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passage. By 1700 the colder-topped clouds had moved southeast of the research area. Figure 3.13 shows !he time-height profile of equivalent dB~ from the vertically-pointing Ka-band radar at the DOT site. A distinct echo band arrived at DOT at 1310 just prior to the cold front, producing the highest echo tops of the storm. Satellite cloud top heights are annotated on Fig. 3.13 at 1300 and 1700, and agree well with the Ka-band profile which shows decreasing echo tops following frontal passage. Storm Period 4: 28-29 January 1991. Between Storm Periods 3 and 4 the long wave ridge off the West Coast amplified, formed a closed high at 500 mb over the Gulf of Alaska by 26 January and allowed a weak southern branch of westerlies to form across New Mexico and Arizona. Utah and the Utah/NOAA research area remained under the influence of the dry northerly winds on the east side of the main ridge axis. High surface pressure over the Great Basin persisted for the lengthy period between storms. By 0500 on 28 January a vigorous short wave was moving rapidly southward along the east side of the offshore ridge. At 500 mb the short wave axis was centered over Oregon. At the surface a major cold front was pushing southward out of Canada, having moved into northern Wyoming by 0500. At 700 mb Utah was in northwesterly flow with moisture associated with the short wave still far to the north. The NWS surface chart for 1700 on 28 January showed the cold front to be just south of Salt Lake City. The 700 mb trough axis (Fig. 3.14) was oriented northeast-southwest and positioned across the northwestern corner of Utah. A small area of moisture associated with the trough was beginning to be advected into northern Utah. Winds at all upper levels over Utah had begun to veer to more westerly directions with the approach of the trough. Following the 700 and 500 mb trough passages Utah returned to dry northerly flow. Figure 3.15 clearly shows the passage of the cold front across the DOT site. Sho~y after 1800 the temperature dropped nearly 50, the wind shifted from 2700 to 3300, and the pressure abruptly started to rise after minimizing at 1800. The dew point depression shows an absence of low level prefrontal moisture, with saturation being reached just as the front arrived. The dry prefrontal conditions are also reflected in the radiometer data which showed cloud liquid existed only an hour prior to and two hours subsequent to frontal passage. Dry, cold air was advected into the area immediately behind the trough, keeping this liquid/precipitation event quite short-lived. Figure 3.16 displays data for 29 January where drying continued, no significant liquid water was observed, and a very minor amount of precipitation was noted early in the period. 3-18 I I I I I I I I I I I I I I I I I I I