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<br />INTERMOUNTAIN WEST CLIMATE SUMMARY, JANUARY 2006 <br /> <br />Regional Standardized Precipitation Index data through 12/31/2005 <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 lon- <br />ger-term SPls (12 months and longer) are useful in hydrological <br />applications. The 12- month SPI for the Intermountain West <br />region (Figure 6) reflects precipitation patterns over the past 12 <br />months (through the end of December 2005) compared to the <br />average precipitation of the same 12 consecutive months during <br />all the previous years of available data. <br />The SPI is mostly in the near normal to wet range around <br />the Intermountain West region as of the end of December 2005. <br />Western Colorado is near normal, while the Rio Grande basin <br />in the south-central part of the state continues to be moder- <br />ately dry. There was some improvement in the Colorado River <br />basin, which moved from near normal to moderately wet last <br />month. Wyoming had some improvements as well, in the Up- <br />per Platte division in the south-central part of the state and the <br />Yellowstone division moving out of dry conditions into near <br />normal conditions. The north-central part of Wyoming remains <br />moderately wet while the rest of the state is near normal. Utah <br />continues to be moderately wet with the western part of the state <br /> <br />very wet. The southeast division SPI decreased slightly since <br />last month from very wet to moderately wet. <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 <br />normal SPI means that the total precipitation for the past 12 <br />months is near the long-term average for one year. An index <br />value of -1 indicates moderate drought severity and means <br />that only 15 out of 100 years would be expected to be drier. <br />An index value of -2 means severe drought with only one year <br />in 40 expected to be drier. (courtesy of the Colorado Climate <br />Center) <br />The SPI calculation for any location is based on the long- <br />term precipitation record for a desired period. This long-term <br />record is fitted to a probability distribution, which is then trans- <br />formed into a normal distribution so that the mean SPI for the <br />location and desired period is zero. Positive SPI values indicate <br />greater than median precipitation, and negative values indicate <br />less than median precipitation. Because the SPI is normalized, <br />wetter and drier climates can be represented in the same way. <br />The SPI is valuable in monitoring both wet and dry periods. <br /> <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 /> CHEYENNE D <br /> II . -1.24 to -0.75 Moderately Dry <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 />GRAND <br />JUNCTION <br /> <br />Figure 6. 12-month Intermountain West regional <br />Standardized Precipitation Index. (data through <br />12/31/05) <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 />