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.