The Standardized Precipitation Index (SPI), developed in 1993, shows precipitation shortfall and excess over a variety of time scales. Longer intervals affect groundwater, stream-flow and reservoir storage. Shorter intervals are helpful in estimating soil moisture, which is very important to farmers and responds fairly immediately to rainfall or the lack of it.

The Daily Gridded Standardized Precipitation Index is being developed by the National Drought Mitigation Center and the High Plains Regional Climate Center, using Applied Climate Information System (ACIS) data from all of the regional climate centers. This version of the SPI helps U.S. Drought Monitor authors fine-tune accuracy at scales smaller than counties and climate divisions. It aggregates and interpolates climate data into a grid that breaks the U.S. down into generally uniform 40-square-kilometer areas. The sub-state lines on the map are county lines, which allow users to orient themselves to familiar geopolitical boundaries. Automated systems update data and maps daily.

The National Drought Mitigation Center has been producing a set of SPI maps every month since 1996. Each month's maps reflect precipitation shortfall or excess at a variety of time intervals. This information is also available at the Western Regional Climate Center with the SPI and related information available for up to six years at different time scales, and at NOAA's National Climatic Data Center for up to five years at different time scales. These SPI products provide spatial resolution at the climate division level.