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<br />INTERMOUNTAIN WEST CLIMATE SUMMARY, JUNE 2006 <br /> <br />Regional Standardized Precipitation Index data through 5/31/06 <br /> <br />Source: Western Regional Climate Center, using data from NOAA National Climatic Data Center and NOAA Climate Prediction Center <br /> <br />The Standardized Precipitation Index (SPI) can be used to <br />monitor conditions on a variety of time scales. 3- and 6-month <br />SPls are useful in short-term agricultural applications and <br />longer-term SPls (12 months and longer) are useful in hydro- <br />logical applications. The 12-month SPI for the Intermountain <br />West region reflects precipitation patterns over the past 12 <br />months (through the end of May 2006) compared to the average <br />precipitation of the same 12 consecutive months during all the <br />previous years of available data. <br />As of the end of May 2006, the SPI around the Intermountain <br />West region ranges from moderately \vet in small portions <br />of northwest Utah and northwest Wyoming to very dry in <br />south central Wyoming (Figure 6). As opposed to April 2006, <br />\vhen several climate divisions in western Utah and northern <br />Wyoming were in wet categories, several divisions have moved <br />into average to drier conditions. The follo\ving divisions moved <br />into one-drier category in May: the Arkansas climate division in <br />Colorado; the Green and Bear, Snake, Wind River, Cheyenne <br />& Niobrara, Upper Platte, Big Horn and Yello\vstone divisions <br />in Wyoming; the Powder, Little Missouri and Tongue division <br />in Wyoming moved into a two-drier category from very wet to <br />average, and the south central, western, north central divisions <br />in Utah moved into a one-drier category. <br /> <br /> <br />Notes <br />The Standardized Precipitation Index (SPI) is a simple statistic <br />generated from accumulated precipitation totals for consecutive <br />months compared to the historical data for that station. Near nor- <br />mal SPI means that the total precipitation for the past 12 months <br />is near the long-term average for one year. An index value of -1 <br />indicates moderate drought severity and means that only 15 out <br />of 100 years would be expected to be drier. An index value of -2 <br />means severe drought with only one year in 40 expected to be <br />drier (courtesy of the Colorado Climate Center). <br />The SPI calculation for any location is based on the long-term <br />precipitation record for a desired period. This long-term record is <br />fitted to a probability distribution, which is then transformed into <br />a normal distribution so that the mean SPI for the location and <br />desired period is zero. Positive SPI values indicate greater than <br />median precipitation, and negative values indicate less than me- <br />dian precipitation. Because the SPI is normalized, wetter and <br />drier climates can be represented in the same way. The SPI is <br />valuable in monitoring both wet and dry periods. <br /> <br />> . +3.00 and above Exceptionally Wet <br /> . +2.00 to +2.99 Extremely Wet <br /> . +1.25 to +1.99 Very Wet <br />~ . +0.75 to +1.24 Moderately Wet <br />~ D -0.74 to +0.74 Near Normal <br />~ CHEVENNE D -1.24 to -0.75 Moderately Dry <br />I . <br /> . -1.99 to -1.25 Very Dry <br /> . -2.99 to -2.00 Extremely Dry <br /> . -3.00 and below Exceptionally Dry <br /> <br /> <br /> <br />I <br />6~ <br /> <br />. <br />GRAND <br />JUNCTION <br /> <br />/ <br /> <br />Figure 6. 12-month Intermountain West regional <br />Standardized Precipitation Index. (data through <br />5/31/06) <br /> <br />On the Web <br />- For information on the SPI, how it is calculated, and other similar products for the entire country, <br />visit http://www.wrcc.dri.edu/spi/spi.html. <br />- For information on past precipitation trends, visit: http://www.hprcc.unl.edu/products/current.html. <br /> <br />RECENT CONDITIONS I 9 <br /> <br />