What a difference a month makes. On Jan. 1, the Sierra Nevada snowpack, the source of nearly one-third of California’s water supply, was a meager 28% of its historic average. October, November and December had brought few storms, leaving ski resorts with many runs closed and water managers around the state beginning to get nervous that California could be heading back into the kind of dry conditions that defined the 2020-22 drought. But since then, winter has arrived. Multiple atmospheric river storms have sent the Sierra Nevada snowpack back to respectable levels. On Wednesday, it was 74% of …