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~ .....'~ v-j~, . ,.> PREPARATION OF WEATHER FORECASTS FOR WINTER OROGRAPHIC CLOUD SEEDING OPERATIONS IN THE COLORADO ROCKIES by WESTERN WEATHER CONSULTANTS, INC. December 1982 The most favorable meteorological conditions for the augmentation of pre- cipitation from cold orographic cloud systems by seeding with ground-based silver iodide (AgI) generators can be separated into four general physical categories, namely; water available, nuclei available, mixing available and time available. The meteorological conditions under the four categories can be determined from rawinsonde data and all other available National Weather Service (NWS) data. The following describes how WESTERN WEATHER CONSULTANTS, INC. (WWCI) evaluates each of these basic categories to arrive at a forecast for cloud seeding operations. 1. WATER AVAILABLE The amount of liquid water available in a cloud is key to determining! seed- ability; the greater the cloud water content, the more water that can poten- tially precipitate out of a cloud system. Parameters useful in estimating! the available water are cloud base height, cloud depth and cloud base moisture. WWCI has established the following guidelines in evaluating this category: Cloud Base Height must be lower than 1,000 ft. above the average elevation of the mountain crest. Cloud Depth must be in excess of 2,000 ft. Cloud Base Moisture must be in excess of 1.5 gm/kgm. The most recent rawinsonde from Grand Junction (GJT) and cloud observations from NWS reporting stations near the target areas are examined to determine the current cloud base height and depth. The base of the lower cloud layer is usually easily identified; on the GJT rawinsonde, the base is determined as the point where the temperature-dew point spread is less than 30C. To estimate cloud depth, we use the rule of thumb that the relative humidity must be 85 percent or greater to be identified as a cloud region. The cloud base moisture ~