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EXECUTIVE SUMMARY This report documents the data collection and subsequent analyses from the first two field programs of the Arizona Snowpack Augmentation Program. The purpose of these field programs was to examine the suitability of winter clouds over the Mogollon Rim area for possible future weather modification activities aimed at increasing water supplies. Only naturally occurring cloud systems were studied during the period of this report, and no cloud seeding was conducted. Measurement programs were carried out during the mid-January to mid-March period of both 1987 and 1988. Two different primary ground-observing sites were used, both near the crest of the high terrain. During early 1987, that site was Happy Jack, south of Flagstaff, while Hannagan Meadow in the White Mountains of eastern Arizona was used in early 1988. The 1987 program was more comprehensive, including aircraft, radar, and other observations not available during 1988. One of the initial analysis efforts was to develop a storm episode classification system. In its final form, each hour of each storm was typed by the scale of the weather disturbance causing it, whether synoptic (affecting a large area) or mesoscale (affecting a local area), and by the presence or absence of significant convection; that is, whether the clouds were predominantly stratiform or convective. This dual classification provided a framework for other analyses. The most important field observations were of cloud liquid water, chiefly collected by a ground-based microwave radiometer. This relatively new instrument can continuously monitor the vertically integrated amount of cloud liquid water above it. Winter precipitation over the Mogollon Rim is produced when the normally supercooled (colder than 0 oc) cloud liquid water is converted into tiny ice particles that grow to snowflake size and settle to the surface. Consequently, it is very important to measure the cloud liquid water overhead as it is the "raw material" or "fuel" for precipitation. It is known that cloud liquid water often forms over the windward side of mountain barriers due to the uplift and associated cooling of moist air. Conversely, the cloud liquid water rapidly evaporates on the lee side due to subsidence and warming of the air, resulting in the well-known "rain shadow" effect downwind of many mountain ranges. In the Arizona studies, the radiometer was located so it could monitor the cloud liquid water passing over the crest just prior to the evaporation zone. The cloud liquid water can therefore be thought of as excess to that used by nature in producing precipitation because any cloud liquid water still existing above the crest would likely soon evaporate. Similarly, small ice particles flowing over a mountain crest soon begin sublimating. Therefore, successful cloud seeding involves converting excess cloud liquid water into tiny ice crystals far enough upstream to permit growth into snowflakes that reach the surface before sublimating in the lee of the barrier. A number of findings resulted from applying the storm classification scheme to the cloud liquid water observations. For example, the large majority of hours with cloud liquid water were produced by synoptic scale disturbances. This suggests that the larger storms have the most potential for snowfall enhancement by cloud seeding. Several of the storm episodes had apparently seedable conditions (excess cloud liquid water) , 1ll