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By using the Palmer Drought Severity Index (PDSI) as calculated from state averages of temperature and precipitation and from numerous single station analyses, it has been demonstrated that droughts (as defined by the PDSI) persist longer in the interior of the United States than in areas farther east or west. The question arises whether this is merely an artifact of the PDSI calculations, or whether there is actually more persistence of abnormally dry (wet) weather in the interior. The sensitivity of the PDSI was tested in relation to changes of derived and prescribed parameters included in the PDSI calculations in order to determine their effect on the spatial characteristics of drought duration. The sensitivity tests indicated a negligible effect. Contingency tables which use the PDSI as the predictor for the following one-month, six-month and 12-month precipitation anomalies (and also anomalies of precipitation minus potential evapotranspiration) however, are generally characterized by significantly greater skill in the interior portions of the United States, confirming the nation that spells of abnormally dry or wet weather do have more persistence in the Rocky Mountain and High Plains states than states farther east or west. Unfortunately, the forecasts derived from the “operational” PDSIs were not a significant improvement from what would have been obtained by using precipitation persistence forecasts.
Thomas R. Karl (Mon,) studied this question.