A New National Drought Atlas from the National Drought Mitigation Center

The first attempt of producing a drought atlas for the United States was done in 1994 using the Historic Climate Network (HCN) data from the Cooperative Observer Program (COOP) of the National Weather Service. The number of stations used numbered 1,036 and this project focused on the Palmer Drought Severity Index (PDSI) calculated from monthly precipitation totals. With drought continuing to be a problematic natural disaster for the United States, an updated and expanded drought atlas for the United States is currently being produced at the National Drought Mitigation Center with support from the Risk Management Agency of the United States Department of Agriculture.

Due to the availability of data in the HCN digital archive, the original drought atlas was mainly constructed using data from 1948 on. With the advent of the Applied Climate Information System (ACIS) from the NOAA Regional Climate Centers, more long-term climate stations are available in the digital climate archive with periods of record extending back into the early 1900's and even the 1800's for many locations used in the new drought atlas. Currently, 2,152 stations are being used in the calculation of the Standardized Precipitation Index (SPI) and the Palmer Drought Severity Index using a weekly time step and Deciles using a monthly time step. Each station used in the drought atlas has at least 40 consecutive years of data with no more than two months of missing data at any time in its record. A unique period of record was established for each station used in the drought atlas and with these criteria the best long-term climate stations in the COOP network are being used to get an accurate assessment of drought occurrence and return probabilities using multiple drought indices.

The focus of the drought atlas is targeted at agricultural producers and decision makers as a means to analyze their potential risk to drought in any particular area for any time of the year. Using regionalization techniques developed and used for the first atlas allows for the estimation of drought frequencies from several locations instead of just one, calculating frequencies using L-Moments statistical techniques. The National Drought Mitigation Center has developed methods to do all of this work in an Arc GIS setting, allowing for the rapid assessment of results and graphical output via a freely available web interface.