Drought continued across the West through the 2016 agricultural season, although other areas across the 48 contiguous United States experienced expansion through late summer and into autumn. However, shrinkage of drought conditions across much of the country occurred during early to midwinter. According to the NOAA's National Centers for Environmental Information, 18 percent of the contiguous United States experienced moderate to exceptional drought conditions at the start of 2016, with an expansion to nearly 32 percent of the nation by the end of November. Persistent and abundant precipitation across the country during December resulted in a decrease in drought conditions by the end of the year when approximately 22 percent of the nation was under moderate to exceptional drought conditions. The circulation pattern responsible for the reduction of the drought conditions in December continued through January, resulting in only seven percent of the contiguous US reporting moderate to exceptional drought conditions by the end of the January. In fact, an "atmospheric river" -- a plume of subtropical humid air running across the North Pacific and into the Western US was responsible for the heavy rain and snow that essentially eliminated the exceptional drought conditions across California, a state that had experienced record drought for nearly five years.
As we approach the beginning of the 2017 agricultural season, only a few areas of the nation 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 central sections of the nation, particularly across Texas northward into Oklahoma and western Kansas, had monthly precipitation totals in January 2017 that were well below "normal" (or the averages from the 1981-2010 climatological reference interval). When coupled with above average precipitation during December 2016, many locations across the "Lower 48" states had much above average December-January precipitation totals that have helped minimize the soil moisture deficit during 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 18 February 2017) shows areas of moderate to extreme drought scattered across the northern Rockies of Montana and Wyoming and adjacent high Plains to the east. Another area of extreme drought was across sections of the Southeast, primarily centered on the Appalachians in southwestern North Carolina. On the other hand, large sections of the West, running from central California northward to Washington and eastward to the western slopes of the Rockies in Montana along with the Great Basin and western Colorado 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. 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 (21 February 2017) shows several areas of extreme (D3) drought scattered across the Southeast, primarily in northern sections of Georgia and in west central Alabama, the Northeast ( western Connecticut) and in the Mid-South (western Arkansas). large section of California and adjacent areas of Nevada. Moderate to severe (D1 and D2) drought was in many areas scattered across the Southwest, the central sections of the nation and along the Atlantic Seaboard from Florida northward to New England. 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 and southern Plains and in the Northeast 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 16 February 2017 by the Climate Prediction Center and valid through May 2017) indicates that the current drought conditions would persist through the forecast period across several widely sections of the nation that include southern Arizona, the eastern slopes of the Colorado Rockies along with the the high Plains in Colorado, Kansas, Nebraska and the Oklahoma Panhandle. The southern sections of the Florida Peninsula should continue to see drought conditions or expect development of drought. Other sections of the nation that have been experiencing drought could have improvement in drought conditions, with a few areas possibly being removed from drought classification. These areas include southwestern California, the Black Hills of South Dakota and Wyoming, the southern Plains of Oklahoma extending into the Ozark Plateau and the mid-Mississippi Valley, the Tennessee Valley in the Southeast and area of the Northeast extending from the Middle Atlantic northward into New England.
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, 2017, The American Meteorological Society.
