WMOD00591

• Title: Investigations of Winter Storms over the Wasatch Plateau during the 1994 NOAA-Utah Field Research Program - Final Report
• Date: 7/1/1996
• State: UT
• Weather Modification - Doc Type: Report

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1100. At 1100 there was also a sharp ,,'eering in the \vind at RRS accompanied by a dramatic increase
in relati\'e humidity and the start of a three hour increase in wind speed. These observations coincide
\\ith the arri\'al of the distinct cloud band seen in Fig. 12. The gradual increase in SLW depth after
1100 seems to reflect the obvious increase in vapor flux over the plateau. The beginning of an AgI
seeding experiment was well timed \\ith the beginning of this storm over the plateau, although the
slight \\'arming trend through the day was not panicularly good for the initiation of ice v.lth the
mixture of AgI in use during the 1994 experiments (threshold nucleation temperature about -60).
Seeding took place with the two DRI generators at AHS and HA.S. with the simultaneous
release of SF6 irom both locations. The seeding period is shown in Fig. 16. The NCAR counts
indicate that the AgI plume affected RRS from shortly after the start until shortly after the end of
seeding. The temperature at RRS was about -6"vo'ith the \vind very consistently from 2400 at speeds
fluctuating around 9 m s.:. A research flight was conducted from about 1412 to 1650. From the
standpoint of SL \V de\'elopment and seeding potential, it appears as though the best portion of this
storm occurred between about 1800 and 2300, when liquid depths were generally greater than 0.4
mm.

The RRS radiometer detected SL W through the early morning of 11 February (Fig. 17). with
a broad maximum occurring in the immediate prefrontal/trough period. Wind speed was also
maximized at 12-15 m S-i during this period from about 0200 to 0600. The trough passage at
mountain top level is clearly seen in Fig. 17 at about 0645, where the v.;nd shifted from 240 Oto 300 ~
\\ind speed dropped 3-6 m s-:, and the temperature dropped about 30. In the posttrough period SLW
dropped quickly below 0.2 mm, with very little observed after 0800. The cooling and drying trend
after trough passage is quite evident in the temperature and vapor trace of Fig. 17.

D. 18 - 22 February 1994 Storm Period
Following an extended dry period after storm period C the western U.S. came under the
influence of a broad upper level trough. There was a surface cold frontal passage early in the period.
but the majority of the active weather and clouds were produced by minor short waves moving
through the long wave trough after 18 February. The period was also generally cold "'o';th 700 mb
temperatures being - 10" or colder after the main trough settled over the Great Basin on 18 February.

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