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About this map: The Palmer Index map depicts long-term (several months) more than short-term (several weeks) drought. Negative values indicate areas of drought--lower numbers being areas of more severe drought. The Palmer Index is standardized to local climate, so it can be applied to any part of the country to demonstrate relative drought or rainfall conditions. However, it is not particularly useful in calculating supplies of water locked up in snow, and thus works best east of the Continental Divide.