The SPI index was developed by T.B. McKee, N.J. Doesken, and J. Kleist in 1993. The data and climate division boundaries are from the National Climatic Data Center. The Western Regional Climate Center uses the NCDC data to calculate SPI values for each climate division. The information is then reclassed and mapped at the National Drought Mitigation Center using a Geographic Information System.
Use the links in the box to the right to learn more about interpreting SPI maps.