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Drought Severity Index, United States

The Palmer Drought Severity Index is the most widely used index to measure drought severity over time. An index value of zero represents the average moisture
conditions observed between 1931 and 1990. Positive values mean wetter than average, negatives mean drier than average.

1895202019201940196019802000Annual drought severity index9-year average

Source: NOAA (National Oceanic and Atmospheric Administration) via the US EPA

Note: A value between -2 and -3 indicates moderate drought, -3 to -4 is severe drought, and -4 or below indicates extreme drought.