The Palmer Drought Index is updated weekly by the Climate Prediction Center. It is based on rainfall, temperature and historic data, and is computed based on a complex formula devised by W.C. Palmer in 1965. Although the Palmer is the main drought index used by the U.S. government, it is slow to detect fast-emerging droughts, and does not reflect snowpack, an important component of water supply in the western United States. Credit: NOAA