 The Thomas Fire was generating massive pyrocumulus clouds, which were gathering heat from the flames below, although the unstable column of air could collapse at any time, which would generate sudden wind shifts near the fire and pose a major hazard to firefighters.This behavior had been previously observed during the massive 2009 Station Fire in Los Angeles.At this point, the Thomas Fire had become the fifth-largest wildfire in modern California history and the largest wildfire recorded in California in December.