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INTERMOUNTAIN WEST CLIMATE SUMMARY, JANUARY 2008 Regional Standardized Precipitation Index data through 12/31/07 The Standardized Precipitation Index is used to monitor mois- ture supply conditions. This index identifies emerging droughts months sooner than the Palmer Index and it can be computed on several time scales. 3- and 6-month SPls are useful in short-term agricultural applications. Longer-term SPls (12 months and longer) are useful in hydrological applications. This month, we describe the 12-month SPI map. At the end of December 2007, several of the climate divisions changed in climate classification as compared to the end of October 2007 (the last IMW Climate Summary). Above average precipitation in Utah and central Wyoming in December helped move some divisions to wetter categories. The South Central and Northern Mountains divisions in central Utah and the Green and Bear Drainage and Wind River divisions in southwest Wyoming changed from the moderately dry category to the near normal cat- egory. The Powder, Little Missouri, and Tongue division moved from the moderately wet to the very wet category. Near average conditions in southeast Wyoming caused the Lower Platte region to change from moderately wet to near normal. There were no changes in climate divisions in Colorado. ~ Notes The SPI is an index based on the probability of recording a given amount of precipitation, and the probabilities are standard- ized so that an index of zero indicates the median precipitation amount (half of the historical precipitation amounts are below the median, and half are above the median). The index is negative for drought, and positive for wet conditions. As the dry or wet conditions become more severe, the index becomes more negative or positive. The SPI is computed by the NOAA National Climatic Data Center (NCDC) for several time scales, ranging from one month to 24 months, to capture the various scales of both short-term and long-term drought. The Colorado Climate Center describes the SPI as valuable in monitoring both wet and dry periods, and it can be applied to other types of data (e.g. streamflow, reservoir levels, etc.). Near normal SPI means that the total precipitation for the past 12 months is near the long-term average for one year. An index value of -1 indicates moderate drought severity and means that only 150/0 would be expected to be drier. An index value of -2 means severe drought with only 2.50/0 of years expected to be drier. A 12-month SPI is used for the Intermountain West region (Figure 6) and compares precipitation patterns for 12 consecutive months with the same 12 consecutive months during all the previ- ous years of available data. The SPI at these time scales reflect long-term precipitation patterns. The graphic in Figure 6 comes from the Western Regional Climate Center, which uses data from the NCDC and the NOAA Climate Prediction Center. ?~ +3.00 and above Exceptionally Wet ;rr- . +2.00 to +2.99 Extremely Wet [c . +1.25 to +1.99 Very Wet D +0.75 to +1.24 Moderately Wet D -0.74 to +0.74 Near Normal D -1.24 to -0.75 Moderately Dry . -1.99 to -1.25 Very Dry -2.99 to -2.00 Extremely Dry -3.00 and below Exceptionally Dry '-- r r/ Jr }. . /,1 r r--,V . GRAND JUNCTION -<~ Figure 6. 12-month Intermountain West regional Standardized Precipitation Index (data from 1/1/07 - 12/31/07). On the Web · For information on past precipitation trends, visit: http://www.hprcc.unl.edu/products/current.html. · For SPI products directly form the NCDC, visit: http://lwf.ncdc.noaa.gov/oa/c1imate/research/prelim/drought/spi.html. These maps use the same data as Figure 5, but the categories are defined slightly differently. RECENT CONDITIONS I 9