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<br />DECEMBER 1983 <br /> <br />ARLIN B. SUPER AND JAMES A. HEIMBACH, JR. <br /> <br />2007 <br /> <br />network, using the Gallatin Range gage as control. <br />These statistics are shown in Fig. 4, for Ridge tem- <br />peratures ~ -9.0oC. It is seen that all probabilities <br />~ O.W are across the southern portions of the zones, <br />roughly along an ESE line from the single southern <br />seeding site. As 700 mb wind directions were generally <br />WNW, this suggests a correspondence between the <br />mean plume position and the lower probability values. <br /> <br />8. Supporting evidence from snow courses <br /> <br />The BRE did not operate gages on the east slope of <br />the Main Ridge as it was assumed that any significant <br />effects of seeding would be further downwind. That <br />is, it was, expected that small ice crystals that formed <br />just upwind of or over the Main Ridge, would be carried <br />several kilometers further east before growing suffi- <br />ciently large to settle as snowflakes. However, aggre- <br />gates of many small crystals with enhanced fall ve- <br />locities were shown to result in the seeding attempts <br />to redistribute lake-effect snowfall (e.g., Holroyd and <br />Jiusto, 1971). It may be that high concentrations of <br />ice crystals, presumably caused by seeding just upwind <br />of the Main Ridge, sometimes resulted in aggregates <br />which fell on the east slope. <br />The westernmost gage, aside from the control gages, <br />was located in the valley between the Main Ridge and <br />Bangtail Ridge (see Fig. 1). After the statistical analysis <br />suggested increases at that gage, it was decided to com- <br />pare data from the two snow courses sampled by the <br />Soil Conservation Service (SCS) on the Main Ridge <br />east slope (Bridger Bowl and Maynard Creek) with <br />nearby courses in a target-control analysis. Both snow <br />courses are approximately east of the midpoint of a <br /> <br />line joining the two seeding generators, and are at 2210 <br />and 1895 m in the locations noted bySl and S2 in <br />Fig. 1. The two courses have continuous records since <br />1965 and 1967 respectively. As a point of information, <br />no snow courses have been maintained on the Bangtail <br />Ridge. <br />A search was made of all other nearby SCS snow <br />course data for any that might be highly associated <br />with S I and S2. Because large natural variations com- <br />monly occur in the percent-of-normal seasonal snow- <br />pack over distances of 100 km or more, it was con- <br />sidered desirable to limit the distance from target to <br />control courses as much as practical. However, it was <br />also important to consider courses in both crosswind <br />directions (north and south), as well as upwind (gen- <br />erally west), to further minimize natural gradients <br />across the region. <br />As a first step, all snow courses with the appropriate <br />period of record within a 60 km radius of S 1 and S2 <br />were selected. The Bridger Range had one additional <br />snow course probably suitable as a control. However, <br />this course, designated by S3 on Fig. 1, was only 7 km <br />north ofS 1. Therefore, it is possible that it may have <br />been affected by the seeding under some wind regimes. <br />Presumably seeding would affect S 1, S2 and S3 in the <br />same direction, so the use of S3 as a .control snow <br />course should be a conservative approach. <br />The Gallatin Range immediately south of the Bridger <br />Range had six snow courses as potential controls, while <br />the Crazy Mountain Range, northeast ofthe Bridgers, <br />had three such courses. These latter three may have <br />been occasionally influenced by seeding although a <br />transport wind more southerly than 240 deg would <br />have been required. Only 5% of all 359 rawins tracked <br /> <br />TABLE 12. Summary of snow course information. <br /> <br /> Direction Approximate distance 13 Season <br />Mountain Snow course Elevation from from Bridger Bowl correlation <br />range name (feet) Bridger Bowl (km) coefficient* <br />Bridger Bridger Bowl 7250 0 <br />Bridger Maynard Creek 6210 ENE 2 <br />Bridger Sacajawea 6550 N 7 0.92 <br />Gallatin Arch Falls 7350 S 45 0.84 <br />Gallatin Devils Slide 8100 S 47 0.89 <br />Gallatin . Hood Meadow 6600 S 39 0.81 <br />Gallatin Lick Creek 6860 S 36 0.76 <br />Gallatin New World 6700 S 29 0.88 <br />Gallatin Shower Falls 8100 S 46 0.92 <br />Crazy Bald Ridge 7500 NE 50 0.79 <br />Crazy Porcupine 6500 NE 48 0.69 <br />Crazy So. Fork Shields 8100 NE 48 0.88 <br />Castle Elk Peak 8000 NNE 75 0.89 <br />Big Belt Boulder Mt. 7950 NNW 90 0.79 <br />Tobacco Root Branham Lakes 8850 WSW 92 0.70 <br />Tobacco Root Lower Twin 7900 WSW 87 0.65 <br />Tobacco Root Middle Mill Creek 7850 WSW 93 0.68 <br />* See text. <br /> <br /> <br /> <br />