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I I I I I I I I I I I I I I I I I I I 16 " \\'as not a significant problem in the series of storms \\'hich passed over the San Juans from 29 to 31 January 1975, on 10 March and on 16-17 Harch. These storms \oJere caused by short \oJaves \vithout significant 10\oJ level cold air advection following the ~torm and/or there was insufficient time for evapo- rative and radiational cooling to form a stable layer in the up\oJind valley. The typical sequence of precipitation rate is ShO\Vl1 at the bottom of Fig. 3.1. The precipitation begins to fall as the cloud base is lowering rapidly. The precipitation rate increases and sometimes has two separate high rates. One of the high rates al\oJays occurs ",hen the instability is greatest over a significant depth. The other high precipitation rate often occurs closer to the front end of the storm when blocking flow is not present and the cloud is very deep but with near neutral stability. The conditions for performing a randomized seeding experiment of v!olf Creek Pass were described in detail by Grant et a1. (1969) and by Hjermstad (1975). In general the conditions \oJere patterned after the results at Climax in that a randomized decision to seed or not seed was made if precipitation \Vas expected to fall on '.Jo1f Creek Pass and if the 500 mb temperature was above -23C. The actual typical sequence for most well organized storms \Vas . that seeding v70uld begin near the front of storms \oJhen blocking f1m\' was present and seeding \vould cease when the 500 mb temperature dropped belmoJ -23C. The 500 mb temperature almost aboJays dropped be1m\' -23C \oJhen the storm \Vas associated with a significant baroclinic zone and/or cutoff low in the upper atmosphere. Since the cold air advection aloft destabilized the atmos- phere, the 500 mb temperature \vould drop below -23C near the time that con- vective instability first developecl. The 500 mb temperature dropped below -23C less often when tllC storm wos associat("d V1ith short Hilves or baroc1 in1c. zones \oJhich pr.Lmari1y pasfjccl \oJc11 to tile north of the San JUClJ1S.