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Storm Period 6: 12-14 February 1991. After the minor event on 2-3 February, the high pressure ridge redeveloped over the western United States. The ridge axis was typically over California or Nevada. The main energy of the westerlies was over Canada. Storms approaching the West Coast between 3 and 12 February, either dissipated before moving onshore or split offshore with the main energy moving northward into Canada. An occasional weak southern branch of westerlies provided little storm potential for the Utah research area. Between 8 and 12 February the long wave ridge increased greatly in amplitude, and eventually allowed a trough to cut underneath it and bring a moderate band of westerlies across the northcentral United States by 12 February. Several rapidly moving short wave troughs moved across the United States during Period 6. The first two short waves propagated across Utah on 12 February, but brought little moisture or cloudiness over the research area. Surface pressure over the Great Basin remained high during the passage of these initial upper level troughs. The West Coast ridge redeveloped behind the first two short waves, but a third short wave with significant moisture and an associated surface trough moved over the ridge and began to move into northwestern Utah by 1700 on 12 February. Figures 3.23 and 3.24 show the surface, 500 and 700 mb charts for 0500 on 13 February. Winds over Utah were westerly at 1700 12 February just ahead of the shortwave, but turned to northwesterly as the trough axis passed at 0500 on 13 February. The somewha~ flattened ridge allowed moist air, with a trajecto- ry from the mid-Pacific ("mid-" refers to midlatitude regions of the northern Pacific Ocean), to continue to advect into Utah following the trough passage. This moist advection pattern (primarily below 500 mb) in northwesterly flow continued well into 14 February. Figures 3.25-3.27 show the progression of events from the DOT site instrumentation. On 12 February the temperature increased, then remained near 00 for the remainder of the day. The wind shifted to the southwest and then gradually veered and increased in speed. Liquid water from midlevel clouds began to be detected at about 2100. The main weather event occurred on 13 February, actually behind the short wave trough as pressure began increasing after about 0500. The increased moist advection behind the trough is obvious in the decreasing dew point depression in Fig. 3.26. An 8-10 h period of liquid water greater than 0.2 mm bracketed a seven hour precipitation episode where rates were generally less than 1 mm h-l. The cloud system was relatively shallow as evidenced by the radar plot in Fig. 3.28 and the cloud top temperatures shown on the top panel of Fig. 3.26. The cloud feature that produced the precipitation is shown in Fig. 3.29, near the start of precipitation at 1000 and near the end of precipitation at 1700. The region of colder cloud tops centered over 3-30 I I I I ,I I I I I I I I I I I I I I I