SACRAMENTO, Calif. — While people were celebrating the end of a sunny New Year’s Eve weekend, the snow in the Sierra Nevada suddenly turned as brittle and spiky as the old ice cream in your freezer. That Jan. 1 phenomenon — when snow vaporized upward at night, refreezing into a layer of fragile crystals — is blamed for this month’s frightening proliferation of avalanches that are occurring across a broad swath of the Lake Tahoe landscape, including one that killed a skier at Palisades Tahoe. There have been more than 50 so far, and it may get worse: This weekend’s rain and heavy snow could add…