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 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 18 February 2006) shows a widespread region of moderate to extreme drought across Wyoming, Arizona, the southern and central Plains, along with the some sections of the Midwest, namely in northern Illinois and adjacent divisions in neighboring states. On the other hand, unusually moist to extremely moist conditions prevail across the Northeast (New England, New York, Pennsylvania and New Jersey) and the Northwest (Washington, Oregon, Idaho and Nevada). 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 (21 February 2006) shows extreme to exceptional drought in the southern Plains, extending from Missouri, Arkansas and Oklahoma into southern Texas. Moderate to extreme agricultural and hydrological drought also extends across the southern Rockies and the Southwest, including New Mexico and Arizona. Another area of moderate to severe drought continues across sections of the Midwest, notably northern Illinois and eastern Iowa, as well as some sections of the northern Plains and central Rockies. 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 revised Seasonal Drought Outlook released on 16 February 2006 by the Climate Prediction Center and valid through May 2006 indicates that the drought should persist across the central and southern Plains, as well as the Southwest. Drought conditions were expected to develop across other sections of the central Plains not already in drought, along with some sections of the Southeast, including southern Georgia and much of Florida.
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.
