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<br />002489 <br /> <br />mixture of characteristics depending upon details of the storm and <br />the terrain. Analyses of winds and temperature indicate that <br />approximately 65 percent of storms may be characterized as "southerly" <br />or "southwesterly" and 25 percent as "westerly." It is estimated <br />that approximately 75 percent of the precipitation results from <br />southerlies and approximately 20 percent from westerlies along the <br />windward slope. There are uncertainties associated with these <br />estimates; however, the findings of both PG&E in the Almanor basin <br />and the CENSARE support these characteristics. <br /> <br />Due to the orientation of the orographic barrier, southerly storms <br />will produce only weak to moderate orographic clouds with vertical <br />velocity generally less than 20 cm/s and correspondingly low cloud <br />liquid water content. The orographic cloud will be largely confined <br />to the windward slope with little potential for carryover precipi- <br />tation in this situation. MBA said that the lifting mechanism in <br />the low- to mid-barrier altitudes should be overwhelmingly convective <br />with vertical air'motion 3 to 8 m/s locally. Cell motions along the <br />barrier systematically reduce the available convective energy so <br />that the depth and intensity of convection decrease beyond the lower <br />slopes. Precipitation rates decrease accordingly, the altitude being <br />dependent on particle trajectories. In this respect, southwest <br />wind cases should behave similarly to the southerly cases, although <br />maximum convection and precipitation will occur at higher elevations. <br /> <br />Over-the-barrier conditions are likely to consist of remnants of <br />convection from the windward slope. Precipitation statistics in most <br />leeward locations support the lack of carryover precipitation. <br /> <br />In the case of the westerly storms, MBA said a strong orographic <br />cloud will develop and persist. Vertical motion of the order of 0.5 to <br />1.0 m/s an~ liquid water content are likely to persist in excess <br />of 0.2 g/m. Typically, convection is expected to be less intense <br />and more shallow than the southerly storm case; however, it may <br />be more continuous in nature with more intense periods during band <br />passages. Barrier-induced convergence and resulting cyclonic <br />curvature will maintain a large incidence angle between the barrier <br />and cell motion. Carryover basins will experience light precipita- <br />tion rates of moderate duration, since most of the convective <br />activity is not likely to survive the ascent up the windward slope. <br /> <br />The warm southerly storms are strongly convective with high liquid <br />water content. They will respond most favorably to high seeding <br />rates over low to mid altitudes. MBA reported that the net effect <br />will be redistribution of snowfall to higher elevations, thus <br />increasing the permanent snowpack. Changes in storm efficiency will <br />be relatively small and may be positive or negative. Additional <br />convective development due to seeding may be possible under some <br />conditi ons. <br /> <br />14 <br />