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Basin COlorado River the Upper n WOSA THE STORM SYSTEMS that pass through the region in the winter take different paths and have different speeds, but they normally move from west to east. The most frequent storm path is to the north of Colorado. In such storms, the winds may blowout of the west over Colorado for several days. Snow falls on the moun- tain ranges that run from north to south. A second situation oc- curs with well-developed storms that pass directly through Colorado. The low level winds are initially from the south, but later turn to the west and northwest. In general, the snowfall amounts from such storms are large over most of the region. A third type of storm often remains stationary for several days, centered in Nevada or northern Arizona. These storms have winds from the southwest over Colorado. They bring relatively warm moist air over areas such as the San Juan Mountains in southwest Colorado, causing heavy snowfall. Storm Tracks and Weather Patterns In any given storm, snow occurs first in the western part of the region and gradually moves eastward. When snow is falling over the Rockies, the Plains States to the east are normally not experiencing precipitation and may even have clear skies. How- ever, when the low-pressure center passes onto the plains, the storm may regenerate by drawing in warm moist air from the south. This air originally comes from the Gulf of Mexico. A new storm center may form southeast of Colorado that can bring large amounts of snow to the plains. 15 J ~ ~ SNOWPACK, CLOUD-'SEEDlNG, AND THE COLORADO RIVER variations in snowfall for different storms, of the. same storm in different areas. Statistical methods would not be suitable for a full-scale WOSA whose goal was water production because they would halve the amount of water produced. To evaluate full-scale WOSA results, we could learn enough about the processes that affect precipitation so that the variability in precipitation in an area could be explained to some extent by physical principles. Some. aspects of the growth of ice crystals, and the expected snowfall patterns, could be stated as mathematical formulas that include a simplified representation of terrain, seeding nuclei distributions, and meteorological conditions (such as moisture content and wind speed). The expected precipitation could be computed and compared with the actual measured results. Over time, the set of formulas (the model) would be adjusted until it could predict how much snow would fall with and without seed- ing, as determined by the statistical methods mentioned. This very promising approach to the evaluation of an operational program is not yet a proven method. With the im- provements that are coming along in our ability to measure snowfall accurately over a large area, the overall evaluation of a water productionWOSA program will soon be possible. How- ever, even if we can assess the effect of seeding over a large area for a given storm, or the effect of seeding over a smaller area for an entire winter season, we will not be able to say how seed- ing affected a single storm at a specific spot. It will be necessary to calibrate each target area initially, and perhaps periodically, using the statistical procedures described above. 14 I ("Ie "+ 1 ii I "1 . I \