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 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 21 February 2004) shows a widespread region of moderate to extreme drought across a large portion of the West, particularly from Montana southward across Wyoming, Idaho, western Nebraska, northern Kansas, southeastern Colorado, central New Mexico and portions of Arizona. Moderate to severe drought also developed across portions of the Midwest, primarily across Minnesota and adjacent areas of surrounding states. Recent rain and snow have essentially eliminated last year's drought conditions across the Pacific Northwest. Numerous winter storms, along with rains from last autumn's hurricane season have produced unusually moist to extremely moist conditions from the Southeast into the Middle Atlantic States. 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 (24 February 2004) shows moderate to extreme agricultural and hydrological drought across much of the central Plains, the Rockies and Intermountain West. Exceptional drought conditions were identified across southern New Mexico and the mountains of southeastern Idaho and western Montana. In addition, this map also shows moderate to severe drought conditions across the northern Plains and Upper Mississippi Valley. An accompanying narrative entitled "National Drought Summary" also provides a five-day forecast and a 6 to 10 day outlook of precipitation and temperature across the country. This site also includes animated Drought Monitor maps for the prior six and twelve weeks.
The Seasonal Drought Outlook released on 19 February 2004 by the Climate Prediction Center and valid through May 2004 indicates that improvement in soil moisture should occur across the West. The eastern Plains and Great Basin would appear to have a greater chance for improving conditions, while much of the Rockies and adjacent high Plains should continue under drought conditions, with only slight improvement from the extreme conditions foreseen.
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, 2004, The American Meteorological Society.
