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| Southwest Montana (climatic division 2) is at an epicenter of the western U.S. 6-year drought. As measured by the Palmer Hydrological Drought Index, the drought here started in the fall of 1999 and has continued uninterrupted to the present, reaching record low levels. With few exceptions, each month has received below-normal precipitation. The cumulative effect of this persistent dryness has been the sixth driest 12-month period (September 2003-August 2004) and the driest 60-month period (September 1999-August 2004) on record. |
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Standardized Precipitation Index
| The Standardized Precipitation Index (SPI) is an index that scales precipitation based on the probability of occurrence, with negative values indicating drier conditions and positive values indicating wetter conditions. It is computed for seven time scales, including 1-, 2-, 3-, 6-, 9-, 12-, and 24-months. The SPI values for these seven time scales for Montana climate division 2 are shown in the graph to the right. The graph indicates that Southwest Montana has experienced dry conditions for each of the time scales, and that the dryness for the longer time scales is more severe, reflecting the long-term nature of the current drought. |
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| Time series graphs for the period of record (1895-present) can be created for each of the seven time scales. Graphs for the shorter time scales (1- and 2-months) show considerable variability, or climatic noise, but the drought signal becomes clearer beginning with the 3-month SPI (see graphs below). |
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| At the longer time scales, the persistent nature of the drought becomes clear. The 12- and 24-month SPI graphs (see below) also indicate that the hydrological, or long-term, drought reached record severe levels. |
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Paleoclimatic Perspective
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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 below left depicts the Palmer Drought Severity Index (PDSI) for southwestern Montana for the instrumental period, January 1900-August 2004. The graph below right shows a record of summer (June-July-August average) PDSI reconstructed from tree-ring data (light red line represents annual values, dark red values are smoothed with a 10-year running mean) for a grid point in southwestern Montana. These tree-ring data are part of a gridded network of PDSI reconstructions recently generated by E.R. Cook and colleagues. In the graph to the bottom right, the gridded tree-ring reconstruction has been scaled to match the variance (year-to-year variability) in the instrumental gridded PDSI data, and the gridded instrumental data have been appended to the end of the reconstruction for the years 1979-2003. The graph shows a record of regional drought from AD 1234 to 2003. The observed summer (June-July-August 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 PDSI records is quite good, particularly for the periods of drought, and considering the two records are not for the same exact regions. |
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In the 20th century, the three major droughts have been the 1930s, 1950s and late 1980s-early 1990s droughts, with a short period of recovery between this last drought and the current period of drought, which started in late 1999 but began in earnest in 2000. A notable feature of the 20th century droughts is the intensity of drought in 1934. This is the 5th lowest summer PDSI value for a single year in the full 770 year record. The driest year was 1382, followed by 1580, 1460, 1863, then 1934.
When 20th century summer droughts are viewed in terms of duration (numbers of years of negative PDSI), they appear to be of a relatively common duration, with the longest period of negative PDSI from 1987 to 1992 (six years). In the 1930s, there were five consecutive years of negative PDSI values. The longest period of drought in the full record, is the 16-year period from 1243-1258, clearly visible in the graph as a period of sustained drought. Other periods of persistent drought were in the mid-17th century (14 years, 1652-1665), in the 1430s (eight years, 1432-1439), and around 1400 (nine years, 1395-1403). In between the latter two periods of drought was a very wet period. However, only 11 near-average years separated the 16-year drought in the 1200s and a period of eight more years of negative PDSI from 1269-1276. |
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Resources: The gridded network of reconstructed PDSI is available from the NOAA Paleoclimatology Branch. The display and downloading of PDSI grid point data is available at: The full gridded PDSI dataset, both reconstructed and instrumental, as well as reconstruction calibration and verification statistics, is available at: The general methodology for the gridded PDSI reconstruction can be found in:
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