Cooler weekend weather in L.A. County expected to give way to heat wave starting Monday

Cooler temperatures are expected to give way to a heat wave in Los Angeles County that could bring periods of elevated fire risk early next week.

Cloudy skies blanketed most of the Los Angeles County region early Saturday, with temperatures ranging from the high 60s to the mid-70s, according to the National Weather Service.

But that is likely to change beginning Monday when downtown L.A. is expected to reach 90 degrees, then 93 Tuesday and Wednesday, and 90 Thursday, said John Dumas, a meteorologist with the weather service.

“We’re going to go from below-normal temperatures to above-normal temperatures and probably stay around the 90-degree mark in the L.A. area for most of the week,” he said.

That’s because a high-pressure ridge is forecast to build over the area, bringing northerly offshore winds that heat up and dry out the air as they travel downhill and squeeze through mountain passes, Dumas said. The winds probably won’t be strong enough to prompt advisories, but they will cut off the afternoon sea breeze that usually mitigates the heat from earlier in the day, particularly in coastal areas, he said.

“This effectively shuts off our natural air conditioning system,” Dumas said.

The National Weather Service is projecting that the combination of elevated temperatures, low humidities and moderate winds will result in elevated to briefly critical fire conditions.

“It hasn’t rained since March or so,” Dumas said. “So it is still very dry out there.”

Crews fighting the Bobcat fire in the Angeles National Forest above Azusa had used this week’s favorable weather conditions as an opportunity to boost containment of the massive blaze to 90%, up from 88% Monday.

The fire’s footprint held at 115,796 acres Friday into Saturday, having destroyed 87 homes and 83 other structures.

Some popular trails in the Angeles National Forest reopened Friday after a monthlong closure because of the active fire and high fire danger. Other roads and trails north of Sierra Madre and Azusa in the burned area remain closed.

The cause of the Bobcat fire, which started Sept. 6, remains under investigation.