, forecasters warned.

"Use extreme caution with anything that can spark a wildfire," the weather service alert said. "Residents near wildland interfaces should be prepared to evacuate if a wildfire breaks out."

Crews continued Tuesday to work to contain the Post fire, which started Saturday afternoon in Gorman and has burned 15,611 acres, according to a Tuesday morning update from the Los Angeles County Fire Department. It is 24% contained.

"Steep rugged terrain and expected weather conditions will challenge firefighters throughout the day," the morning update said. Strong winds continue to push the fire south and plumes of unhealthy smoke along with it, with heavy smoke expected Tuesday to affect communities in and around Santa Clarita.

The area's red flag warning, an alert for critical fire weather conditions, is in effect through Tuesday evening, when the Post fire burn area should begin to see weakening winds, after days of treacherous conditions. But until then, fire officials warned that it's possible that red-hot firebrands may ignite spot fires more than half a mile ahead of the main fire front.

Climate change is transforming the character of the West's hottest periods—making them more frequent, more persistent, more humid and more lethal. Greenhouse gas emissions from the burning of fossil fuels also play a role, not only by fueling higher air temperatures but also by warming ocean water.

Now the state's largest fire of the year, the Post fire is threatening power lines, dams and oil pipelines, fire officials said.

But it's only one of the more than 20 fires that have ignited since Saturday across California, where warm temperatures and strong winds have sparked an unofficial kickoff to fire season.

Red flag warnings remain in effect for much of the the San Joaquin Valley and into the northern Sacramento Valley through noon Tuesday.

The Sites fire in Colusa County, which started Monday afternoon, grew to 10,000 acres by Tuesday morning, "influenced by winds from the north at 10–20 mph and low relatively humidity," according to an update by the California Department of Forestry and Fire Protection, or Cal Fire.

The Aero fire in Calaveras County, which also ignited Monday, was 5,425 acres Tuesday morning and 20% contained. Burning in an area that hasn't seen a large fire since 2003, Cal Fire officials said it was burning in grass but also oak woodlands—which can create more intense fire conditions. Officials have issued evacuation orders for two nearby communities, around Copper Town Square and O'byrnes Ferry Road, with several others warnings in place.

The Point fire in Sonoma County's wine country remained just over 1,000 acres, and was 40% contained Tuesday morning.