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Climate of 2005 - September
Oregon Division 8 Drought


National Climatic Data Center, 7 October 2005
Regional Overview / Paleo Perspective

Top of Page Regional Overview
Precipitation Ranks for
Oregon Division 8, 2004-2005
Period Rank
Sep 1st driest
Aug-Sep 1st driest
Jul-Sep 1st driest
Jun-Sep 2nd driest
May-Sep 42nd driest
Apr-Sep 42nd driest
Mar-Sep 39th driest
Feb-Sep 15th driest
Jan-Sep 8th driest
Dec-Sep 7th driest
Nov-Sep 5th driest
Oct-Sep 6th driest
Graphic showing precipitation departures, January 1998 - present

Graphic showing Palmer Z Index, January 1998 - present

Graphic showing precipitation, September 1895-2005
Graphic showing Palmer Hydrological Drought Index, January 1900 - September 2005

Top of Page Paleoclimatic Perspective


2005 Drought, Pre-Instrumental Perspective, Northeast Oregon

60-month October-September Oregon Division 8 precipitation
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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).
Water Year October-September Oregon Division 8 precipitation, 1895-2005
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Oregon Division 8 PHDI
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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.
Paleoclimatic tree-ring reconstruction for Wyoming Division 4 for 1260-2005
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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.

Resources:
  • 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.

References:
  • 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.

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