At 6:30 am on November 8, a wildfire of astounding proportions and speed broke out in Northern California. Dubbed the Camp Fire, at one point it was burning 80 acres a minute.

When it hit the town of Paradise, home to 27,000 people, those buildings became yet more fuel to power the blaze. It destroyed over 18,000 structures. For perspective, the previously most destructive wildfire in state history, Tubbs Fire that raged through the city of Santa Rosa last year, destroyed 5,500 total structures.

The death toll so far stands at 88. That makes it by far the deadliest wildfire in California history. Hundreds are still missing.

“We're seeing urban conflagrations, and that's the real phase change in recent years,” says Stephen Pyne, a wildfire expert at Arizona State University. It used to be that fires destroyed exurbs or scattered enclaves. “But what's remarkable is the way they're plowing over cities, which we thought was something that had been banished a century ago.”

The Camp Fire horror show, which burned 70,000 acres in 24 hours, and reached over 150,000 acres, is a confluence of factors. The first is wind—lots of it, blasting in from the east. “We have a weather event, in this case a downslope windstorm, where, as opposed to the normal westerly winds, we get easterly winds that are cascading off the crest of the Sierra Nevada,” says Neil Lareau, an atmospheric scientist at the University of Nevada, Reno.

A windstorm barreling from the east set the stage for this disaster. It’s a normal phenomenon that comes from the jet stream, which this time of year grows stronger. North and south “meanders” in the jet stream, known as troughs and ridges, get amplified. These cold air masses travel through the Great Basin in Nevada and spill over the Sierra Nevada Mountains in eastern California. Big meanders set up very-high-pressure areas that accelerate winds.

“Then they get local accelerations on top of that as they flow down the mountain ranges, kind of like water over a dam,” Lareau says. Some areas in California are particularly prone to downsloping winds. “Unfortunately, right where the Camp Fire is is one of those places.”

“I always like to say nothing good comes from an east wind in California,” Lareau adds.

As the air descends at an accelerating pace, it warms up and drives the relative humidity down. Which brings us to our second factor in the horror show: fuel—lots of it. It may be November, but California is still extremely dry, which means plenty of vegetation that’s primed to go up in flames.

The east winds further dehydrate the vegetation. This is where something called the evaporative demand drought index comes in. “You can think about it as how thirsty the atmosphere is,” Lareau says. “How strongly does the atmosphere want to pull water out of the vegetation and out of the ground?”

Very strongly, in the case of the Camp Fire and those downslope winds. So it isn’t just a matter of things being generally dry for the season in Northern California—ground and vegetation moisture fluctuates day to day, too. Scientists can calculate this in part by going out and cutting vegetation, weighing it, drying it out, and weighing it again.

“This tells us those fuels have been drying out really, really rapidly over the past few days and into this event,” Lareau says. Just take a look at the eerily prescient tweet below from meteorologist Rob Elvington the day before the Camp Fire broke out.

So you’ve got hot, dry gusts of 40 or 50 miles per hour from the northeast pushing the fire, and the fire is itself creating wind, further accelerating the conflagration. As it moves along, embers fly out of the front of the fire. “As the fuels get drier, a smaller and smaller spark can leapfrog the fire through the landscape,” Lareau says. “That's just another way this thing comes up and bites you.”