|
|
| The West North Central Region covers a large area, including parts of the northern Rockies and northern and central Great Plains. July was wet over parts of the region, and dry over other parts, giving the region as a whole near normal precipitation -- a rank of 51st wettest (60th driest). At longer time scales (6, 12, 24 months), however, most of the region has been dry with only the eastern edges experiencing overall wet conditions. The northern High Plains area has been especially dry during the last 3 months and last 3 years. Long-term impacts of drought on vegetation can be seen in satellite vegetation indices (VHI, greenness index). |
|
  |
 |
 |
Paleoclimatic Perspective
|
The northern High Plains area (western South Dakota, southwestern North Dakota, eastern Wyoming, and southeastern Montana) has experienced unusually dry conditions both in the short-term and long-term. The last 5 months have averaged consistently drier than normal (see graph below left), and the late spring-early summer (May-July) months have been drier than normal for the last 3 years (see graph below right).
The persistent dryness of the last 3 years gives August 2001-July 2004 a rank of fourth driest such 36-month period in the 1895-2004 record (graph below left). Long-term drought conditions (as measured by the Palmer Hydrological Drought Index) quickly reached severe levels (early 2003), comparable to but not surpassing the levels of the 1930s, early 1960s, and late 1980s droughts (graph below right).
|
|
When the 20th and 21st century record of drought for this region is compared to the multi-century record of drought reconstructed from tree rings, recent droughts can be evaluated in a longer-term context. |
| The graph to the right shows a northern High Plains summer (June-July-August average) Palmer Drought Severity Index (PDSI) record reconstructed from tree-ring data (light red line represents annual values, dark red values are smoothed with a 10-year running mean). These data are part of a gridded network of PDSI reconstructions recently generated by E.R. Cook and colleagues. These gridded tree-ring reconstructions have been scaled to match the variance in the instrumental gridded PDSI data, and the gridded instrumental data have been appended to the end of the reconstructions for the years 1979-2003. |
 |
|
The graph shows a record of regional drought from AD 900 to 2003 (red). The observed summer (June and July averaged) PDSI from divisional data is shown in blue (light blue line represents annual values, dark blue line represents smoothed values) for the years 1900-2004. The match between the two sets of drought records is very good and slight differences are due to the gridded versus divisional data and the months averaged. Three major periods of drought are evident in this record: the 1930s Dust Bowl, the 1950s-early 1960s, and the 1980s-early 1990s droughts. The current period of drought has not yet persisted long enough to match any one of these three droughts, in either duration or intensity. When viewed over the past 11 centuries, the three 20th century droughts are of relatively moderate severity, in terms of both duration and intensity. The low values of the 1930s drought in the smoothed record are not matched until the late 16th century, but droughts more recent than that have been more persistent (e.g., around 1700, and in the mid-19th century). Four notable periods of intense and prolonged drought occurred in the 900s, 1100s, mid-1200s, and the late 1300s. Intense but less persistent droughts occurred in the mid-1400s and late 1500s. |
 |
 |
|
The same general pattern of drought is seen in subsections within the northern High Plains region, but some subsection-specific differences are also evident. In eastern Wyoming (graph above left, light thin lines represent annual values, dark thick lines represent smoothed values), for the same period of time, the major droughts in the 1100s and mid-1200s are present, but drought in the mid-1400s is more severe than the one in the late 1300s. In addition, a very prolonged, but moderate drought occurred over a large part of the 19th century. In the reconstruction for northwestern South Dakota (graph above right, light thin lines represent annual values, dark thick lines represent smoothed values), this period of prolonged drought in the 19th century is also evident. Major droughts in the mid-1200s and mid-1400s are in common with the other regional reconstructions. A notable aspect in this record is the suggestion of a change in drought regime prior to about 1300. Annual PDSI values are much more consistently negative, and the smoothed values suggest a regime of more prolonged drought, in this part of the record. This indication of a regime shift is also evident in the eastern Wyoming reconstruction, although less pronounced. Resources: The new gridded network of reconstructed PDSI will soon be available from the NOAA Paleoclimatology Branch. In the meantime, the original PDSI reconstruction network is available at: The general methodology for the gridded PDSI reconstruction can be found in:
|
NCDC /
Clim. Monitoring /
Climate-2004 /
Jul /
U.S. Regional Drought /
Search /
Help
Downloaded Sunday, 05-Jul-2009 15:36:49 EDT
Last Updated Wednesday, 20-Aug-2008 12:22:44 EDT by Richard.Heim@noaa.gov
Please see the NCDC Contact Page if you have questions or comments.