2009-03-02_PERMIT FILE - C1980004 (11)

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4 - 76 4.6 METEOROLOGY . The climate of west-central Colorado is characterized by dry air masses, which are modified Pacific air masses that move eastward across the Rocky Mountains. Winter snows and summer showers or thunderstorms result in unusually even distribution of precipitation throughout the year. Prevailing winds vary greatly throughout the Upper Colorado River Basin, and are markedly affected by differences in elevation and by the orientation of mountain ranges and valleys with respect to general air movements. Five years of upper air observations at Grand Junction show that surface based inversions occur on 84 percent of the mornings. During the afternoons they are not as common, occurring 11 percent of i:he time in winter but less than 3 percent of the time in other seasons. The area is subject to a relatively high frequency of stagnation situations, mostly in winter. National Weather Service Records at Fruita indicate an annual average precipitation of 8.8 inches. Vegetation on the tract suggests an annual precipitation of approximately 11 inches at the bottom of East Salt Creek Valley and perhaps 15 inches on the higher ridges above 7,000 feet. Evaporation is estimated to be about 45 inches annually. • It is assumed that prevailing wind direction is along the major canyon or from the north northeast. The wind rose from Grand Junction weather station has been rotated to reflect the major canyon axis and is shown in Figure 4.6.1. Average wind speed at the Grand Junction station is 8.1 mph. The rotated wind rose was taken directly from the Fnal West Central Colorado Coal EIS in the absence of site specific wind measurements. A remote on•site meteorological station was established in Spink Canyon to gather site specific weather data. The station parameters included wind speed, wind direction, temperature and solar radiation. Thelremoteness of the station, winter snow conditions and reliance on generators for power made data gathering difficult. Records are therefore incomplete and further data collection be necessary to provide a thorough baseline description of site specific conditions with any tlegree of certainty. To date, no information has indicated that conditions vary significantly the generalized description above; however, the data is considered incomplete and not a part of this application. The monitoring will be continued and the station will be to an area near the mouth of Munger Canyon if found to be necessary. . Monthly and annual mean, maximum and minimum water equivalent, precipitation totals are shouvn in Table 4.5.1, as is monthly average temperature. M~ Volume 1 4-29-96