Photo: The Canadian Press

Chris Shiery waits to evacuate in Sonoma, Calif., Wednesday.

Gusting winds and dry air forecast for Thursday could drive the next wave of devastating wildfires that are already well on their way to becoming the deadliest and most destructive in California history.

Winds up to 72 km/h were expected to pummel areas north of San Francisco where at least 23 people have died and at least 3,500 homes and businesses have been destroyed. The conditions could erase modest gains made by firefighters.

"It's going to continue to get worse before it gets better," state fire Chief Ken Pimlott said Wednesday.

Entire cities had evacuated in anticipation of the next round of flames, their streets empty, the only motion coming from ashes falling like snowflakes.

In Calistoga, a historic resort town known for wine tastings and hot springs, 5,300 people were under evacuation orders. Tens of thousands more have been driven from their homes by the flames. A few left behind cookies for firefighters and signs reading, "Please save our home!"

The 22 fires, many out of control, spanned more than 686 square kilometres as the inferno entered its fourth day. Strategic attacks that have kept wildfire destruction and death tolls low in recent years haven't worked against the ferocity of the blazes.

"We are literally looking at explosive vegetation," Pimlott said.

"Make no mistake," he added later, "this is a serious, critical, catastrophic event."

Residents in the community of Boyes Hot Springs in Sonoma County were told to clear out Wednesday, and the streets were quickly lined with cars packed with fleeing people.

Officials said 8,000 firefighters and other personnel were battling the blazes, with more resources pouring in from Arizona, Nevada, Washington and Oregon.