Standardized Precipitation Index

The SPI is a drought index, developed by then-Colorado state climatologist Thomas McKee and others, based on the historical probabilities of receiving a given amount of precipitation over a given period. The SPI was designed to explicitly capture that it is possible to simultaneously experience relatively wet conditions on one or more time scales, and relatively dry conditions at other time scales. Consequently, a separate SPI value is calculated for different time scales.

The SPI maps shown on the Dashboard are developed by the National Drought Mitigation Center (NDMC) and the High Plains Regional Climate Center (HPRCC). The HPRCC SPI maps shown here, updated daily, are also available at timescales of 1, 2, 4, 6, 9, and 24 months. The Western Regional Climate Center (WRCC) produces a set of national SPI maps showing values for each climate division, updated monthly, at timescales of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 18, 24, 30, 36, 48, 60, and 72 months. 

The historical probabilities of receiving a given amount of precipitation are standardized so that an SPI of zero indicates the mean historical precipitation (1981-2010), and other SPI values correspond to the number of standard deviations that the observed precipitation value departs from the long-term mean, for a normally distributed random variable. Since precipitation is not normally distributed at most timescales in most regions, a transformation is first applied so that the transformed precipitation values follow a normal distribution. 

An SPI value of -1 indicates moderate drought severity and means that only 16% of all periods would be expected to be drier. An SPI value of -2 indicates severe drought, with only 2.5% of periods would be expected to be drier. For more information on the SPI and how it is calculated see this page at the Western Regional Climate Center.