National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center (CPC)
The experimental Global Gridded Standardized Precipitation Index (SPI) is derived from the NOAA CMORPH dataset and includes timescales of 1, 3, 6 and 9 months. The NOAA CMORPH precipitation dataset is a gridded dataset derived from combining numerous microwave-based estimates from low orbiter satellites. The dataset is also archived and managed through the NOAA Climate Data Record program at NOAA's National Centers for Environmental Information (NCEI).
While the CMORPH precipitation dataset is operational, the derived SPI products are currently experimental as the results are evaluated and the stability of the processing system is improved. The SPI is available with both the gamma and Pearson Type III types of distribution fittings, using the Python Drought Indices open-source package.
Joyce, R. J., J. E. Janowiak, P. A. Arkin, and P. Xie, 2004: CMORPH: A method that produces global precipitation estimates from passive microwave and infrared data at high spatial and temporal resolution.. J. Hydromet., 5, 487-503. https://ftp.cpc.ncep.noaa.gov/precip.orig/CMORPH_V1.0/REF/Joyce_et_al_2004_JHM_CMORPHP.pdf
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.