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 19 February 2005) shows severe to extreme drought conditions across the Northwest, especially along the Columbia Valley of northern Oregon and across southern sections of Montana and much of Wyoming. Moderate drought was also found across south Florida. With the recent storm systems spreading across California and the Southwest, the southern half of California, Nevada, Utah, western Colorado, Arizona and New Mexico have had extremely moist soil conditions. Storm tracks that stretched from the southern Plains to the eastern Great Lakes and the Middle Atlantic States have also produced very moist or extremely moist soil conditions in these areas. 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 (22 February 2005) shows moderate to extreme agricultural and hydrological drought across many areas of the West, primarily across the interior Pacific Northwest, the Intermountain West, the northern and southern Rockies and the western high Plains. Exceptional drought conditions were identified across southeastern Montana and northern Wyoming. 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 17 February 2005 by the Climate Prediction Center and valid through May 2005 indicates that improvement in soil moisture should occur across the Southwest, including the Four Corners Area and the Great Basin, along with portions of the high Plains. While improvement in soil moisture is foreseen, large reservoirs should remain low. The drought is seen to persist across the interior Northwest and the northern Rockies, while some limited improvement was anticipated across the adjacent high Plains, the central Rockies and the northern sections of the Great Basin.
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, 2005, The American Meteorological Society.
