The 9-month SPI provides an indication of precipitation patterns over a medium time scale.
Droughts usually take a season or more to develop.
SPI values below -1.5 for these time scales are usually a good indication that fairly significant impacts are occurring in agriculture and may be showing up in other sectors as well. Some regions of the country may find that the pattern displayed by the map of the Palmer Index closely relates to the 9-month SPI maps. The Palmer Index maps are updated each week, although the patterns usually do not change significantly. The SPI maps are updated at the end of each month.