Standardized Precipitation Index (SPI) Products

The standardized precipitation index data set may be accessed via
World Wide Web. Link to: https://www.wrcc.dri.edu/spi/spi.html

Five quantities are computed as part of the Standardized Precipitation
Index procedure. They follow as a consequence of an observation made
by Dr. Tom McKee, State Climatologist for Colorado, that users are
interested in one or several among the following types of information:

What is the absolute amount of precipitation?

What is the absolute departure from climatology?

What is the relative departure from climatology?

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: https://www.wrcc.dri.edu/spi/explanation.html

More drought information is available from the National Drought
Mitagation Center. https://www.drought.unl.edu/index.htm

(Summary adapted from the WRCC Home Page)