Tuesday, September 23, 2014

National Drought Mitigation Center

Interpretation of 9-Month Standardized Precipitation Index Map

The 9-month SPI provides an indication of precipitation patterns over a medium time scale. Droughts usually take a season or more to develop. SPI values below -1.5 for these time scales are usually a good indication that fairly significant impacts are occurring in agriculture and may be showing up in other sectors as well. For example, the 9-month SPI map for the end of May 1996 shows that many climate divisions from Utah and Arizona to Texas have SPI values less than -1.5. As a result, President Clinton authorized $70 million in aid for the drought-stricken states of Texas, Oklahoma, Kansas, New Mexico, and Colorado.

Some regions of the country may find that the pattern displayed by the map of the Palmer Index closely relates to the 9-month SPI maps. For other areas, the Palmer Index is more closely related to the 12-month SPI. The Palmer Index maps are updated each week, although the patterns usually do not change significantly. The SPI maps are updated at the end of each month.

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