Drought, a period of abnormally dry weather, can have far-reaching and costly impacts on agriculture and other water-dependent activities. Prolonged drought cuts crop yields, reduces the flow of rivers and impedes navigation, decreases the hydroelectric power potential, lowers water tables, stresses municipal water supplies, and increases the risk of wildfire. Drought can occur anywhere at any time but drought is most frequent in areas where the average precipitation is already relatively low.
What constitutes a drought? One of the most important indices used by the National Weather Service to assess the severity of a drought is the Palmer Drought Severity Index. The Palmer Index, developed by Wayne Palmer in the 1960s, incorporates temperature and rainfall data in a formula to determine abnormal dryness or wetness over prolonged time intervals, such as a month to years. As such, this drought index primarily reflects meteorological drought. (For drought types, see p. 99 of the WES Textbook.) The National Weather Service and U.S. Department of Agriculture jointly compute the Drought Index weekly for each of 344 climatological divisions across the United States. A map of the current Drought Index is available that shows those divisions experiencing drought with negative index values and varying shades of red, while those regions with excess precipitation have positive values and varying shades of green. Unfortunately, the index is slow to detect fast-emerging droughts and does not reflect the contributions of snowpack, an important component of water supply in the West.
The most recent map (weekly index values ending 6 October 2001) shows a broad region of moderate to extreme drought across much of the West, with the Pacific Northwest and the northern Rocky Mountain States continuing to experience extreme drought. East of the Rockies, several areas also were under moderate to severe drought conditions to include northern New England and northeastern Arkansas, while moderate drought was also reported in portions of the Carolinas. Near normal conditions prevailed over most of the rest of the nation, except for several regions across the central Gulf Coast, the Midwest and Dakotas where recent rains have produced unusually moist soil conditions. Some divisions in Kansas, Missouri, Nebraska and North Dakota are experiencing excessively moist conditions.
In the last several years, the National Drought Mitigation Center, a group consisting of several governmental agencies along with the University of Nebraska-Lincoln, has maintained a Drought Monitor site that provides weekly updates of current drought information and forecasts of the potential for drought across the nation. Their current summary map of drought conditions attempts to improve upon the Palmer Drought Severity Index and synthesize five other indices and a certain amount of subjectivity to arrive at six drought severity categories. They attempt to show the impacts of the drought upon agriculture and wildfire potential. Their current map (2 October 2001) shows extreme drought in portions of Montana, Idaho, northern Nevada, northern California, and the Pacific Northwest. Drought is moderate to severe in west Texas and northern New England. They also include animated maps for the prior six and twelve weeks.
The Seasonal Drought Outlook released on 14 September 2001 by the Climate Prediction Center and valid through December 2001 indicates that drought is likely to persist in the West, west Texas, and northern New England.
Palmer, W.C., 1988. The Palmer Drought Index: When and how it was developed. Weekly Weather and Crop Bulletin, 75 (28), 5.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.
