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Firefighters from Southland agencies remained in Ventura and Santa Barbara counties on Monday to assist in efforts to contain what has grown into the fifth largest wildfire in modern California history, authorities said. 

Media: FoxLA

A relentless Southern California wildfire grew by 4,000 acres overnight as stiff winds propelled the monster inferno across rugged and extremely dry terrain, leaving at least 18,000 homes and other structures in the posh communities of Santa Barbara and Montecito under threat.

By dawn Thursday, the Thomas Fire had burned 242,500 acres — nearly 380 square miles — since starting in Ventura County on Dec. 4. Gusty Santa Ana winds and single-digit humidity in the region “will continue to support fire growth to the west, east and north sides of the Thomas incident,” the California Department of Forestry and Fire protection, also known as Cal Fire, said its latest update on the conflagration.

The sprawling blaze, which on Thursday morning moved up to the fourth largest in modern state history, is also burning through Cal Fire’s firefighting budget, costing nearly $69 million to battle it.

“That’s the suppression costs, which includes the food, the fuel, paying for the firefighters and all their equipment. And the bills keep coming in,” said Scott McLean, a Cal Fire spokesman, adding that more than 8,100 firefighters were battling the blaze.

Gov. Jerry Brown and President Trump have both declared a state of emergency, freeing up state and federal resources to use in the fight.

The fire was 30 percent contained Thursday morning after devouring 972 structures and damaging 221 others in its now 11-day run over a 45-mile stretch from Santa Paula in Ventura County to the Santa Barbara city limits.

About 100,000 people remain under mandatory evacuation orders and a Red flag warning, due to the strong winds and dry weather, is in place through 10 a.m. Friday.

“When you have these really low humidities with any type of wind, you end up fighting a lot of spot fires and it’s kind of like playing whack-a-mole,” said Jason Hodge, a firefighter in Ventura County. “You’re running from spot to spot.”

McLean said significant winds in the area are again expected throughout Thursday and Friday and temperatures are forecast to be in the 80s.

“It’s going to be a significant battle today and tomorrow,” McLean told The Chronicle Thursday.

He said more favorable firefighting conditions are expected for the weekend with humidity rising significantly.

Much of the inferno has seeped into burn scar areas from fires that have erupted in the last decade, Hodge added.

“We are finding that when the fire reaches those older fire (areas), it essentially loses all of its momentum and either puts itself out or allows us to go direct on it and put out the line,” Hodge said.

Elsewhere in the region, the Creek Fire in Los Angeles County and the Lilac Fire in San Diego are both nearly 100 percent contained. The Rye Fire in Los Angeles County was 100 percent contained Tuesday evening.

The cause of all three conflagrations are still being determined, but investigators are looking at whether equipment owned by Southern California Edison, the electric utility company that serves much of Ventura County, was somehow involved in igniting the Thomas Fire.

“The investigations now include locations beyond those identified last week as the apparent origin of these fires,” the power company said in a statement this week. “SCE believes the investigations now include the possible role of its facilities.”

Sarah Ravani is a San Francisco Chronicle staff writer. Email: sravani@sfchronicle.com Twitter: @SarRavani