Projections of precipitation are less certain than those for temperature, however projections indicate that the southwestern U.S. may experience chronic future precipitation deficits, particularly in the spring. With continued greenhouse gas emissions, higher temperatures would cause more frequent and severe droughts in the Southwest. Higher temperatures sharply increase the risk of megadroughts—dry periods lasting 10 years or more. Snowpack supplies a major portion of water in the Southwest, but with continued emissions, models project substantial reductions in snowpack. Under high emissions scenarios (RCP 8.5), models project annual declines of river flow in southern basins (the Rio Grande and the lower Colorado River), and either no change or modest increases in northern basins (northern California and the upper Colorado River). Uncertainty in the magnitude and timing of future southwestern drying is high, and fewer days with precipitation may lead to increased year-to-year variability.