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 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 5 October 2002) shows a
widespread region of moderate to extreme drought across much of the West,
particularly in California, Nevada, Arizona, Oregon, Idaho and Colorado. Severe to extreme drought also persists in
the Mid-Atlantic States from North Carolina northward to New Jersey. On the
other hand, unusually moist to extremely moist conditions prevail over northern
Wisconsin and Minnesota, in Mississippi and adjacent portions of Louisiana and
Tennessee, and in southern Texas. Some
areas of the South have received recent heavy rains from Tropical Storm Isidore
and Hurricane Lili during the last two weeks resulting in the change from
drought to moist conditions. Near
normal soil moisture conditions prevailed over the remainder of the nation. Many locations throughout the Pacific
Northwest and the Rocky Mountain States were under severe drought conditions
for much of the summer. According to the National Climatic Data Center,
approximately 48 percent of the coterminous
U.S. experienced moderate to extreme drought (based on the Palmer Drought
Severity Index) as of the end of August. Significant monetary losses were felt,
with lost crops and the losses from some of the worst wildfires in more than a
half of a century.
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 and 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 current map (8 October 2002) shows moderate to severe hydrological drought along the Eastern Seaboard, with pockets of extreme drought in North Carolina and southwestern Virginia. The map also indicates that a portion of the high Plains and much of the Rockies and Intermountain West also are experiencing moderate to severe agricultural and hydrological drought, with extreme drought in portions of southwestern Wyoming, southern Colorado, southern Utah and northern Arizona. A pocket of extreme drought is also found in south central Nebraska. This site also includes animated maps for the prior six and twelve weeks.
The Seasonal Drought Outlook released on 25 September 2002 by the Climate Prediction Center and valid through December 2002 indicates that drought is likely to persist with only spotty relief across much of the West, extending from California across the Great Basin and the northern Rockies on to the northern Plains. Some slight short-term improvement is expected across the Four Corners region of the Southwest, along with the high Plains of Kansas and Nebraska. Expansion of the drought area is likely across the Pacific Northwest. Drought is also expected to persist across the Ohio Valley, the Middle Atlantic and the Eastern lakes, with only spotty relief anticipated. On the other hand, major relief is possible across a large area of the Southeast is foreseen.
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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URL: WES/supl.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2002, The American Meteorological Society.
