The SPI at these time scales reflects long-term precipitation patterns. A 12-month SPI is a comparison of the precipitation for 12 consecutive months with the same 12 consecutive months during all the previous years of available data. Because these time scales are the cumulative result of shorter periods that may be above or below normal, the longer SPIs tend toward zero unless a specific trend is taking place. The 12-month SPI for the end of February 1996 compared the precipitation totals for the March 1995–February 1996 period with similar periods in history. Most of the climate divisions across the United States were in the “near normal” category. The large positive SPIs across South Dakota indicate the very wet conditions that existed for this 12-month period. Indeed, 1995 was the wettest year on record for the entire state of South Dakota, and this is still shown in the 12-month SPIs for the state.
SPIs of these time scales are probably tied to streamflows, reservoir levels, and even groundwater levels at the longer time scales. In some locations of the country, the 12-month SPI is most closely related with the Palmer Index, and the two indices should reflect similar conditions.