Current and Archived Maps - view climate division based 1-, 3-, 6-, 9-, 12-month, and year-to-date SPI maps (updated monthly) going back to 1996.
The Standardized Precipitation Index (SPI), developed by T.B. McKee, N.J. Doesken, and J. Kleist and in 1993, is based only on precipitation. One unique feature is that the SPI can be used to monitor conditions on a variety of time scales. This temporal flexibility allows the SPI to be useful in both short-term agricultural and long-term hydrological applications. The NDMC’s overview of drought indices contains an in-depth look at the SPI.
Calculating the SPI
The Western Regional Climate Center uses precipitation data from the National Climatice Data Center (NCDC) 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. The maps are based on preliminary precipitation data, and the data’s source and methods used in incorporating the data into a final product must be considered carefully when analyzing these maps. “Interpretation of SPI Maps” explains how to interpret the maps at various timescales. The SPI program and sample files are available for downloading.