What’s driving Northern California’s freak ‘fire siege’?

More than 300 fires are the result of rare ‘trifecta’ of fire conditions

The weekend’s record-bursting heat wave and freak summer lightning storm have left an already parched Northern California with a rash of rapidly spreading wildfires — more than 300 blazes — something rarely seen before and possibly unprecedented in scope, climate scientists say.

And if that’s not alarming enough — “It probably gets worse before it gets better,” said Daniel L. Swain, a climate scientist at the UCLA Institute of the Environment and Sustainability.

Onshore winds Wednesday threaten to whip up what is already “a really serious fire siege across Northern California,” he said.

Northern California hasn’t been jolted by lightning storms like the one that hit last weekend in more than a decade, Swain said. In 2008, lightning sparked more than 2,000 wildfires from Monterey County to the Oregon border, including the Inyo Complex Fire that scorched the Big Sur coast, forcing evacuations and closing Highway 1.

But last weekend’s lightning storm — remnants of Tropical Storm Fausto — collided with a massive heat wave that settled over the southwestern U.S. and burst temperature records across California, including what may be the hottest temperature ever recorded on earth, 130 degrees in Death Valley.

In June 2008, storms delivered some 5,000 lightning strikes over a 48-hour period. In last weekend’s storms, firefighters say there were at least 10,800 lightning strikes. Those strikes have sparked at least 367 fires across California, including three major blazes in the Bay Area. More than 30,000 residents have been evacuated in the Bay Area from fires in Sonoma, San Mateo and Santa Cruz counties.

“The problem is it was so unbelievably hot, and the amount of dry lightning and also if you think back to this past winter, it was very dry across most of Northern California,” said Swain, a research fellow at the National Center for Atmospheric Research.

“That’s the trifecta. It’s hard to imagine anything worse.”

Normally, the coastal fog keeps fires along the Northern California coast at bay. But Swain and other experts say the relatively dry winter, heat wave and “dry lightning” from the storm that produced thousands of lightning strikes but little rainfall allowed “holdover” fires to fester into the week.

Also known as “sleepers,” they are fires that start with a lightning strike that leaves a tree or bush smoldering, said Brent Wachter, a fire meteorologist at U.S. Forest Service’s northern operations center in Redding.

“The fire can burn inside the tree and not really reveal itself, and eventually the tree may fall or a limb,” Wachter said. “It could be two or three days after the event, sometimes seven to 14 days. In the right conditions, it starts to activate.”

“These aren’t something you plan for every year, these conflagrations are pretty rare,” Wachter said. “You’ve got to go back to 2008 to find a comparison, and that’s over a decade ago. You’ve got to have all those factors lined up.”

Will this week’s infernos be worse? It’s hard to say.

As in 2008, with visibility reduced by thick smoke, it’s hard to know where all the fires are.

“The number of fires in the Bay Area from this weekend is extraordinary,” Swain said. “I don’t know if it’s unprecedented, because we don’t know how many there are. It’s actually overwhelming, there’s so much fire going on in Northern California.”

And the outlook for the next few days is worrisome. With the heat wave dissipating, onshore breezes from the Pacific Ocean are picking up. That’s ordinarily a blessing, bringing cooler, moister air to the parched region.

But wind and fire are never a good mix. Though these aren’t the withering hot, dry Santa Ana type desert winds that blow from the eastern high deserts in the fall and fuel California’s peak wildfire season, the onshore breezes are a concern with active fires burning summer dry brush and woodland.

“Even though there’s a slight cooling, that will bring strong winds,” Swain said. “It’ll be a bit of a reprieve with the cooler temperatures. But it will increase fire risk.”