A set of atmospheric rivers that brought heavy rains and floods to California also dumped a record amount of snow on Mammoth Mountain in January — 20½ feet, the most in the resort town’s history, local tourist officials announced. “What a time it is to be at Mammoth,” the announcement said on MammothMountain.com, which represents area resorts. “Conditions are all-time, get out there and have the ‘best pow day of your life.’ ”