Drought abated across many sections of the nation during the first half of the 2017 agricultural season. Above average precipitation in many locales during the first four months of 2017 resulted in only 4.5 percent of the contiguous United States experiencing moderate to exceptional drought conditions, the smallest areal extent in nearly two decades. By midsummer, hot and dry weather resulted in the expansion and deepening of drought conditions across northern Plains. Drought expanded into the Midwest and Pacific Northwest by the end of summer. With below average precipitation in some areas during autumn, drought expanded across the southern tier of states through the end of the year, resulting in slightly more than one quarter of the "Lower 48" states reporting drought or abnormal dryness by the end of December.
As we approach the beginning of the 2018 agricultural season, the Southwest and southern Plains were experiencing serious drought conditions, which would adversely impact the soil moisture available to growing crops, an important factor during this upcoming agricultural season. 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. Many cities across the nation, had monthly precipitation totals in January 2018 that were below "normal" (or the averages from the 1981-2010 climatological reference interval). Only stations in the northern tier of states had above average precipitation. When coupled with above average precipitation during December 2017, coastal stations along the Pacific Northwest and many locations across the eastern half of the nation had much above average December-January precipitation totals. On the other hand, below average precipitation across the Plains and the Southwest was adding to the soil moisture deficit in these regions during the first two-thirds of the meteorological winter season.
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 or water manager 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, a meteorologist for the U.S. Weather Bureau, 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 17 February 2018) shows areas of moderate to extreme drought scattered across the Southwest, that included Arizona and California, along with the northern Plains and upper Midwest. On the other hand, sections of the Midwest and the Northeast had unusually moist to very moist conditions. Interior sections of the Northwest also had very moist soils. 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. 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 (20 February 2018) shows areas of extreme (D3) drought scattered across the Southwest, centered upon the 4 Corners States of Arizona, Utah, Colorado and New Mexico, as well as across the southern Plains, running from Texas northward into Kansas. Sections of Missouri were also in severe drought. Drought conditions may have ameliorated across the Ozark Plateau and the Mississippi during this week as abundant rain fell after the early Tuesday cut-off for data collection. Moderate to severe (D1 and D2) drought was found across wide areas of the Southwest. The drought across most of these areas would have both short and long-term consequences, as indicated by the block letters "S" and "L." 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"). Scattered areas across the northern Plains were reporting short-term abnormally dry (DO) and moderate drought (D1) conditions. 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 15 February 2018 by the Climate Prediction Center and valid through May 2018) indicates that the current drought conditions would persist or expand through the forecast period across the southwestern quadrant of the nation, extending from the high Plains of Texas, Oklahoma and Kansas westward across the southern and central Rockies, the Great Basin and the Sierras to California's Pacific Coast, as well as northward into interior Oregon. Drought was expected to continue across the Dakotas in the northern Plains, as well as across coastal sections of Georgia and northern Florida. Several areas that have been experiencing drought could have improvement in drought conditions, with a few areas possibly being removed from drought classification. These areas include sections of the southern Plains of Oklahoma and north Texas extending eastward into the Ozark Plateau and the mid-Mississippi Valley, as well as the Tennessee Valley in the Southeast. Drought conditions could also dissipate across the Plains of eastern Montana and the Black Hills of South Dakota. A Seasonal Drought Outlook Discussion is included describing the forecasters' confidence.
Palmer, W.C., 1988 (12 July): 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.
