Monday, May 21, 2018

National Drought Mitigation Center

Interpretation of 3-Month Standardized Precipitation Index Map

The three-month SPI provides a comparison of the precipitation over a specific 3-month period with the precipitation totals from the same 3-month period for all the years included in the historical record. In other words, a 3-month SPI at the end of February compares the December–January–February precipitation total in that particular year with the December–February precipitation totals of all the years.

A 3-month SPI reflects short- and medium-term moisture conditions and provides a seasonal estimation of precipitation. In primary agricultural regions, a 3-month SPI might be more applicable in highlighting available moisture conditions than the slow-responding Palmer Index. A 3-month SPI at the end of August in the U.S. Corn Belt would capture precipitation trends during the important reproductive and early grain-filling stages for both corn and soybeans. Meanwhile, the 3-month SPI at the end of May gives an indication of soil moisture conditions as the growing season begins.

It is important to compare the 3-month SPI with longer time scales. A relatively normal 3-month period could occur in the middle of a longer-term drought that would only be visible at longer time scales. A good example of this can be illustrated by the four Climate Divisions in southeastern Nebraska. The 3-month SPI for the end of March 1996 showed these Climate Divisions in the “near normal” category, largely because of above-normal precipitation in January 1996.

However, dry conditions across this region began in June 1995, and can be seen in the 6-month SPI for the end of March 1996. Looking at longer time scales, as in this example, would prevent a misinterpretation that any “drought” might be over.

As with the 1-month SPI, the 3-month SPI may be misleading in regions where it is normally dry during that 3-month period. Large negative or positive SPIs may be associated with precipitation totals not very different from the mean. This caution can be demonstrated with the Mediterranean climate of California, where very little rain falls from April to September (see climographs for Los Angeles, San Francisco, and Fresno). Thus, a 3-month SPI for the end of August in any Climate Division in California will compare the historic precipitation totals for June–July–August. Because this is a time period with little rain, these historic totals will be small and relatively small deviations on either side of the mean could have large negative or positive SPIs.

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