Why It’s So Hard to Predict Exactly Where Wildfires Will Strike

After at least 21 people were killed and more than 2,000 structures were destroyed by fast-moving wildfires raging across northern California this week, some are wondering: Is there a way to better predict wildfires and warn residents before they start?

Unfortunately, experts say the nature of California fires makes it almost impossible to create an accurate early-warning system like those that exist for other natural disasters, including hurricanes and tornadoes.

“With wildfires, there is nothing typically natural that would give an indication ahead of time that a fire is going to spark,” Daniel Berlant, assistant deputy director for Cal Fire, told TIME. “Unlike a hurricane where you’re looking at weather forecasts to make predictions, we don’t have that luxury.”

That’s in part because more than 90% of wildfires are triggered by human activity, whereas tornadoes and hurricanes are the direct result of certain atmospheric conditions. (The other 10% of wildfires are sparked by lightning strikes.) Because it’s impossible to track every backpacker starting a campfire or teenager lighting off fireworks, predicting human-caused wildfires is an impossibility.

Nevertheless, forecasters can and do issue warnings when conditions are ripe for wildfires to begin. Dry climates, strong winds and lightning in the forecast can all make wildfires more likely to occur and spread.

During those weather conditions, the National Weather Service will issue “Red Flag Warnings,” which indicate that a wildfire is likely to occur over the next 24 hours. When Red Flag Warnings are issued, firehouses will usually increase their staff to prepare for a potential wildfire outbreak. Meanwhile, residents are urged to act extremely cautiously during any activity that could spark a fire, clear vegetation around their property and avoid mowing the grass.

It’s not yet clear what started the northern California fires. Brooke Bingaman, a forecaster for the National Weather Service in Sacramento, told TIME that a warning was in place for much of the area on Sunday and Monday, but the winds had been forecasted the week before. Another warning is currently in place in Sacramento through Thursday evening.

Ultimately, because so many wildfires are caused by people, experts say the real key to prevention is simply teaching people how to avoid starting them.

“Don’t park on dry grass, don’t drag chains down the road, don’t leave your campfire unattended, and make sure it’s dead out before leaving it,” Robin Broyles, a spokesperson for the National Interagency Fire Center, wrote in an e-mail to TIME. “The best way to prevent wildfires is to keep them from occurring in the first place.”