The Daily Gridded SPI, developed by the NDMC and High Plains Regional Climate Center, is calculated at the station level and updated daily. Results of the Daily Gridded SPI can be viewed as a shaded map or dot map for the entire United States or six predetermined geographic regions (High Plains, Midwest, Northeast, South, Southeast, and West). Maps are available at the state level for the eight states in the High Plains region.
What Data Is Used?
Weather stations are selected for SPI calculation dynamically. There is no fixed list of stations for any state at any time. The selection is based on the availability of daily precipitation data historically and during the current time period. Stations must meet historical criteria and have current data available to calculate the Daily Gridded SPI.
Making the Map
The Daily Gridded SPI is generated by using Grid Analysis and Display System (GrADS). The map is available at multiple SPI time scales including 30-day, 60-day, 90-day, 6-month, 12-month, year-to-date (since Jan. 1), and water year (since Oct. 1). An archive of the Daily Gridded SPI is available back through 2003.
The map can be viewed as a dot map or a shaded map. The shaded map is interpolated using a Cressman objective analysis, and it has a grid resolution of 0.4 degrees.