Drought - August 2006


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Contents Of This Report:
Map showing Palmer Z Index

Top of Page National Overview

  • Based on the Palmer Drought Index, severe to extreme drought affected about 18 percent of the contiguous United States as of the end of August 2006, a decrease of about 9 percent compared to last month. By contrast, about 5 percent of the contiguous U.S. fell in the severely to extremely wet categories.
  • About 40 percent of the contiguous U.S. fell in the moderate to extreme drought categories (based on the Palmer Drought Index) at the end of August.
  • On a broad scale, the previous two decades (1980s and 1990s) were characterized by unusual wetness with short periods of extensive droughts, whereas the 1930s and 1950s were characterized by prolonged periods of extensive droughts with little wetness (moderate to extreme drought, severe to extreme drought).
  • A file containing the national monthly percent area severely dry and wet from 1900 to present is available for the severe to extreme and moderate to extreme categories.
  • Historical temperature, precipitation, and Palmer drought data from 1895 to present for climate divisions, states, and regions in the contiguous U.S. are available at the Climate Division: Temperature-Precipitation-Drought Data page in files having names that start with "drd964x" and ending with "txt" (without the quotes).


Top of Page Detailed Drought Discussion


By the end of August, drought was concentrated in the Plains from Texas to the Dakotas and across the Southeast from Texas to Georgia (August 29 Drought Monitor). In the drought areas, soil moisture was low, evaporation was high, vegetative health was poor, and streamflow in the Southeast was especially low. Monsoon rain and Tropical Storm Ernesto alleviated dryness in the Southwest and along the Atlantic coast.

Map showing Palmer Z Index

Drought impacted many sectors of the economy. Crops were highly stressed or dying, livestock was dying or prematurely sold because of a lack of feed and water, and water restrictions were common in many areas. Disaster conditions have been declared by the governors of several states. The USDA plans to distribute about $100,000,000 to farmers and ranchers. Companies whose sales indirectly depend on precipitation, such as lawn mower manufacturers, have partially shut down because of reduced demand. Low water in the Mississippi River affected transportation of farm products, petroleum, steel and ore. Impacts in drought-stricken areas have been collected and summarized by county at the National Drought Mitigation Center's Drought Impact Reporter.

Corn field
Highly stressed corn field east of Kansas City (photo courtesy of University of Missouri Extension)

Empty Platte River
Empty Platte River (photo courtesy of the High Plains Regional Climate Center)


The August precipitation pattern at the primary stations in Alaska was above normal across most of the state; the northwest coastal stations were below normal. In Hawaii dryness continued throughout most the State. In Puerto Rico the month was predominantly dry in the central and eastern interior areas and along the southern coast (based on National Weather Service radar estimates of precipitation).

Map showing August State Precipitation Ranks

Some regional highlights:
Western U.S. percentage area wet and dry, 1996-2006


Top of Page State/Regional/National Moisture Status


A detailed review of drought and moisture conditions is available for all contiguous U.S. states, the nine standard regions, and the nation (contiguous U.S.):

STATES:
Alabama Arizona Arkansas California Colorado Connecticut
Delaware Florida Georgia Idaho Illinois Indiana
Iowa Kansas Kentucky Louisiana Maine Maryland
Massachusetts Michigan Minnesota Mississippi Missouri Montana
Nebraska Nevada New Hampshire New Jersey New Mexico New York
North Carolina North Dakota Ohio Oklahoma Oregon Pennsylvania
Rhode Island South Carolina South Dakota Tennessee Texas Utah
Vermont Virginia Washington West Virginia Wisconsin Wyoming


REGIONS:
Northeast Region East North Central Region Central Region
Southeast Region West North Central Region South Region
Southwest Region Northwest Region West Region
Map showing the nine U.S. standard regions
NATIONAL:
Contiguous U.S.




Top of Page Pre-Instrumental Perspective


August 2006 Paleoclimatic Analysis for North Central Texas

Severe drought conditions have gripped much of Oklahoma and north Texas since September 2005. In Texas Climate Division 3 (north central), precipitation for the past 12 months was only 64% of normal, the driest September-August period since 1956. The U.S. Drought Monitor for late August/early September showed north central Texas to be in category D4, "exceptional drought". The average monthly Palmer Hydrological Drought Index (PHDI) for Texas Division 3 for June-August 2006 was -4.34, which according to the PHDI scale is "extreme drought".
Texas Division 3 September-August 60-month precipitation, 1895-2006
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Precipitation Ranks for
Texas Division 3, 2005-2006
Period Rank
Aug 48th driest
Jul-Aug 17th driest
Jun-Aug 14th driest
May-Aug 7th driest
Apr-Aug 7th driest
Mar-Aug 23rd driest
Feb-Aug 25th driest
Jan-Aug 28th driest
Dec-Aug 18th driest
Nov-Aug 17th driest
Oct-Aug 8th driest
Sep-Aug 4th driest
1996-2006 Texas Division 3 monthly precipitation anomaly
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Texas Division 3 Palmer Hydrological Drought Index, 1900-2006
large image

The graph below shows the summer (June-August) PDSI, 1896-2006, for Texas Division 3 (annual values in light blue, 5-year weighted average in dark blue). The value for 2006 is the lowest since 1956 (-5.91), which was the most severe summer drought on record. The most persistent drought was in the 1950s, when summer PDSI was negative (in fact, below -2) for six years in a row (1951-1956; yellow arrow).

Paleoclimatic tree-ring reconstruction and observed PDSI for Texas Division 3 for the total period 1650-2006
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larger image (175 KB)
The graph to the left also shows a 329-year tree-ring reconstruction (1650-1978) of summer PDSI for a gridpoint (32.5 N, 97.5 W) within Texas Division 3 (annual values in light red, 5-year weighted average in dark red). The reconstruction is one of over 200 developed by Cook et al. (1999, 2004) across a gridded network covering much of North America, and is based on tree-ring data from 32 sites in Texas and surrounding states. The correlation between reconstructed gridpoint PDSI and Division 3 instrumental PDSI over their common period (1896-1978) is 0.810, indicating a very high degree of shared variance.

The tree-ring reconstruction can put the droughts of the last century in north central Texas into a much longer perspective. The frequency of severe one-year droughts appears to have remained fairly constant over the past ~350 years. Both the instrumental and reconstructed PDSI records indicate that "severe" or "extreme" summer drought (PDSI below -3) occurred in about 1 in 9 years. "Extreme" summer drought (PDSI below -4) such as 2006 is seen in about 1 in 20 years in both the instrumental and reconstructed records.
The severe and sustained 1950s drought appears to be unusual even in the context of the multicentury tree-ring record. From 1951 to 1956, the average instrumental summer PDSI was -3.60 (the average reconstructed summer PDSI was -3.16). The closest analog in the paleorecord is the six-year drought 1859-1864 (orange arrow), for which the average reconstructed summer PDSI was -2.87. There is one seven-year drought in the paleorecord, from 1772-1778 (red arrow), with an average reconstructed summer PDSI of -2.07.
The most notable aspect of the current drought was the profound dryness in fall and early winter; precipitation from September-December 2005 in Division 3 was only 24% of normal, the lowest level in the 111-year instrumental record. This extreme dryness led to the spectacular grass fires in Oklahoma and Texas in December and January. But this spectacle aside, the instrumental and tree-ring records of PDSI show the 2006 drought to be an infrequent, but not extraordinary, event.

Resources:
  • Divisional climate data, including precipitation for Texas Division 3 as shown above, can be obtained from NCDC.
  • The summer PDSI reconstructions from Cook et al. are available online from the NOAA Paleoclimatology Branch.

References:
  • Cook, E. R., D. M. Meko, D. W. Stahle, and M. K. Cleaveland, 1999. "Drought reconstructions for the continental United States." Journal of Climate, 12:1145-1162.
  • Cook, E. R., C. A. Woodhouse, C. M. Eakin, D. M. Meko, and D. W. Stahle, 2004. "Long-Term Aridity Changes in the Western United States." Science, 306(5698):1015-1018.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Drought for August 2006, published online September 2006, retrieved on April 20, 2014 from http://www.ncdc.noaa.gov/sotc/drought/2006/8.