National Oceanic and Atmospheric Administration's (NOAA) National Centers for Environmental Information (NCEI)
The U.S. Gridded Standardized Precipitation Index (SPI) is derived from the nClimGrid-Monthly dataset and includes timescales of 1, 2, 3, 6, 9, 12, 24, 36, 48, 60 and 72 months. The nClimGrid-monthly dataset is a gridded dataset derived from spatially interpolating data from the Global Historical Climatology Network (GHCN), starting in 1895.
The SPI is available with both the gamma and Pearson Type III types of distribution fittings, using the Python Drought Indices open-source package.
Guttman, N. B., 1999: Accepting the standardized precipitation index: A calculation algorithm. J. Amer. Water Resour. Assoc., 35, 311–322, https://doi.org/10.1111/j.1752-1688.1999.tb03592.x.
Wu, H., M. J. Hayes, D. A. Wilhite, and M. D. Svoboda, 2005: The effect of length of record on the standardized precipitation index. Int. J. Climatol., 25, 505–520, https://doi.org/10.1002/joc.1142.
Wu, H., M. D. Svoboda, M. J. Hayes, D. A. Wilhite, and F. Wen, 2007: Appropriate application of the standardized precipitation index in arid locations and dry seasons. Int. J. Climatol., 27, 65–79, https://doi.org/10.1002/joc.1371.
Cumbie-Ward, R. V., and R. P. Boyles, 2016: Evaluation of a high-resolution SPI for monitoring local drought severity. J. Appl. Meteor. Climatol., 55, 2247–2262, https://doi.org/10.1175/JAMC-D-16-0106.1.
Vose, R. S., and et al, 2014a: NOAA’s Gridded Climate Divisional Dataset (CLIMDIV). NOAA National Climatic Data Center, accessed 4 June 2018, https://doi.org/10.7289/V5M32STR.