In the parched southwestern United States, few forecasts are as important as the future height of Lake Mead, which tells federal authorities how much water to release to the 20 million people living downstream of the giant reservoir. This year, the U.S. Bureau of Reclamation is testing out a new tool it hopes will make those projections a little better: A model that can predict — months in advance — the summer rainfall associated with the North American Monsoon.