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- 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
- 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
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
|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
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.
Highly stressed corn field east of Kansas City (photo courtesy of
University of Missouri Extension)
Empty Platte River (photo courtesy of the High Plains Regional
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).
Some regional highlights:
State/Regional/National Moisture Status
August 2006 Paleoclimatic Analysis for North
|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).
large image (50 KB)
larger image (175
|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
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
- 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
- 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.