Like St. Elmo's Fire, it's a Hollywood movie as well as an actual weather event. The real Express is a subtropical jet stream of warm, moist air that originates over the tropical Pacific and gets pulled north by pressure differences over the Gulf of Alaska, sending fat-bottomed clouds streaming to the West Coast several times each winter. "It's like a fire hose of moist air pumping two to five times the amount of precipitation you'd get in a normal winter storm," says Tanner. Since warm air holds more moisture than cool air, this means big snows in the Sierra. It also caused the November 2006 deluge that dumped 18 inches of rain on Mount Rainier National Park in 48 hours.