National Environmental Public Health Tracking Network 
Standard Precipitation Index (SPI) Metadata 


Publication Date 


01/11/2017 


Background 


The Standardized Precipitation Index (SPI) is a widely used index to characterize 
meteorological drought ona range of timescales. On short timescales, the SPI is closely 
related to soil moisture, while at longer timescales, the SPI can be related to 
groundwater and reservoir storage. The SPI can be compared across regions with 
markedly different climates. It quantifies observed precipitation as a standardized 
departure from a selected probability distribution function that models the raw 
precipitation data. The raw precipitation data are typically fitted to a gamma ora 
Pearson Type III distribution, and then transformed to a normal distribution. The SPI 
values can be interpreted as the number of standard deviations by which the observed 
anomaly deviates from the long-term mean. The SPI can be created for differing periods 
of 1-to-36 months, using monthly input data. For the operational community, the SPI has 
been recognized as the standard index that should be available worldwide for 
quantifying and reporting meteorological drought. The dataset includes one-month SPI 
values for every contiguous US county and the District of Columbia monthly from 1895- 
2016. 


The dataset has been compiled to estimate wetness and dryness of a particular area. 
This is important for the agriculture as well as health sectors. The data can be used to 
examine local and national trends in drought information. 


Data Values 


Range from -3 (dry) to +3 (wet). 0 indicates normal conditions. Missing data is noted as 
-99,99, 


Geographic Scale 


Data includes all counties in the lower 48 states plus the District of Columbia. 


& Scope 
Time Period January 1, 1895 — December 31, 2016. Known to be accurate as of time period end date. 
Raw Data Data downloaded from the National Oceanic and Atmospheric Administration (NOAA) 
Processing server and were originally provided as monthly values at a 5km grid. Distance weighting 
functions were applied to constrain the drought values to a specific US county. 
No data were lost or omitted during calculation. All data that were available were used. 
Data will be updated on an ad hoc basis, when necessary. 
Additional Keyantash, John & National Center for Atmospheric Research Staff (Eds). Last modified 
Information 02 Mar 2016. "The Climate Data Guide: Standardized Precipitation Index (SPI)." 


Retrieved from https://climatedataguide.ucar.edu/climate-data/standardized- 
precipitation-index-spi. 


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. 179-18