NOTE: This document is a repeat of that appearing as a Supplementary Information File for DataStreme Earth's Climate Systems Week Six.
Drought continued across the West and southern Plains through the 2014 agricultural season. Welcome precipitation summer rains helped ease or eliminate drought conditions in the northern Plains and the Mississippi Valley. However, areas across the Southeast and along the Atlantic Seaboard began to suffer from a lack of precipitation in late summer, resulting in a return to dry conditions across the region. 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 contiguous United States had above average "normal" (or the averages from the 1981-2010 climatological reference interval) for August 2014, locations along the Pacific Coast (Oregon and central and northern California) along with the southern Plains (Oklahoma and Texas) and the Southeast had below normal August precipitation. The Pacific Coast also saw a lack of significant precipitation in the meteorological summer of 2014 (June-August). However, some areas across the Intermountain West and nation's midsection 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 20 September 2014) shows a widespread region of moderate to extreme drought across the West, primarily in California, Nevada and Arizona, along with sections of Wyoming, Utah, Idaho and Montana. Moderate to severe drought conditions were also indicated across scattered areas in the Southeast, including sections of West Virginia. On the other hand, northern sections of the nation extending eastward from the high Plains of eastern Montana and the Dakotas across the Upper Midwest to the the eastern Great Lakes had unusually moist to very moist conditions. Near normal soil moisture conditions prevailed over the remainder of the nation.
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). They attempt to show the short-term impacts of the drought upon agriculture and wildfire potential and long-term impacts on hydrology and ecology. (A description of the categories used in the drought classification scheme appearing on the National Drought Monitor map is available.) Their most recent map (23 September 2014) shows a large area of exceptional drought stretched across a large section of California and adjacent sections of western Nevada. A few areas of exceptional drought were also detected across north Texas. Areas of severe to extreme drought stretched across a large section of the western United States, particularly across the Pacific Coast States, the Intermountain West and the Southwest, including the southern Plains. For many of these areas, the drought had both short-term and long-term impacts. Short-term drought that would typically have durations of less than six months affect 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 "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 US Seasonal Drought Outlook (released on 18 September 2014 by the Climate Prediction Center and valid through December 2014) indicates that the extensive drought conditions would continue across western sections of the nation, primarily across a large section of the Pacific Coast (California and Oregon) along with Great Basin (Nevada and Utah) and the interior Northwest (including Washington and Idaho). Sections of the Southwest and the southern Plains, extending from southern California eastward to Texas and northward to Kansas could see some improvement in drought conditions during the rest of the year, including some areas that could removed from drought listings. The remainder of the nation does not appear to be headed for drought conditions.
Palmer, W.C., 1988 (12 Jul): The Palmer Drought Index: When and how it was developed. Weekly Weather and Crop Bulletin, 75 (28), 5.
Return to DataStreme ECS website
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2014, The American Meteorological Society.
