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Show of 190-240° from the Park Range. However, when warm frontal over-running snow does occur, the rate can be very heavy due to the high water content of the relatively warm southwesterly flow aloft. , Target area precipitation rates very widely within the same precipitation event for the reasons discussed above and also due to release of conditional instability by orographic lifting. Figure 67 shows the relationship between precipitation rate and the number of hours of precipitation occurrence. It indicates something less than 100 hours of precipitation rate of> 0. 10 in 1 ·i ;i h r -1 . About 50% ·of the winter precipitation occurs from 200 of the 1000 hours of precipitation. Almost all of these 200 hours of relatively high precipitation rate occur with Buffalo Pass temperature readings > -SC (at 10, 300 ft. MSL). The curve of cumulative percent of total precipitation versus total precipitation occurrence time is shown in Figure 68. Rapidly moving dynamic waves frequently produce strong time-rates-ofchange of upper level temperature, and Figure 69 shows a temperature · range which can occur within a 12 hour period during the passage of a sharp, rapidly moving upper trough, having strong advection of exceptionally warm air ahead of it followed by strong advection of cold air behind the surface front. Extremely cold 700 mb air ( < - l 9C) is quite rare over the target area, and the most frequent range of 700 mb temperature during precipitation is from -4C to -13C. A derived climatology of 12-hour temperature changes at 700 mb and 500 mb is included in Section 5. Mt. Harris soundings indicate that most precipitation-p roducing air masses possess some layers which reach or slightly exceed the moist adiabatic lapse rate. Thus, there is usually a convective precipitation component, which is reflected in the unsteady precipitation rates observed. Convective band passages at 2. 5 to 4 hour intervals are quite commonly observed. " 3. 3. 3 Preferred Radar Echo Patterns and Mt. Harris Wind Anomalies From a large number of upper wind measurements at Mt. Harris it was found that, for large scale northwesterly flow aloft, the Mt. Harris obse·rved wind up to at least 14, 000 ft~ MSL is more westerly than would be predicted. This phenomenon is related to the east-west oriented Elkhead Mountain Range located approximately 25 miles northwest through 18 miles north from Mt. Harris (and reaching 11, 000 ft. MSL elevation). As air crosses . this mountain range from the north or northwest, a lee side trough of low r pressure forms, with a resultant backing of winds on ±he downwind side of the trough. Mt. Harris PPI radar photographs during conditions of large scale northwesterly flow aloft frequently reveal a persistent east-w:est oriented line echo lying just north of Mt. Harris and e xtending eastward. 109 |