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<br />cumulative result of repeating thunderstorm rainfall at lower elevations on the <br />plains. Thus the 1995 runoff featured a record snow pack and record late snow <br />melt during periods of weather patterns which were somewhat similar to the <br />weather pattern of June 1965 which was used in the general storm PMP <br />calculations. <br /> <br />Record snow melt floods in the mountains and plains flash flooding similar <br />in magnitude to the 1965 flash floods did not oc:cur in 1995. The reason <br />appeared to be due to the runoff moderating impact of the very cool <br />temperatures and persistent cloud cover in the mountain areas associated with <br />the general weather pattern. Several times when brief warm-ups occurred and <br />the runoff began to accelerate, general storm events with rapid cooling occurred <br />and slowed the runoff. On the plains numerous small flash flooding events <br />occurred but all at elevations below 6,000 feet msl. Higher elevations were too <br />cold to support thunderstorm formation and development. <br /> <br />HMS concludes that the coincident occurrence of an extreme snow melt <br />event with a general storm PMP event does not appear to be a reasonable <br />consideration given the sequence of events during Spring 1995. The <br />24-hour lag response of the runoff after two to three days of abnormal warmth <br />could occur easily but the occurrence of a general storm PMP would act to <br />quickly cool down high altitude temperatums and rapidly slow the runoff down. <br />Thus the simultaneous occurrence of an extmme snow melt and \~eneral storm <br />PMP is not likely. <br /> <br />The occurrence of a local storm PMP and an extreme snow melt is a trickier <br />question since the local storm PMP would require only a short 6-hour period of <br />cloud cover over the basin to occur. It is possible that an extreme snow melt <br />could be produced by a 2 to 4 day period of record hot, dry weather followed by <br />the arrival of a weather disturbance capable of producing a local storm PMP <br />event. A local storm PMP event could occur during the ensuing 24..hour lag <br />period response time of an extreme snow melt runoff to the cooler, <br />cloudier weather accompanying the local storm PMP. Thus HMS would <br />recommend that BV consider calculating the effects of the IOI:al storm PMP <br />event during an extreme snow melt condition. <br /> <br />45 <br />