On Wednesday and Thursday, high seasonal winds will tear through California, drying out vegetation and fanning wildfires. The conditions could easily spell a devastating, deadly conflagration. In preparation, early Wednesday morning the utility PG&E— whose equipment sparked last year’s Camp Fire, which killed 86 people and destroyed the town of Paradise—will begin preemptively shutting off power to a staggering 800,000 customers.

Those customers are not happy, and for good reason: Losing power is a hassle for anyone, but it’s potentially deadly for those who rely on electrical medical devices. Businesses lose business, food spoils in warming fridges, and critical infrastructure goes offline. But this is no shot in the dark—meteorologists can predict where and when those winds will grow dire, so PG&E can target their shutoffs. It’s a calculus that climate change is making increasingly familiar. But blaming the climate alone would be letting California off the hook. Its policies and building habits are also responsible for the darkness that must now descend on northern portions of the state.

California’s wildfire problem grows from a clash of contrasts. In the atmosphere at this time of year, pressure builds up in air masses over the Great Basin, east of the state. At the same time, a low pressure region takes shape near the coast. Because air tends to move from high- to low-pressure areas, winds start accelerating from the northeast toward the coast. The greater this pressure gradient, the stronger the winds.

As the winds move over the Sierra Nevada in eastern California, they flow like water over rocks in a stream, compressing and warming. Slicing through valleys, the winds gather more speed, desiccating the air. “If you imagine the atmosphere over your head as a sponge, you can’t wring it out anymore,” says Daniel Swain, a climate scientist at UCLA.

At ground level, the warm air screaming through the mountains sucks away whatever moisture might be left in the vegetation—which is increasingly little as the climate warms in California and autumns grow increasingly dry. What’s left is a parched landscape that’s primed to burn, and winds of 60 or 70 miles per hour can speedily turn a spark into a fast-moving wildfire. Such was the case in last year’s Camp Fire: Winds picked up embers and blew them perhaps a mile ahead of the main conflagration, setting a multitude of small fires throughout the town of Paradise, overwhelming firefighters.

Because meteorologists know why and when and where these winds form, they can use models to give perhaps a week’s warning of a major wind event, like one coming up. Ground-level data, like topography, sharpens the forecast to show where winds might be fiercest. So PG&E is cutting off power in the particularly dangerous zones it has identified in Northern California, where high winds might rustle power lines and shower sparks onto the wind-parched vegetation below. Specifically, says PG&E spokesperson Ari Vanrenen, they’re looking for humidity levels below 20 percent and sustained winds above 20 mph or gusts over 45 mph.

If the conditions align, PG&E initiates what it calls public safety power shutoffs, and they’re tortured decisions. “It is sort of unprecedented for such a large utility doing this preemptively,” says Swain. “They are probably an unfortunately necessary stopgap fire-prevention measure right now, but they come with serious risks as well.”

A utility like PG&E is mandated to provide power, because doing so isn’t just a matter of modern conveniences—it can be a matter of life and death. That’s especially true in the Golden State's mountain towns that are most at risk of catastrophic wildfire, many of which are retirement communities. The elderly may rely more heavily on medical appliances and be more vulnerable to heatstroke without air conditioning. By preemptively cutting off power, you’re also potentially cutting off communication—if the power goes out and a wildfire starts, and TVs and internet routers don’t work, people could be at risk. Electric water pumps too would go offline, potentially hampering firefighting efforts.