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<br />19 <br /> <br />~ <br /> <br />4.2 14-15 December 1977 CASE STUDY <br /> <br />A large 50 kPa trough in the Gulf of Alaska with strong westerly <br /> <br />flow into the California coast above 70 kPa existed on 14-15 December. <br /> <br />20 and 30 kPa flows were fairly zonal from the central Pacific into the <br /> <br />Sierra during this storm, with a strong jet axis just north of the <br /> <br />latitude of the target area. A vigorous short wave trough at middle <br /> <br />levels, emanating from the Aleutian low, passed over northern California <br /> <br /> <br />after 1200 GMT on 15 December. Figure 6 ts g c~P9~tte of th~ 59 and <br /> <br /> <br />70 kPa observations for 1200 GMT, 15 December showing the strong wester- <br /> <br /> <br />ly flow into the Sierra near the time of frontal passage. <br /> <br />Features of the 14-15 December storm worth noting are many in <br /> <br />number. In comparison to many winter Sierran storms this one was warm, <br /> <br />with rain falling at elevations up to 2400 m early in the storm. Pre- <br /> <br />cipitation at Blue Canyon is strongly related to cloud top temperature <br /> <br />in the target area. The major precipitation event is forced by in- <br /> <br /> <br />creased low-level upslope winds 6 hours in advance of the front, during <br /> <br /> <br />a period of clearing high clouds. Another sharp drop in cloud height <br /> <br /> <br />follows the frontal passage, typical of most of these storms. An <br /> <br /> <br />isolated orographic cloud remained over the Sierra behind the front. <br /> <br />It lasted for several hours, producing showery precipitation due to its <br /> <br />convective nature. At this time, CTTs ranged from -15 to -20oC. Moore <br /> <br />(1978) gives a more complete description of the synoptics and micro- <br /> <br /> <br />physics of this storm. <br /> <br /> <br />In the pre-frontal environment on 14 December, multi-layered <br /> <br /> <br />clouds were evident from project soundings up to 1800 GMT and were also <br /> <br />observed by University of Wyoming aircraft flights. By 2100, no cloud <br /> <br />layering was seen on the soundings and satellite observed CTT had fallen <br /> <br />. <br />. <br /> <br />~ <br /> <br />,- <br /> <br />. <br /> <br />i <br />