Wildfires already well on their way to becoming the deadliest and most destructive in California history could gain momentum today and erase even the modest gains firefighters have made.
Steady winds with gusts up to 45mph (72kph) with nearly non-existent humidity are expected to descend on the areas north of San Francisco where at least 23 people have died and at least 3,500 homes and businesses have been destroyed.
"It's going to continue to get worse before it gets better," state fire Chief Ken Pimlott said yesterday.
Entire cities had evacuated in anticipation of the next wave, their streets empty, the only motion coming from ashes falling like snowflakes.
They included Calistoga, the historic resort town of wine tastings and hot springs, whose 5,300 people are under evacuation orders. Tens of thousands more were also driven from their homes by the flames. A few left cookies for firefighters and signs reading "Please save our home!"
The 22 fires spanned more than 265 square miles (686 sq km) as they entered their fourth day, many of them completely out of control. Modern, strategic attacks that have kept destruction and death tolls low in recent years just have not worked against their ferocity.
"We are literally looking at explosive vegetation," Mr Pimlott said. "Make no mistake, this is a serious, critical, catastrophic event."
The community of Boyes Hot Springs in Sonoma County was also told to get out on Wednesday, and the streets were quickly lined with cars packed with people fleeing.
"That's very bad," resident Nick Hinman said when a deputy sheriff warned him that the driving winds could shift the wildfires toward the town where 11,000 people live. "It'll go up like a candle."
Ash rained down on the Sonoma Valley, covering windscreens, as winds began picking up towards the potentially disastrous forecast speed of 30mph (48kph). Countless emergency vehicles sped toward the flames, sirens blaring, as evacuees sped away. Residents manhandled canvas bags into cars jammed with possessions or filled their fuel tanks.
State fire spokesman Daniel Berlant said 22 wildfires were burning yesterday, up from 17 the day before. As the fires grow, officials voiced concern that separate blazes would merge into even larger infernos.
"We have had big fires in the past. This is one of the biggest, most serious, and it's not over," Governor Jerry Brown said at a news conference, alongside the state's top emergency officials.
They said 8,000 firefighters and other personnel were battling the blazes and more resources were pouring in from Arizona, Nevada, Washington and Oregon.
Flames have raced across the wine-growing region and the scenic coastal area of Mendocino farther north, levelling whole neighbourhoods and leaving only brick chimneys and charred appliances to mark where homes once stood.
In Boyes Hot Springs, residents had watched the ridges over the west side of town for days to gauge how close the billowing smoke and orange flames of the wildfires had come. Yesterday, the ridges were obscured by the growing clouds of smoke.
With fires advancing from several sides in Sonoma Valley, law enforcement officers on loan from other areas of Northern California barred residents of evacuated communities from returning to see how the homes and businesses had fared. Manned roadblocks were set up between Sonoma and devastated areas of Santa Rosa.
Alejandro Rodriguez had been evacuated from one tiny Sonoma Valley town, only to have deputies come to the neighbourhood where he had relocated and tell residents there to pack up and go.
"I want to see my house, see if anything's left," Mr Rodriguez said, gesturing at officers at one roadblock. "They won't tell us nothing."
Sonoma County Sheriff Robert Giordano said hundreds of people were still reported missing, but officials believe many will be found. Chaotic evacuations and poor communications over the past few days have made locating friends and family difficult.
The sheriff also expects the death toll to climb.
"The devastation is enormous," he said. "We can't even get into most areas."
Helicopters and air tankers were assisting thousands of firefighters trying to beat back the flames. Until now, the efforts have focused on "life safety" rather than extinguishing the blazes, partly because the flames were shifting with winds and targeting new communities without warning.
Fires were "burning faster than firefighters can run, in some situations", emergency operations director Mark Ghilarducci said.
In Southern California, cooler weather and moist ocean air helped firefighters gain ground against a wildfire that has scorched nearly 14 square miles (36 sq km).
Orange County fire officials said the blaze was 60% contained and full containment was expected by Sunday, although another round of gusty winds and low humidity levels could arrive later today.