Complex sentence: In the vicinity of Aurora there had been light snowfall in November, then mild weather until Christmas Eve when it began a heavy and rapid snowfall for days, the temperature dropped below zero and the passes over the Sierras were closed.
Split into simpler sentences: In the vicinity of Aurora, there had been light snowfall in November, then mild weather until Christmas Eve, when there began a heavy and rapid snowfall for days. The temperature dropped below zero and the passes over the Sierras were closed.