As the 2018 agricultural season winds down across northern sections of the nation, moderate to exceptional drought continued across many areas of the Southwest, the Northwest and the Plains. According to the NOAA's National Centers for Environmental Information, approximately 23 percent of the contiguous United States was experiencing moderate to exceptional drought conditions by the end of August 2018 after starting the year of 2018 with a seven percent areal coverage of these drought conditions across the "Lower 48".
A lack of significant and timely rain over an extended interval often is an indicator of drought conditions. One could inspect the tables of monthly precipitation (in hundredths of an inch) that are furnished by the National Weather Service several days after the conclusion of each month for nearly 250 selected U.S. cities. While many of the cities across the midsection of the United States had below average "normal" (or the averages from the 1981-2010 climatological reference interval) for August 2017, locations along the Texas and Louisiana Gulf Coast had above normal August precipitation, courtesy of Hurricane Harvey. The northwestern quadrant of the nation. along with some sections of the Midwest (centered on Iowa) and the Carolinas also saw a lack of significant precipitation during the meteorological summer of 2017 (June-August). However, the Gulf Coast and the lower Mississippi Valley reported above average summer precipitation totals.
What constitutes a drought? The answer depends upon whom you ask. At least four types of drought can be defined. To the farmer, an agricultural drought represents an extended interval with a serious soil moisture deficiency during critical crop growth periods. A hydrologist would classify a hydrological drought as an extended interval containing abnormally low stream flow, lake levels and ground water reservoirs. Most meteorologists would consider a meteorological drought to occur when the accumulated precipitation is well below a prescribed amount that would depend upon the region or season. A fourth type would be socioeconomic drought, where the shortage of water affects humans, typically in terms of economic activities. Economic goods that may be affected may include water, agricultural food products and hydroelectric power.
Typically, the severity of a drought depends upon the lack of soil moisture, which is influenced by one or more of the following factors to include a lack of precipitation, low atmospheric humidity, high air temperature, strong winds, a lack of clouds and intense sunlight. In addition, the drought severity depends upon its duration and the size of the affected area.
The start of a drought usually is subtle in that few can tell when a spell of dry weather really constitutes the incipient phase of a drought. Similarly, the end of a drought is also difficult to assess, since one rain event does not necessarily "break a drought". The National Weather Service uses several indices to assess the severity of a drought. One of the most frequently used drought indices is the Palmer Drought Severity Index developed by Wayne Palmer in the 1960s. This Palmer Index, with unit-less values ranging from below -4 (severe drought) to above +4 (extremely moist),. incorporates temperature and rainfall information in a formula to determine abnormal dryness or wetness over prolonged time intervals, such as a month to years. 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. 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.
The most recent map (weekly index values ending 22 September 2018) shows a widespread region of severe to extreme drought across several wide areas of the western half of the nation, primarily from California, Oregon and in interior Washington eastward across the Four Corners (Arizona, Colorado, New Mexico and Utah), as well as to the northern Plains that include the Dakotas and northwestern Minnesota. Severe drought was also found in northern sections of Upstate New York. On the other hand, climate divisions along the Appalachians running from northern Virginia to eastern Pennsylvania and across the Midwest and the central Plains from southern Wisconsin westward to northern Nebraska had unusually moist to extremely moist conditions. Southeastern North Carolina also had extremely moist conditions due to Hurricane Florence. Near normal soil moisture conditions prevailed over the remainder of the nation. NOTE: No drought indices were available for several large areas across the northern and central Rockies indicated by black shading.
Beginning in 2000, the National Drought Mitigation Center, a group consisting of several governmental agencies along with the University of Nebraska-Lincoln, has maintained a US 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 (D0 to D4). The drought specialists attempt to show the short-term impacts of the drought upon agriculture and wildfire potential, along with the long-term impacts on hydrology and ecology. (A description of the categories used in the drought classification scheme appearing on the US Drought Monitor map is available.) Their most recent map (25 September 2018) shows a large area of exceptional drought centered on the Four Corners region, which is surrounded by areas of severe to extreme drought across the Southwest. Sections of Oregon in the Northwest and the Missouri Valley in the Plains were also experiencing areas of severe to extreme drought. Widely scattered areas across the 48 contiguous United States were also reporting extremely dry to moderate drought conditions. The severe to exceptional drought had both short-term and long-term impacts. A short-term drought would typically have durations of less than six months affecting agriculture and grasslands, while long-term drought exceeds six months and would affect rivers, lakes and groundwater (or "hydrology"), along with trees and other natural perennial vegetation (or "ecology"). An accompanying narrative entitled "This Week's 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 US Seasonal Drought Outlook (released on 20 September 2018 by the Climate Prediction Center and valid through December 2018) indicates that the extensive drought conditions would continue across across the northern Plains, the interior Northwest and a large area of the Southwest, extending from southeastern Colorado westward across the Four Corners to southern California. Drought conditions were also expected to develop across sections of the Tennessee Valley in Alabama. On the other hand, a few sections of the Northwest, the southern Plains, the southern Rockies, the lower Missouri Valley and northern sections of Upstate New York and Vermont Plains could see some improvement in drought conditions during the rest of the year, including some areas that could be removed from drought listings. The remainder of the nation does not appear to be headed for drought conditions. Note: a Seasonal Drought Outlook Discussion is included describing the tools used to construct the Seasonal Drought Outlook and the forecasters' confidence in this product.
Palmer, W.C., 1988 (12 Jul): 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@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.
