Intense, fast-moving fires have been raging across much of California since Sunday night. The blazes have barreled through communities like freight trains, turning homes to dust in a blink and leaving at least 23 people dead. The largest of the fires are in the state’s wine country north of San Francisco.

“We’ve never seen anything like this,” said Trina Grant, 40, who grew up in California but was not prepared for the ferocity of this year’s fire season. Her parents, Arthur and Suiko Grant, died on Monday when flames consumed their home.

Why have these fires been so destructive?

Wildfires often break out in California in October after the state’s dry, sunny summers. The fires are worse this year because of record heat over the summer and high winds now, which can swiftly turn the smallest fires into fast-moving infernos.

Weather experts note that this year’s outbreak was a long time in the making.

Drought parched California for years, leaving it littered with fuel in the form of dry vegetation. Then the winter of 2016 and the spring of 2017 brought record amounts of rainfall, which spurred new plant growth. That was followed by months of extreme heat that withered the new growth and turned it into more tinder.