It takes three or four months to assemble final, quality-controlled data. Using preliminary data for the SPI makes it possible to use it as part of a drought early-warning and monitoring system. However, preliminary data are based on measurements gathered from 450 to 550 stations nation-wide each month.
Final data includes measurements from more than twice as many reporting stations.
Stations tend to be closer together in the eastern and central states, so there might be one or two stations per climate division. But in the west, some climate divisions may have no stations reporting in a particular month, or may lack a first or second order station altogether, so SPI values are interpolated from surrounding divisions. This means that local anomalies are not as well-reflected by preliminary data as they would be by final data. Those SPI maps are part of the state's drought early warning system. The National Drought Mitigation Center is working to compute a contour-based SPI map first for Nebraska, where the data is nearly available, and then for other regions in the United States.