Drought, a lengthy 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 identify locales experiencing drought and assess its severity is the Palmer Drought Severity Index. The Palmer Index, developed by Wayne C. Palmer in the 1960s, incorporates temperature and rainfall data in a formula to determine abnormal dryness or wetness over prolonged time intervals, ranging from one month to years. As such, this drought index primarily reflects meteorological drought. (For the distinction among types of drought, see p. 99 of the DataStreme 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, which conform to the crop reporting districts. 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 30 September 2006) shows a widespread region of moderate to extreme drought across much of the West, northern Rockies and the northern Plains, except for southern sections of Arizona and New Mexico and West Texas. Some sections of the Southeast, namely those climate divisions along the eastern Gulf Coast, including Florida also were in moderate to severe drought conditions. On the other hand, unusually moist to extremely moist conditions prevail across southern New Mexico and west Texas in the Southwest, the Northeast, including New England and the eastern Great Lakes. Near normal soil moisture conditions prevailed over the remainder of the nation.
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, together with 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 most recent map (3 October 2006) shows extreme to exceptional agricultural drought extending across the Southern Plains, especially in the Red River Valley separating Oklahoma and north Texas. Moderate to extreme agricultural and hydrological drought can also be found across the central and northern Rockies, along with the adjacent western high Plains of Nebraska and the Dakotas. The Arrowhead region of northeast Minnesota was also experiencing extreme drought conditions. Abnormally dry conditions, as well as moderate drought, were also found across the Southeast. An accompanying narrative entitled "National Drought Summary" also provides a five-day forecast and a 6- to 10-day outlook for precipitation and temperature across the country. This site also includes animated Drought Monitor maps for the prior six and twelve weeks. The Drought Impact Reporter is an interactive tool that permits exploration of the reported drought impacts across the nation. The goal is to help in risk management that could ultimately help shape drought related policy at the state and federal levels.
The US Seasonal Drought Outlook released on 21 September 2006 by the Climate Prediction Center and valid through December 2006 indicates that the drought should persist across the northern Rockies, while the Plains could experience a variety of degrees of improvement. Sections of the Southeast could also have some improvement in their drought conditions. Drought conditions were expected to develop across interior sections of the Pacific Northwest.
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, 2006, The American Meteorological Society.
