Canada is not included in the contour maps because of the limited number of climatological stations currently available. Until sufficient data are made available to increase the station density so that contours can be more accurately drawn, Canadian data will be provided only on the dot maps. The maps below are based on station data for Canada, Mexico, and Alaska, and on climate division data for the contiguous U.S. plotted on the divisions' center points.
The Standardized Precipitation Index (SPI) is a probability index that considers only precipitation. The SPI is an index based on the probability of recording a given amount of precipitation, and the probabilities are standardized so that an index of zero indicates the median precipitation amount (half of the historical precipitation amounts are below the median, and half are above the median). The index is negative for drought, and positive for wet conditions. As the dry or wet conditions become more severe, the index becomes more negative or positive. The SPI is computed by NCDC for several time scales, ranging from one month to 24 months, to capture the various scales of both short-term and long-term drought.
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http://www.ncdc.noaa.gov/temp-and-precip/drought/nadm/indices.php
