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 29 percent of the contiguous United States as of the end of August 2007, an increase of about 5 percent compared to last month. By contrast, about 16 percent of the contiguous U.S. fell in the severely to extremely wet categories.
  • About 44 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

August 2007 was characterized by a contrast in extreme weather. Very hot and dry conditions plagued most of the Southeast U.S., and dry weather persisted across the northern Great Lakes and much of the West. Meanwhile, heavy rain developed along a quasi-stationary front from the central Plains to southern Great Lakes, splitting the northern Great Lakes and Southeast drought areas. Torrential rains from thunderstorms and the remnants of Tropical Storm Erin inundated parts of the southern Plains, acting as a wet barrier between the parched Southeast and drier portions of the Far West. Drier-than-normal conditions occurred across much of Alaska and the Hawaiian Islands, and parts of Puerto Rico.
Map of Statewide Precipitation Ranks, August 2007
For the three core drought areas, August marked a continuation of persistent dryness that has plagued the regions for several months. Every month since December 2006 has averaged drier than normal for the Southeast Region, giving the region the driest year-to-date on record. In the Lake Superior area (Upper Michigan, northern Wisconsin, and northeast Minnesota), the last four months have been persistently dry and most of the last 12 months have been very dry (May-August, September-August). Idaho, Montana, Nevada, and California have been hardest hit by persistent dryness in the West. Persistent dryness shows up in Hawaii precipitation maps for the last 3 months and even out to 12 months.
Map of 9-Month SPI, December 2006-August 2007
Several states have had the driest month or season, or placed in the top ten driest category, at the end of this month (August, June-August, March-August, September-August). This is especially true for Tennessee (driest Oct-Aug through May-Aug), North Carolina (driest Dec-Aug through Jun-Aug), Florida (driest Sep-Aug, Jan-Aug, Feb-Aug), and Alabama (driest Jan-Aug).
Graph of Tennessee Statewide Precipitation, February-August 1895-2007
About two-thirds of the western U.S. (Rockies westward) fell in the moderate to extreme drought category, and about half fell in the severe to extreme category (as defined by the Palmer Drought Index) by the end of this month. Moderate to exceptional drought affected about 83 percent of the Southeast region and 46 percent of the contiguous U.S., according to the federal U.S. Drought Monitor.
Map of Palmer Drought Index, August 2007

Wildfires remained active and difficult to contain across the northern Rockies and northern Intermountain West. As of August 31, year-to-date wildfires across the U.S. had burned about 7.0 million acres of vegetation (128 percent of the 10-year average), mostly in the Northwest. The relentless spread of large wildfires prompted Governor C.L. Otter to declare a statewide disaster emergency in Idaho.

The severe heat wave, which persisted beneath a strong upper high pressure system much of the month across southern and central parts of the nation, intensified evaporation. More than 30 all-time high station temperature records were tied or broken and more than 2000 new daily high temperature records were established. More than ten days of triple digit temperatures were recorded at numerous stations across the southeastern states. This heat combined with the general lack of rain to exacerbate drought conditions in the Southeast. Many communities had implemented mandatory water restrictions. Streams and lakes shrank, soils dried out (USDA observed and CPC modeled), and vegetation and crops withered (USDA observed and compared to average).

Map of USDA Observed Percent of Pasture and Range Land in Poor or Very Poor Condition, August 26, 2007

According to the Alabama State Climatologist, 96 percent of the state reported short to very short soil moisture, and Oaks were dying on ridges which will have serious consequences in the next two years. Historic low pools were expected for Alabama reservoirs. Brown's Ferry Number 2 nuclear power plant shut down due to the Tennessee River being too weak and too warm for cooling the reactor. Climate Division 4 in northeastern Alabama had record dry conditions since the beginning of this year.
Graph of Palmer Drought Index for Alabama Division 4 (Northeastern Valley), January 1900-August 2007
The lack of rain in recent months has dried out vegetation so much that the North Carolina Division of Forest Resources imposed a statewide ban on open burning and cancelled all burning permits. As reported by the USGS, the hot and dry weather conditions in August brought record lows to many of North Carolina's rivers and streams. The lowest average August streamflow in 110 years of record was recorded on the Tar River at Tarboro. The lowest average August streamflow on record was measured at 12 other monitoring stations in the state, with 9 of those sites located in central North Carolina. The drought brought groundwater levels in unpumped wells in western North Carolina and in the outer coastal plain to near the levels observed during the 1998-2002 drought. The heat and low streamflows also raised stream water temperatures, having an adverse effect on fish and biological communities.

As noted by the Midwest Regional Climate Center, drought conditions continued to expand and intensify in parts of the Midwest during August. Precipitation was only 10 to 25 percent of normal from the Missouri bootheel into western Kentucky and less than 25 percent of normal in the Michigan Upper Peninsula. Marquette, MI received only 0.85 inch of rain in August. This was the driest May-August on record in Marquette, with only 7.20 inches of rain. The previous record was 8.28 inches in May-August 1998. Water levels in Lake Superior reached a new record low for August as a result of the combination of warmer than normal weather and below normal precipitation over the last 20 months over the Lake Superior drainage basin. The average Lake Superior water level at the end of August 2007 was 600.4 feet above mean sea level, a foot lower than a year ago and two inches lower than the previous record of 600.5 feet in 1926. The all-time record low water level for Lake Superior is 599.5 feet in March and April 1926. Water levels in Lakes Michigan and Huron were about two feet below normal level, but still about nine inches above the record low water levels. In early August, the USDA declared 24 drought-stricken Minnesota counties to be agricultural disaster areas.

And in the Northeast, state officials declared a drought watch across most of Pennsylvania in early August.

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.):

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

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
Contiguous U.S.

Top of Page Pre-Instrumental Perspective

August 2007 Paleoclimatic Analysis for Central Tennessee

Much of Tennessee has experienced moderate drought conditions since late 2005, but conditions dramatically worsened this spring and summer. The U.S. Drought Monitor for late August/early September showed most of the state to be in category D4, "exceptional drought". Conditions were worse in some local areas. Tennessee Division 3 (central Tennessee) has experienced dry conditions since 2005 and has been persistently dry since February 2007. The Palmer Drought Severity Index (PDSI) for central Tennessee for August 2007 was -5.28, the lowest value ever recorded for any month during the instrumental period.
Precipitation Ranks for
Tennessee Division 3, 2006-2007
Period Rank
Aug 2nd driest
Jul-Aug 1st driest
Jun-Aug 1st driest
May-Aug 1st driest
Apr-Aug 1st driest
Mar-Aug 1st driest
Feb-Aug 1st driest
Jan-Aug 1st driest
Dec-Aug 1st driest
Nov-Aug 1st driest
Oct-Aug 1st driest
Sep-Aug 1st driest
Graphic showing precipitation departures, January 1998 - present

Graphic showing Palmer Z Index, January 1998 - present
Graphic showing precipitation, January-August, 1895-2007
Graphic showing Palmer Drought Severity Index, January 1900 - August 2007

The graph to the right shows the summer (June-August) average PDSI, 1895-2007, for Tennessee Division 3 (annual values in light blue, 5-year weighted average in dark blue). The value for 2007 (-4.31) is the lowest summer PDSI in the instrumental record. The most persistent drought was from 1940-1946, when summer PDSI was negative for seven years in a row, including three consecutive years (1941-1943) when summer PDSI was below -3 (yellow arrow).
Paleoclimatic tree-ring reconstruction and observed PDSI for Tennessee Division 3 for the total period 1550-2007
large image (50 KB)

larger image (190 KB)

The graph above also shows a 429-year tree-ring reconstruction (1550-1978) of summer PDSI for a gridpoint (35°N, 87.5°W) on the southern boundary of Tennessee Division 3 (annual values in light red, 5-year weighted average in dark red). This 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 29 sites across the Southeast. The correlation between reconstructed gridpoint PDSI and instrumental Division 3 PDSI over their common period (1895-1978) is 0.673, indicating a high degree of shared variance.

The tree-ring reconstruction can put the droughts of the last century in central Tennessee, including the current one, into a much longer historical perspective. "Extreme" summer drought (PDSI below -4) such as 2007 occurs very infrequently (less than once per century) in both the instrumental record and the tree-ring reconstructed records. One year in the tree-ring record (1839 with -5.43) had a reconstructed summer PDSI lower than the observed value for 2007, and another was very similar (1708 with -4.28). The tree-ring record confirms that the severity of this summer's drought is very unusual, but also suggests that it is not unprecedented.

The most persistent period of reconstructed summer drought (1564-1573; red arrow) exceeds the duration of the 1940s drought by three years. However, the core of the 1940s drought appears to be unusual even in the context of the multicentury tree-ring record. No 3-year period in the tree-ring record has an average summer PDSI approaching that observed from 1941-1943 (-3.56).


  • 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 Centers for Environmental Information, Monthly Drought Report for August 2007, published online September 2007, retrieved on July 18, 2024 from