This Supplemental Information file is updated from Week 6.
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 24 February 2007) shows a widespread region of moderate to extreme drought across northern Minnesota, including adjoining sections of northwestern Wisconsin, along with the some divisions across Wyoming. Moderate to severe drought was also reported across eastern sections of California and adjacent sections of Arizona. On the other hand, unusually moist to extremely moist conditions prevail across the Northeast (New England, New York and Pennsylvania), the Midwest (Michigan, Ohio, Indiana and southern Missouri), the southern Plains (western Kansas), and the southern Rockies (Colorado and New Mexico). 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 (27 February 2007) shows extreme to exceptional drought across Texas from the Rio Grande Valley northward into the Hill Country. Moderate to extreme hydrological drought also extends across the central Rockies, the high Plains of Nebraska and the Dakotas and the Upper Midwest, primarily west of Lake Superior. Moderate to severe agricultural and hydrological drought continues across the Arizona and New Mexico in the Southwest, along with portions of the Southeast, from the southern Appalachians southward across the Florida Peninsula. 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 revised Seasonal Drought Outlook released on 15 February 2007 by the Climate Prediction Center and valid through May 2007 indicates that the drought should persist across the Southwest, along with the Upper Mississippi Valley, primarily across northern Minnesota. Drought conditions were expected to develop across the Hawaiian Islands.
Short-term improvement to definite improvements in soil moisture should occur in sections of the California, the Rockies, the Plains, the southern shore of Lake Superior, the Cumberland Plateau of Tennessee and the Florida Peninsula.
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, 2007, The American Meteorological Society.
