Santa Ana winds are strong, dry gusts that blast through Southern California's canyons and passes, from the Santa Clara River Valley near Interstate 5 to Cajon Pass along Interstate 15 and the Santa Ana Mountains in Orange County. They form when high pressure builds over the Great Basin and funnels air through mountain gaps, sometimes topping 100 mph in canyons like Fremont Canyon. They're most common from October through March — exactly why they're the biggest wildfire risk factor forecasters watch each fall and winter.