Standardized Precipitation Index (SPI) Products

How frequent are such departures from climatology? How can this be expressed as a single number comparable across climates? The Standardized Precipitation Index was designed to explicitly express the fact that it is possible to simultaneously experience wet conditions on one or more time scales, and dry conditions at other time scales, often a difficult concept to convey in simple terms to decision-makers.

Consequently, a separate SPI value is calculated for a selection of time scales, covering the last 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 18, 24, 30, 36, 48, 60, and 72 months, and ending on the last day of the latest month. A more detailed description of this data set is available on-line. Link to: wrcc.dri.edu/spi/explanation.html More drought information is available from the National Drought Mitagation Center. drought.unl.edu/index.htm (Summary adapted from the WRCC Home Page)