What's sparking the devastating California wildfires

A wet winter followed by hot summer produced dry vegetation fueling blazes.

ByEMILY SHAPIRO
December 8, 2017, 11:58 AM

— -- As multiple wildfires ravage Southern California this week, experts are pointing toward a dangerous combination of last winter's rain, this year's dry and hot conditions and the Santa Ana winds as a cause.

At least six fires are fanning across swaths of the southern part of the Golden State, spanning from San Diego to Ventura counties. The blazes have burned more than 141,000 acres and forced over 212,000 people from their homes.

PHOTO: Firefighters battle to save one of many homes from a wildfire in Sylmar, Calif., Dec. 5, 2017.
Firefighters battle to save one of many homes from a wildfire in Sylmar, Calif., Dec. 5, 2017.
Gene Blevins/Reuters

Here is what's making this year's wildfires particularly fierce:

Heavy rain followed by hot temperatures led to dried-out vegetation

After five years of drought, California had a lot of rainfall last winter, leading to a lot of vegetation growth, said ABC News meteorologist Max Golembo. That was followed by very high temperatures in the summer and fall, including some record heat events, he said. Those hot temperatures dried out much of the vegetation that grew last winter and spring, which is now creating fuel for the wildfires, Golembo said.

That "specific sequence -- of a wet winter followed by warm, hot conditions to dry the fuel out" is what's needed to spark these larger fires, said Park Williams, a climate scientist and an assistant professor at Columbia University's Lamont-Doherty Earth Observatory.

Add to that the fact that this fall and winter "there has not yet been any precipitation," Williams said.

"Usually by now, Southern California has gotten a storm or two. And once the first rain comes that eliminates the chances of really big wildfires occurring."

Santa Ana winds

PHOTO: The Thomas Fire burns along a hillside near Santa Paula, Calif., on Dec. 5, 2017.
The Thomas Fire burns along a hillside near Santa Paula, Calif., on Dec. 5, 2017. More than a thousand firefighters were struggling to contain a wind-whipped brush fire in southern California on December 5 that has left at least one person dead, sent thousands fleeing, and was choking the area with thick black smoke. / AFP PHOTO / Kyle GrillotKYLE GRILLOT/AFP/Getty Images
Kyle Grillot/AFP/Getty Images

Golembo and Williams also pointed to a major Santa Ana wind event for intensifying the flames.

Santa Ana winds occur when a high pressure system over Southern California pulls air out of "the desert of the Southwest, and brings that air to the west out over the coast," Williams said.

PHOTO: A man uses a hose to extinguish the Creek Fire as it burns along a hillside near homes and horses in the Shadow Hills neighborhood of Los Angeles, Calif., on Dec. 5, 2017.
A man uses a hose to extinguish the Creek Fire as it burns along a hillside near homes and horses in the Shadow Hills neighborhood of Los Angeles, Calif., on Dec. 5, 2017.
Kyle Grillot/AFP/Getty Images

"Because it's coming from the desert, the air is really dry. Then that air has to come up and over the mountains ... and then is brought all the way down to sea level," Williams said. "When that air comes down to sea level, it gets compressed, and when dry air gets compressed, it warms. So this air that was out over the desert, when it hits L.A. and the areas around L.A., it is really warm and really dry."

That warmth and dryness is key for fueling the fires, Golembo said.

December is the prime month for Santa Ana winds in Southern California, Golembo said.

ABC News Live

ABC News Live

24/7 coverage of breaking news and live events