Overview: The SPI is an index based on the probability of precipitation for any time scale.
Who uses it: Many drought planners appreciate the SPI’s versatility.
Pros: The SPI can be computed for different time scales, can provide early warning of drought and help assess drought severity, and is less complex than the Palmer.
Cons: Values based on preliminary data may change.
Developed by: T.B. McKee, N.J. Doesken, and J. Kleist, Colorado State University, 1993.
Monthly maps: generated by the National Drought Mitigation Center; at the Western Regional Climate Center.
Daily Gridded SPI: http://drought.unl.edu/MonitoringTools/DailyGriddedSPI.aspx