With another round of winter storms hitting California this week, the question isn’t just how much rain and snow they will dump, but how cold they will be. The coldness of storms can make the difference between one that adds to the fast-rising snowpack — an essential source of water for the state — and one that also leaves a wet mess. Northern California was pulled out of a five-year drought by a series of storms over the last few weeks that deposited huge amounts of snow over hundreds of miles of the state’s greatest mountain range, the Sierra Nevada.