NOTE: This Supplemental Information File is a repeat of last week's Supplement, except where changes were made to account for changes in the drought appearing on more recent maps .
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 discharge 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 NOAA's 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 various 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 e xperiencing 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 23 February 2008) shows a widespread region of moderate to extreme drought across portions of the northern Rockies and northern Plains, as well as across sections of the Southeast. On the other hand, unusually moist to extremely moist conditions prevail across the southern Rockies, central Plains, Northeast, and the Pacific Northwest. 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 (26 February 2008) shows extreme to exceptional agricultural drought extending across the Southeast, especially across sections of North Carolina, Tennessee, Mississippi, Alabama and Georgia. Moderate to severe agricultural and hydrological drought continues in the W est, primarily in southern California and across the Great Basin eastward across the Rockies into western Dakotas as well as southern Texas. 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 assist in risk management that could ultimately help shape drought related policy at the state and federal levels.
The US Seasonal Drought Outlook released on 21 February 2008 by the Climate Prediction Center and valid through May 2008 indicates that drought conditions were expected to develop or persist across the Southeast, Texas and sections of the Southwest. Much of the Great Basin along with sections of the Appalachians and the Tennessee Valley could experience varying degrees of improvement.
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 WES Central Staff and Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2008, The American Meteorological Society.
