California’s snowpack has almost completely disappeared, and that’s not a good sign for water supply. A key survey found no measurable snow in the Sierra, with statewide levels sitting at just 18 percent of normal. Officials say a hot March and more rain than snow melted things down weeks ahead of schedule. That April snowpack reading is a big deal because it helps predict how much water the state will have this summer. Now, experts say it’s another sign of how unpredictable weather patterns are becoming.