Data:  CAMS_OPI monthly precipitation on a 2.5 x 2.5 deg. lat/lon grid
Base Period:  All available data (i.e., Jan 1979-Present) are used to estimate the gamma distributions on which the SPI is based.
Source:  Climate Prediction Center, NCEP
Description:  The Standardized Precipitation Index (SPI; McKee 1993) is the number of standard deviations that observed cumulative precipitation deviates from the climatological average. It can be calculated for any time scale; the 1-, 3-, 6-, 9-, and 12-month time scales are shown here. To compute the index, a long-term time series of precipitation accumulations over the desired time scale are used to estimate an appropriate probability density function. The analyses shown here are based on the Pearson Type III distribution (i.e., 3-parameter gamma) as suggested by Guttman (1999). The associated cumulative probability distribution is then estimated and subsequently transformed to a normal distribution. The result is the SPI, which can be interpreted as a probability using the standard normal distrubtion (i.e., users can expect the SPI be within one standard deviation about 68% of the time, two standard deviations about 95% of the time, etc.) The analyses shown here utilize the FORTRAN code made available by Guttman (1999).

Extreme values of the SPI based on this version of the CAMS OPI dataset should be interpreted with caution. Studies have indicated that the SPI should be based on a time series of at least 50 years in length. While the CAMS OPI provides a global, near real-time analysis of monthly precipitation, its base time series length does not meet this recommendation. Global SPI analyses based on the global precipitation dataset from the University of East Anglia (1900-1998) can be found here.

Shading starts at +/- 1.0 with green and brown shades indicating unusually wet and dry conditions, respectively. Regions with an annual average precipitation of less than 0.2 mm/day have been "masked" from the plot. 

References
Guttman, N. B., 1999: Accepting the Standandardized Precipitation Index: a calculation algorithm. J. Amer. Water Resour. Assoc., 35(2), 311-322.

McKee, T.B., N. J. Doesken, and J. Kliest, 1993: The relationship of drought frequency and duration to time scales. In Proceedings of the 8th Conference of Applied Climatology, 17-22 January, Anaheim, CA. American Meterological Society, Boston, MA. 179-184.