IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis
Box 3.6 Figure 1

Box 3.6, Figure 1. Percentage of the USA west of the Rocky Mountains (the 11 states west of and including Montana to New Mexico) that was dry (top) or wet (bottom), based on the Palmer Drought Severity Index for classes of moderate to extreme drought or wet. From NOAA, NCDC.