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28/23 to 29/03: In the next period E~mbedded convection forms while cirrus persists even though the cloud top height decreases and hence, the cloud top temperature increases. The atmosphere destabilizes because of differential advection of ee (aee/at < 0 aloft and aee/at > 0 below). During this period the level above which the winds are from 1600 through 2800 lowers rapidly while the cloud base typically lowers to the ground from evaporating precipitation particles. If the unblocked wind level lowers below 2.5 km, it is probably possible to over- seed this period of the storm. This is bl~cause the cirrus above the convection already more than adequately seeds the cmlvective clouds. In this particular storm the 500 mb temperature went below seeding specifications at 28/23. The preliminary data suggests that convection embedded in high cirrus typically con- tains ice crystal concentrations in excess of 100/9-. 29/03 to 29/12: The next and last pE~riod in the storm sequence prior to dissipation is quite typical. The clouds are predominantly convective and are capped by an inversion which continues to lower as the synoptic trough passes. When the inversion lowers to mountain top level (3.5 krn) the airflow is blocked and the clouds dissipate. As the level of free convection lowers the air is drawn from ever lower levels until finally, the air and AgI is definitely flushed up and over the mountain. During this period the AgI is probably distributed throughout the cloud by the free convection. Based on observations in real time over the San Juans and in Wyoming we have the strong ~npression that convective clouds which are not embedded in cirrus have substantial supercooled water in them and typical ice crystal concentrations of 1 to 10/Q,. If these impressions are confirmed by detailed analysis, they have a significant implication. This period of the storm has a high seeding potential despite the cold 500 mb temperatures. V. Swmnary and Seedability Implications Convection, if present in an orographic cloud, serves to concentrate the vertical velocities within the updra.ft cores. In so doing it transports AgI under seeding conditions from the PBL upwa.rds and distributes it throughout the cloud. If the convection extends upward into an overlying cloud containing high concentra- tions of natural ice crystals, then the subsiding motion around the embedded convection transports the cirrus crystals downward. When seeding an orographic cloud containing embedded convectioTL that extends into a cloud of cirrus crystals, overseeding is quite likely. Without convection the seeding material will remain in the PBL. If the horizontal pressure gradient is sufficient, then the PBL will move up and over the mountain barrier and only the shallow layer near the ground will be seeded. Much of the time the flow below the mountain is blocked on the San Juans and the seeding material does not sE!ed the clouds until sometime later when either the pressure gradient becomes suffieient to move the seeded layer over the mountain and/or the convection transports the material upward and over the mountain. The convection serves several other roles, mainly of a cloud physics nature. The condensation rate is increased by the increased vertical velocities and also the location of the condensation is changed. For a stratiform cloud the maximum condensation rate is located just upwind of the mountain crest, while with con- vection the location is displaced several kilometers upstream, most normally near the base of the mountain. The condensation rates and locations quite likely modify the precipitation process with riming and graupel normally associated with con- vection. 25