WMOD00315

• Title: Investigations of Winter Storms Over the Wasatch Plateau During the 1991 Utah/NOAA Field Program
• Prepared For: Utah Department of Natural Resources Division of Water Resources
• Prepared By: Arlen W. Huggins, Melanie A. Wetzel, Patricia A. Walsh
• Date: 10/1/1992
• State: UT
• Weather Modification - Doc Type: Report

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time. The consistent but rather low depths of liquid water, and the absence of precipitation
suggests Period 2 was affected by an altocumulus or altostratus layer. Visual observations from
DOT at 1000 and 1733 on 17 January confrrmed that a uniform, low overcast existed, with
cloud bases a few hundred feet above the site and ice crystals occasionally falling to the surface.
The surface pressure was very steady until about 1200 on the 18th, when it began to fall
as the surface high was beginning to be displaced eastward. Clear conditions over the DOT site
were noted at 1330 on 18 January. Satellite images and Mt. Pleasant soundings were not
available for Period 2.

Stonn Period 3: 19 January 1991. This storm period was marked by the passage of
a vigorous short wave and surface cold front. At 0500 on the 19th the 500 mb trough axis was
oriented northeast-southwest and positioned across central Idaho and Nevada. At 700 mb a
closed low was beginning to form over central California and the axis of a trailing short wave
was moving southeastward through central Idaho (Fig. 3.9). Utah was on the leading edge of
the short wave where winds backed from northwesterly to west-northwesterly as the wave
approached. Midlevel moisture was also being advected over the research area by 0500.
Although not well analyzed on the NWS surface charts for 19 January, there was strong
evidence of a surface cold frontal passage in the data from the DOT site shown in Fig. 3.10.
At 1330 there was a 30 temperature decrease; an abrupt wind shift (from 2400 to 32(0), and a
sharp rise in surface pressure. Cloud liquid water was observed almost entirely prefrontally and
precipitation was measured only for the four hours preceeding and during frontal passage.
A prefrontal sounding taken from Mt. Pleasant at 1000 (Fig. 3.11) shows moderate
westerly Plateau-level winds with cloud base near the height of the top of the Plateau (about 740
mb). Figure 3.10 shows that saturation near the surface was only reached for a few hours just
prior to frontal passage, suggesting cloud base was slightly above the DOT site for most of the
storm. Figure 3.11 also depicts a very stable environment from the surface to 700 mb, with a
weak northeasterly wind at the surface. Such conditions can adversely affect the transport of
seeding material from valley-based generators to clouds over the Plateau. This prefrontal
situation has been documented on numerous occasions, both in the Wasatch Plateau region
(Super and Huggins, 1992b) and near the Tushar Mtns. of southern Utah (Long et ai., 1990).
The postfrontal sounding at 1400 was unstable from 2400 to 3000 m with west-northwesterly
winds at all levels.
Clouds over the research area are shown in the GOES infrared images at 1300 and 1700
(Fig. 3.12). The colder cloud tops were over the area at 1300, near the time of the frontal

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