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GOES satellite images from 1700 and 2100 in Fig. 3.17 show the distinct frontal cloud band, where, the DOT site was at the leading edge of the band at 1700 and near the trailing edge by 2100. The greatest cloud liquid was at the leading edge of the band where cloud top temperature was -11.60 C over DOT at 1700. Visual observations documented these clouds as altocumulus. Very little liquid existed in the trailing edge where the cloud top was -39.10 C at 2100. The lack of prefrontal moisture reflected the continental nature of the air mass associated with this frontal feature. Heggli and Rauber (1988) documented similar liquid water characteristics in fast-moving meridional storms affecting the Sierra Nevada of California. Stonn Period 5: 2-3 February 1991. The offshore ridge which dominated the weather pattern for the first two weeks of the field program, finally moved eastward between 1 and 3 February in advance of a fairly deep trough. The trough weakened as it moved onshore, but it was evident at 500 mb along the Utah/Colorado border (Fig. 3.18), and at 700 mb over eastern Colorado (Fig. 3.19), by 0500 on 3 February. The surface pattern was relatively unaffected by this upper level trough. A persistent Great Basin high remained intact as the trough moved across the research area. A cold front moved into California on 2 February, but it was not identified later in the surface analysis (Fig. 3.18) on 3 February. Figures 3.20 and 3.21 show the effects of the trough passage at the DOT site. The wind shifted from southeasterly to southwesterly near midday on 2 February as the trough approached. The wind continued to veer to a more westerly direction as the trough moved through, and finally shifted to the northwest by 0700 on 3 February on the back side of the trough. The pressure trace suggests the trough axis passed the DOT site late on 2 February or early on 3 February. Although minor amounts of liquid water were detected, the dew point depression was 8-100 during the liquid period, suggesting cloud base was considerably above the top of the Plateau. Visual observations from the DOT site noted a thin altocumulus layer early on the morning of 3 February, and clear skies by 1000. The 0600 sounding from Mt. Pleasant shown in Fig. 3.22 reveals the cloud layer was centered at 650 mb (-~ C). A relatively deep inversion layer existed over the valley with dry conditions and light and variable winds. The midlevel winds had already shifted to northwesterly, indicating the trough axis at 600-700 mb had passed by this time. No radar or satellite data existed for this very minor weather event. The clouds consisted primarily of cirrus on 2 February, then thin altostratus and altocumulus late on 2 February and early on 3 February. 3-24 I I I I I I I I I I I I I I I ,I I 'I ,I