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 1 October 2005) shows a widespread region of moderate to extreme drought across the northern Rockies and adjacent Plains, except for the windward slopes of the Cascades. Some sections of the Midwest, namely in northern Illinois and adjacent divisions in neighboring states also were in moderate to extreme drought conditions, as were scattered areas of the Middle Atlantic States, such as in south central Pennsylvania and western Maryland. On the other hand, unusually moist to extremely moist conditions prevail across the Southwest, the Upper Midwest and in the Southeast that received soaking rains that accompanied the remnants of hurricanes and tropical storms moving across the region during the last 6 weeks. 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 (4 October 2005) shows moderate to extreme agricultural and hydrological drought across sections of the Midwest, notably northern Illinois and eastern Iowa, interior sections of the Pacific Northwest, especially in Washington State and Oregon, and across the central and northern Rockies. In addition, this map also shows abnormally dry conditions to moderate drought across the western Plains, Texas, the mid Mississippi Valley and the Middle Atlantic States. Undoubtedly, the heavy rain associated in part from the remnants of former Tropical Storm Tammy that fell across the Middle Atlantic States after this map was posted could alleviate some of the drought conditions there. 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 6 October 2005 by the Climate Prediction Center and valid through December 2005 indicates that the drought should persist across the West, while limited or short-term improvement in soil moisture should occur in some sections of the Pacific Northwest, the Plains and the Midwest during the fall and early winter. Sections of the Great Lakes, the lower Mississippi Valley and coastal sections of the Middle Atlantic States should experience greater improvement. What was thought to have been the development of drought conditions over the Appalachians when this Outlook was first issued in mid-September have no been erased by this past weekend's rain.
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.
