Drought - September 2005
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U.S. Drought Highlights:
- On the national scale, severe to extreme drought affected about 7 percent of the contiguous United States as of the end of September 2005.
- Moderate to extreme drought affected about 18 percent of the contiguous U.S.
- The month was drier than normal across much of the country, with 13 percent of the contiguous U.S. very dry (in the bottom 10 percentile of the historical record). Especially dry areas included the Southeast to southern New England, and much of the Great Plains and Far West.
- Rains from cold fronts and tropical systems brought short-term relief to the drought areas from Arkansas to the Great Lakes.
- Many of these September dry areas have been drier than normal for the last 2 to 6 to 9 months. Long-term moisture deficits (last 24 to 36 to 60 months) persisted across parts of the West into the northern High Plains and central Plains.
Please Note: The data presented in this drought report are preliminary. Ranks, anomalies, and percent areas may change as more complete data are received and processed.
On the national scale,
- severe to extreme drought affected about 7 percent of the contiguous United States as of the end of September 2005, an increase of about 1 percent compared to last month
- about 18 percent of the contiguous U.S. fell in the moderate to extreme drought categories (based on the Palmer Drought Index) at the end of September
- 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
- about 15 percent of the contiguous U.S. fell in the severely to extremely wet categories at the end of September
- a file containing the national monthly percent area severely dry and wet from 1900 to present is available
- 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).
September 2005 was drier than normal across much of the country, with 13 percent of the contiguous U.S. very dry (in the bottom 10 percentile of the historical record). Especially dry areas included the Southeast to southern New England, and much of the Great Plains and Far West.
The September precipitation pattern at the primary stations in Alaska was mostly wetter than average. Across Hawaii, the precipitation pattern was mixed, with more stations wetter than average than drier. In Puerto Rico, the precipitation signal was also mixed, based on National Weather Service radar estimates of precipitation. September streamflow averaged near normal for Puerto Rico and wetter than normal for the Hawaiian Islands.
Many of these September dry areas have been drier than normal for the last 2 to 6 to 9 months. Long-term moisture deficits (last 24 to 36 to 60 months) persisted across parts of the West into the northern High Plains and central Plains.
Some regional highlights:
- Three states had the driest September in the 111-year record:
- Four additional states had the tenth driest, or drier, September:
- Six states had the tenth driest, or drier, August-September:
- Four states had the tenth driest, or drier, July-September:
- Illinois ranked tenth driest, or drier, for several seasons:
- Apr-Sep: 10th driest
- Mar-Sep: 6th driest
- Feb-Sep: 6th driest
- During September, parts of Kansas and eastern Iowa were declared drought disaster areas by the USDA (High Plains Journal, 9/21; Quad-City Times, 9/14). In parts of North Carolina, some public water supply systems were under voluntary water conservation during the month because of drought related issues.
- By the end of the month, 60% or more of the pasture and range land was in poor to very poor condition in Arkansas (61%), Connecticut (71%), Louisiana (76%), Pennsylvania (78%), California (86%), and Rhode Island (100%).
- End-of-month and month-averaged soil moisture conditions were drier than normal across a broad swath from the mid-Atlantic to southern New England states, much of the Great Lakes region into the central Plains, and parts of the southern Plains, Pacific Northwest, and northern and central Rockies, based on model computations (CPC-1, CPC-2, MRCC). The models also indicated dry soil moisture conditions in parts of Alaska and Hawaii, and near the surface and at depth from Iowa to the western Great Lakes.
- According to early October USDA observations, more than 50 percent of the topsoil moisture was rated short to very short (dry to very dry) across the Pacific Northwest and Rocky Mountain states, parts of the central and southern Plains, and from the Southeast to southern New England states. This is drier than the 5-year and 10-year averages across much of the area.
- Streamflow levels were below seasonal norms across much of the Southeast to southern New England states, and parts of the Pacific Northwest, northern and central Rockies, central and southern Plains, and Great Lakes, as computed by models and based on USGS observations.
- Wildfire activity continued across the western U.S. during September, as numerous large fires burned in Alaska, the northern Rockies, and Columbia Plateau regions. As reported by the National Interagency Fire Center, over 8.1 million acres had burned across the U.S. by the end of the month, ranking the 2005 season as the second highest since 1960 for acres burned, just 0.3 million acres behind 2000.
- Reservoir levels in the West (provided by the USDA) reflected the long-term precipitation deficits in many states. About 16 percent of the western U.S. (Rockies westward) fell in the moderate to extreme drought category (as defined by the Palmer Drought Index) as of the end of September, reflecting continued dryness in the Pacific Northwest (June, July, August, September).
2005 Drought, Pre-Instrumental Perspective, Northeast Oregon
The northeast corner of Oregon has been experiencing persistent drought conditions since 1999. Precipitation in Oregon Division 8 since October 2000 is only 76% of the 60-month normal (based on 1950-2000). The 60-month period ending September 2005 was among the driest such October-September 60-month periods in the last 111 years (see graph to left).
In the last several months, short-term drought in the region has turned extreme. Based on preliminary reports, there was no recorded precipitation in Division 8 in August and September — which is unprecedented in 100 years of record. July-September 2005 was the driest July-September in the 111-year record, and overall June-September precipitation was 30% of normal, ranking second driest. This extraordinarily dry summer and early fall ensured that the 2005 water year (previous October-current September) would be the seventh in a row with near to much below-normal precipitation (12.44 inches, or 68% of normal).
To assess the severity of the current drought, we can compare it to the century-long instrumental record (see graph to left), but a longer window onto past droughts requires proxy data such as tree rings. The graph below shows a 275-year tree-ring reconstruction (1705-1979) of water year precipitation for Oregon Division 8 (annual values in light red; 5-year weighted mean in dark red). The instrumental precipitation record on which it is calibrated is also shown (annual values shown in light blue; 5-year weighted mean in dark blue).
The reconstruction was developed by Garfin and Hughes (1996) from a network of 18 tree-ring sites in Oregon, California, Nevada, and Idaho. The correlation between the reconstructed and instrumental records over their common or calibration period (1896-1979) is 0.586, indicating moderately high shared variance but some uncertainty around the reconstructed annual values. The reconstruction does capture well the multi-year and decadal-scale variability in the instrumental record.
The most significant feature in the last 100 years in both the instrumental and reconstructed precipitation records is a severe and extended drought in the 1930s, indicated on the graph with the longer yellow bar. Both records show that water year precipitation was below normal for 10 years in a row (1928-1937). The current drought, indicated by the shorter yellow bar, is now in its seventh year (1999-2005), and its average annual anomaly is similar to the 1930s drought. The full reconstruction does not show any events that rival the 1930s drought or the current drought. The longest reconstructed drought prior to 1900 is six years long (1839-1844; indicated by the orange bar), and it was less severe than the two more recent events.
The driest water year in the last century, according to the instrumental record, was 1977 (11.41 inches), with 2001 (11.63 inches) a close second. The reconstruction shows three years in the pre-instrumental period (1717, 1721 and 1729; indicated by orange triangles on the graph) with reconstructed precipitation less than in 1977. Taking into account the uncertainty in the reconstruction (as estimated from the differences between the reconstructed and instrumental values during the calibration period), there is an 80% probability that the actual precipitation in 1729 was lower than in 1977.
So in terms of duration and severity, the ongoing (1999-2005) drought in northeast Oregon appears to have been exceeded in the past 300 years only by the 1930s drought. The driest years in the current drought (2001) and the past century (1977) were probably exceeded in severity by only a few years in the two preceding centuries.
- Divisional climate data, including precipitation for Oregon Division 8, can be obtained from NCDC.
- The Eastern Oregon precipitation reconstruction is available online from the NOAA Paleoclimatology Branch.
- Garfin, G. M., and Hughes, M. K., 1996. "Eastern Oregon Divisional Precipitation and Palmer Drought Severity Index from Tree Rings." Report to the U.S. Forest Service Intermountain Research Station, USDA Forest Service Cooperative Agreement PNW 90-174.