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. ,. .. . . . . . . . . ,. . . . . . . . . . . . . It . " . II) . . .. .. . .. ,. Ie . ,. . t 4.0 Storm Types Over 30 of the identified storm events were analyzed to determine the weather pattems and types of storm systems likely to produce significant snowfall during the winter season. It was found that these heavier precipitation amounts occur during a variety of weather patterns, which tend to vary according to the time of year. During the winter period (December-February), many ofthese precipitation events resulted from strong zonal flow, with mid- and high-level Pacific moisture moving over Colorado. Satellite images during these events were often very impressive, with high cloud cover (very bright on the IR imagery) covering large portions of the western U.S. Deep upper-level trough situations were also responsible for some ofthe precipitation events, with storm systems over the south-western United States and southern Rockies favoring the spring and fall seasons. Typically the more significant precipitation amounts were found to occur in south-westerly flow in advance of the surface frontal system (sometimes well in advance), although some did occur in northwesterly flow following the frontal passage. A thick shield of high-level cloud cover was evident in most of these significant precipitation events, suggesting a large amount of high-level moisture. Strong upper-level winds and ajet core near Colorado were very often associated with these precipitation events. 5.0 700mb Wind Flow and Temperature Characteristics Upper-air soundings (National Weather Service, twice daily balloon observations of temperature, dew-point and winds) from Grand Junction, Colorado (GJT) were used to determine the 700-mb (approximately 10,000') wind velocity and temperature, as well the stability of the atmosphere. Although Denver is approximately the same distance away from the project area, the Grand Junction soundings were primarily used because ofthe greater similarity of terrain than Denver, which frequently experiences airmasses primarily affecting the Great Plains. Denver soundings were used in a few cases when the sounding from Grand Junction was not available. The 700-mb wind flow is used to represent the transport of potential seeding material, and 700-mb temperature is used to gauge the seedability of the storm events. Silver iodide, the chemical commonly used to conduct cloud seeding projects, becomes an active ice nucleant at temperatures colder than -5 nC. Winter cloud seeding research has shown that ground releases of silver iodide upwind of mountain barriers rises to heights of approximately 1000' above the top ofthe barrier depending upon atmospheric stability. Therefore, the 700mb temperature, in conjunction with the project's mean barrier height, can be used as an index of whether the silver iodide particles are likely to reach effective temperatures in a given storm. Wind roses for the 700mb level were prepared for all events combined and for individual months. These wind roses are provided in Figures 1 to 8 . These wind roses are plots of the frequency ofthe direction and speed of the 700 mb winds. The direction is reported as that from which the wind is blowing (for example, a 2250 wind would be a wind blowing from the south- west). The results of the analysis are summarized in Table 3. The 700-mb wind during these stonn events strongly favored a southwesterly direction, with a direction between southerly and westerly in approximately 64% of the 166 soundings examined. Wind direction was between 6