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<br />27 <br /> <br />4.3 <br /> <br />17-18 December 1977 CASE STUDY <br /> <br />. <br />. <br /> <br />Upper winds were fairly zonal in the eastern Pacific and western <br /> <br />U.S. on 17-18 December, but less so than during the 14-15 December <br /> <br />~ <br /> <br />case. A short wave embedded in the flow was just off the California <br /> <br />coast at 0000 GMT on 18 December. At the surface, a cold front passed <br /> <br />the target area around 2115 GMT on 17 December in advance of the upper <br /> <br />wave. Ahead of the front, a wide band of cloudiness covered with <br /> <br />cirrus moved over the target area, giving satellite-observed CTTs around <br /> <br />-44oC at 1545 GMT (Fugure 10). At 1500 GMT, the Sheridan sounding had <br /> <br />approximately a 1300 m separation between upper and lower clouds with <br /> <br />the separation centered near the 57 kPa level. Mean CTT of the lower <br /> <br />deck was -12oC and of the upper deck was -4loC in the target area. A <br /> <br />~ <br /> <br />separation between cloud decks lowered by 1800 GMT as precipitation <br /> <br />began to increase with the approach of the cold front (Figure 11). By <br /> <br />~ <br /> <br />1845, over 8 rom per hour rain rates were occurring at Blue Canyon. Pre- <br /> <br />cipitation rates at Truckee and Donner Summit peaked about an hour later, <br /> <br />just ahead of the front. <br /> <br />The frontal band of cloudiness is seen on the 1945 GMT visible <br /> <br />image, Figure 12. The band itself was moving ~o the southeast with the <br /> <br />cold front. The clouds comprising the band moved from the west-south- <br /> <br />west with the upper winds. Slight troughing off the coast at 20 and 30 <br /> <br />kPa was responsible for the cirrus which covered the frontal band and <br /> <br />flowed in off the Pacific continuously, parallel to the surface front. <br /> <br />High clouds did not clear out with the passing front in contrast <br /> <br />to most days studied. The 2045 GMT IR image (Figure 13) revealed the <br /> <br />~ <br /> <br />extent of cirrus cover behind the front, which was just within the <br /> <br />target area at this time. With frontal passage at 2115 GMT, it was seen <br />