Mother Nature's Christmas gift of several feet of fresh powder in the Sierra was just a start.Starting late Sunday, even more snow is forecast to fall, said Gina Beninato, a forecaster with the National Weather Service in Reno. And that means ideal conditions at Lake Tahoe ski resorts.For the first time this season, Squaw Valley, Northstar-at-Tahoe and Boreal are fully open thanks to cold, strong storm systems coming out of Alaska and meeting up with moisture-laden air.Snow is expected for the high Sierra again Sunday and Monday, Beninato said. Areas in lower elevations such as Grass Valley and Nevada City …