Drought continued across the West and southern Plains through the 2014 agricultural season, although other areas of the nation experienced some recovery from the major drought of 2012. Following a reduction in the drought during 2013, below average precipitation in early 2014 caused an expansion of drought conditions. However, welcome summer rains helped ease or eliminate drought conditions in many areas. According to the NOAA's National Climatic Data Center, 31 percent of the contiguous United States was experiencing moderate to exceptional drought conditions at the start of 2014, but at the end of the year 28.7 percent of the the nation was under drought conditions, which was the smallest extent in three years.
As we approach the beginning of the 2015 agricultural season, drought continues across sections of the West. This drought will impact the soil moisture available to growing crops, an important factor as 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. Numerous cities in California and Nevada in the West, the central Rockies and the central Plains had monthly precipitation totals in January 2015 that were below "normal" (or the averages from the 1981-2010 climatological reference interval). A few cities across the Arizona, New Mexico and west Texas reported above average precipitation totals for January.
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 14 February 2015) shows a widespread region of moderate to extreme drought across the Southwest, including California, Nevada and Arizona. Other areas that had moderate to extreme drought included the northern Rockies, primarily in Montana and Wyoming and across South Florida. On the other hand, sections of the Northwest along with a few scattered areas of the northern Plains, the Great Lakes and coastal New England 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 (17 February 2015) shows severe to extreme drought extending along the West Coast States, primarily in California where exceptional drought prevailed, across western Nevada and southeastern Oregon in the Intermountain West and in Oklahoma and north Texas in the southern Plains . Scattered sections of the Mid-South also experienced continued moderate to severe drought. The drought across most of these areas would have both short and long-term consequences. 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 19 February 2015 by the Climate Prediction Center and valid through May 2015) indicates that the current drought conditions would persist across a large section of the Southwest extending from the southern Plains of Texas westward across New Mexico and Arizona to California and northward across Nevada and into interior Oregon along the West Coast. The Pacific Northwest that includes most of western Washington could also experience development of drought conditions. The northern Plains and Upper Midwest could also see the continuation or development of drought during this upcoming spring. On the other hand, sections of the southern and central Plains along with the Mid-South and lower Ohio Valley could experience improvement in drought conditions, with a few areas possibly being removed from drought classification.
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@meteor.wisc.edu
© Copyright, 2015, The American Meteorological Society.
