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I I I I I I I I I I I I I I I I I I I period, but Figs. 3.3-3.5 show the data that were available from the DOT high altitude site (see Fig. 2.1). Precipitation occurred from the beginning of the period through 2200 on 15 January. The radiometer also detected significant depths of liquid water from the beginning of its data collection through the end of 15 January. The pressure trace from the Mt. Pleasant Airport suggests that a surface trough, accompanied by a precipitation maximum, may have crossed the area as early as 1200 on 15 January, although the minimum pressure was observed at about 1530. The greatest liquid depths and nearly all of the precipitation occurred prior to, or during the passage of the short wave trough. No precipitation, and only very minor amounts of cloud liquid were observed on 16 January as the upper level ridge became re-established over the West Coast and surface pressure again increased over the Basin. Soundings from Mt. Pleasant were not yet being taken and satellite images were not being archived, so the local atmospheric conditions and cloud cover were not extensively documented. Visual observations from the DOT site indicated that cloud base was slightly above DOT (2698 m) at 1030 on 14 January. Rimed snow particles and periodic occurrences of rimed stellars and other cold habits were observed through 1200 on 15 February. After 1200 riming decreased, as did snowfall and breaks in the sky were noted by 1600. Snow showers with rimed particles then occurred after 1600, likely from weak convection following the trough passage. Mostly clear conditions were noted after 1000 on 16 January. Stonn Period 2: 17-18 January 1991. There was no pronounced synoptic feature associated with this storm period. The 500 mb ridge centered over the West Coast amplified on 17 January and high pressure at the surface continued over the Great Basin. On 18 January the ridge was beginning to be noticeably weakened by a short wave trough, and the surface high was starting to be pushed to the east. The cloudy episode comprising Period 2 was caused by a region of moisture centered at 700 mb being advected over the top of the ridge and into Utah from the northwest. Figure 3.6 shows the 700 mb pattern for 0500 on 17 January with a 25 kt northerly wind at Salt Lake City and a region of dew point depressions less than 30 covering much of Idaho, Nevada and Utah. There was no evidence of height falls to suggest a minor short wave accompanied this moisture. Data from the DOT site are shown in Figs. 3.7 and 3.8. The temperature was generally colder than _50 C and winds were generally less than 3 m S-l. During the lengthy period of liquid water winds were northwesterly to northerly and 2-3 m S.l. The temperature and dew point traces indicate that the air near the surface at DOT remained unsaturated even when the radiometer detected liquid water, suggesting that cloud base was above the Plateau most of the 3-7