All the rain that has led to swollen rivers and flooding in parts of San Diego and large portions of Southern California has coincided with multiple snowstorms that blew across the Sierra Nevada in the northern half of the state. That may translate to a second consecutive year of robust output from the state’s hydroelectric power plants, which would help bolster the electric grid this summer. But officials at the California Independent System Operator, which manages the power system for about 80 percent of the state, aren’t celebrating yet. “It’s always encouraging to have a wet winter and a g…