California is, once again, burning. A freak summer thunderstorm last weekend swept through the northern part of the state, sparking nearly 400 wildfires, many of which have turned into conflagrations of stunning scale. As of Friday morning, two groups of wildfires, the SCU Lightning Complex near San Jose and the LNU Lightning Complex near Napa, have burned 230,000 acres and 220,000 acres, respectively—that’s 700 square miles total. Fire crews are stretched far too thin to deal with this many wildfires at once, and have barely contained any of those blazes. In Santa Cruz, the CZU August Lightning Complex has burned an additional 50,000 acres, and has led to mass evacuations. Five people have been killed.

At the core of the crisis is some very weird weather, all across the board. For one thing, California isn’t supposed to be peppered with lightning strikes in August. But as Tropical Storm Fausto moved north from Baja California last week, it hit cooler waters and fell apart, sending a stream of moisture into Northern California. “To have that much thunder and lightning come through California, that in itself is rare,” says NBC Bay Area meteorologist Rob Mayeda. “But to see more than 350 wildfires start just like that, that’s a convergence zone of very unfortunate conditions.”

Last weekend’s thunderstorms lit so many fires because they didn’t always dump water to accompany the lightning strikes. Well, some did … but if that water falls from too high in the atmosphere, and the weather is warm and dry enough, it’ll actually evaporate before it hits the ground. Unfortunately for California, the state was already suffering a heat wave when the thunderstorms rolled through. So when lightning struck some 11,000 times across the state in 72 hours, there wasn’t always rain to snuff out the sparks.

Plus, those storms were moving at 35 or 40 miles an hour. “So they would drop lightning bolts down and it would rain, but then it moves out so fast,” Mayeda says. “So it was wetting the ground, but it wasn’t sustained enough to prevent it from putting it out, so to speak. It’s the one-two punch of the heat first, and then fast-moving storms.”

So now the California landscape has broken out in hives of fire. Already parched thanks to drought and a heat wave, the vegetation is extremely dry and flammable. Then Tuesday night, a so-called nocturnal drying event, or NDE, took hold across the Bay Area. In the mountains, very dry air forms right above the cool, wet marine layer, which usually helps plants hold on to their moisture. But this particular NDE dropped humidity levels down to 15 percent at night, further desiccating vegetation, and fast: dead plants might respond to the drying in only an hour. The drier the fuel, the hotter it burns.

Then the wind, a critical component in spreading wildfires, went haywire. “Even though the surface winds were pretty weak, there was a low-level jet above the mountains,” says Craig Clements, a fire weather researcher (and fire chaser) at San Jose State University. This turbulence produced high winds that then mixed down at the surface, creating gusts. Firefighters battling the blazes—already burning out of control, thanks in part to the effect of the NDE—had the fires go squirrelly on them as the blazes reacted to changes in wind. “They were reporting erratic fire behavior,” Clements says. “And that’s due to potentially this low-level jet that was occurring over the region, and this dry air that had moved into the region at night.”