With snow failing to pile up in the Sierra Nevada, Californians are starting to think about the unthinkable again — another round of drought to shrivel lakes, parch the landscape and roil power markets. The mountain range hasn’t been this bare since the depths of the monster dry spell that was declared over only last year. “The snow numbers are abysmal,” said Brian Fuchs, a climatologist at the National Drought Mitigation Center. Some ski resorts are struggling to keep trails open.