The Standardized Precipitation Index (SPI) is a drought index based only on precipitation. The SPI can be used to monitor conditions on a variety of time scales. This temporal flexibility allows the SPI to be useful in both short-term agricultural and long-term hydrological applications. The SPI was developed in 1993 by T.B. McKee, N.J. Doesken, and J. Kleist.

The SPI value represents an estimation of the unusualness of a precipitation total, based on the climate division's history. Larger absolute values (both above and below zero) represent increasingly unusual events. A thorough description of the SPI is available from the National Drought Mitigation Center.

The following table gives a brief description of the significance of various SPI values, including an estimate of how likely it is to exceed SPI values.

SPI Value	Brief Description	Estimated Likelihood
2.00 and Greater	Extremely Wet	about 2.3% of events (roughly 1 out of 40) are expected to exceed 2.00
1.50 to 1.99	Very Wet	about 6.7% of events (roughly 1 out of 15) are expected to exceed 1.50
1.00 to 1.49	Moderately Wet	about 16% of events (roughly 1 out of 6) are expected to exceed 1.00
-0.99 to 0.99	Near Normal	about 68% of values (roughly 2 out of 3) fall in this range
-1.00 to 1.49	Moderately Dry	about 16% of events (roughly 1 out of 6) are expected to exceed -1.00
-1.50 to -1.99	Severely Dry	about 6.7% of events (roughly 1 out of 15) are expected to exceed -1.50
2.0 and Less	Extremely Dry	about 2.3% (roughly 1 out of 40) of events are expected to exceed -2.00
* - Note: The values calculated in the drought products use precipitation values averaged across climate divisions. Local conditions may vary within the divisions.