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003190 Some exploratory statistical experiments have suggested that a 5 to 15 pct increase in seasonal snowfall can be achieved. Perhaps the best known experiment is the Climax, Colorado, program of the 1960s, the only winter experiment to date that incorporated a confirmatory phase after the exploratory phase. Some of the initial suggestions did appear to be confirmed (Mielke et al., 1971), and the results generally were accepted by the scientific community for a time. Mielke et al. (1981) dealt with a possible type I statistical error (a chance selection of seeded and nonseeded days that favored an erroneous statistical conclusion of increased snowfall caused by seeding). However, serious challenges to the Climax experiments appeared in the open scientific literature (Hobbs and Rangno, 1979; Rhea, 1983; Rangno and Hobbs, 1987). The Climax statistical results now are viewed with skepticism by many scientists, in part because of the lack of supporting physical observations. The Climax experiments were conducted in complex terrain with a modest budget before the development of much of the instrumentation in common use today. Hence, routine monitoring of the key physical mechanisms involved was not then practical. Several winter seeding experiments were attempted in the West during the late 1960s and early 1970s, but major funding cutbacks terminated all but Reclamation's CRBPP (Colorado River Basin Pilot Project) after about 1973; the CRBPP continued through 1975. Most of the experiments were inconclusive. Two exceptions are the Bridger Range Experiment, which will be discussed later, and the Cascade Project. The Cascade Project was a pioneering effort in physical evaluation of seeding experiments which produced some of the most convincing . evidence available that seeding can modify snowfall when properly applied under some conditions (Hobbs, 1975a; Hobbs and Radke, 1975; Hobbs, 1975b). The Cascade Project did not address the magnitudes of the changes that might be achieved on a seasonal basis over a large area. To do so would have required a multiple winter statistical experiment for which funding was not available. , , ~ The CRBPP, discussed by Elliott et al. (1978), found no significant difference in precipitation from seeded and nonseeded days. However, at least two serious design problems were discovered involving inadequate T&D of the seeding agent and inability to forecast cloud top temperatures and winds. Thus, CRBPP results must be considered inconclusive, meaning the evidence supported neither the conclusion that seeding could enhance snowfall nor the conclusion that seeding was incapable of increasing snowfall. Elliott et al. (1978) reanalyzed the data set and concluded that precipitation increases of 10 pct or more could have been achieved if seeding had been limited to only favorable cloud conditions. Also, a limited number of physical studies during the last season of the CRBPP provided evidence that seeding potential exists in the latter stage of winter storms over the San Juan Mountains associated with the release of convective instability (Cooper and Marwitz, 1980). l Since the end of the CRBPP, the only major winter orographic cloud seeding project conducted in the West was the SCPP (Sierra Cooperative Pilot Project) reviewed by Reynolds and Dennis (1986). The main purpose of the SCPP was to determine when seeding of cold winter clouds over the central Sierra Nevada of California produced increases, decreases, or no change in precipitation, and to document the magnitude of the changes. In addition, extensive physical studies were conducted of three-dimensional airflow and microphysical processes (both cloud liquid water and ice processes) occurring in natural winter cloud systems. Examples include Heggli et al. (1983), Heggli and Rauber (1988), Marwitz (1987a), and Marwitz (1987b). These studies led to the development of an analytical targeting model (Rauber et al., 1988) that was used the last 3 yr of the program (1984-1986). The latter years of the SCPP emphasized 7