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.”