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<br />21 <br /> <br />o <br />to -40 C in the target (Figure 7). Blue Canyon precipitation increased <br /> <br />after 2100 with the decreasing CTT, yet remained marginal at Donner <br /> <br />. <br />. <br /> <br />Summit (Figure 8). As a slight break in high cloudiness passed over <br /> <br />~ <br /> <br />the region, the increased CTTs were accompanied by a decrease in Blue <br /> <br />Canyon precipitation totals at 0045 GMT, 15 December. CTT again de- <br /> <br />creased as cloudiness associated with the lifting ahead of the trough <br /> <br />pa~sed over the target. A secondary maximum of precipitation occurred <br /> <br />at Blue Canyon, centered around 0315 GMT under the coldest clouds of <br /> <br />the entire storm period. It is difficult to determine whether layered <br /> <br />cloudiness was present from the 0300 Sheridan sounding, on which the <br /> <br />relative humidity decreased smoothly with increasing height. The good <br /> <br />relationship between observed CTT and Blue Canyon precipitation after <br /> <br />0300 GMT would indicate that a microphysical connection between the <br /> <br />.., <br /> <br />cirrus and lower clouds was taking place, inferring natural seeding. <br /> <br />Heaviest precipitation fell at around 0600 GMT on 15 December at <br /> <br />both Blue Canyon and Donner Summit (Figure 8). At that time, a strong <br /> <br />low-level jet appeared at 85 kPa on the Sheridan sounding, from 2250 at <br />-1 <br />26 ms . The strong orographic component to the low-level winds combined <br /> <br />with abundant moisture to produce the observed precipitation. CTT in- <br /> <br />000 <br />creased during this time from -40 C to -30 C, warming to -25 C by 0745. <br /> <br />Figure 9 is the 0445 GMT IR image showing the cold, high clouds moving <br /> <br />off to the east of the Sierra, with an extensive orographic cloud <br /> <br />remaining behind. <br /> <br />With the approaching cold front, precipitation increased moderately <br /> <br />at Donner Pass and significantly at Blue Canyon, up to 1145 GMT. After <br /> <br />I <br /> <br />frontal passage between 1215 and 1245 GMT, cloud heights and precipita- <br /> <br />tion rates decreased sharply due to subsidence behind the front. A <br /> <br />. <br />