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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these two features a pattern of broken \,,"armer-topped clouds existed over much of northern Utah.
A brief drying trend at the midlevels of the atmosphere occurred from 0500 through 1700 on 18
February follo\\'ing the passage of the second upper level short wave. By 1700 on 18 February the
third minor wave was over central i\ievada, and the dew point depressions over Utah had increased
to 6-10" at 700 mb.
The data from RRS for 18 February are shown in Fig. 19. The cooling trend at plateau top
level began as a sharp temperature drop at 0100, with a decrease in wind speed after 0400. This
might have been associated with the initial front/trough passage. SL W did not begin developing over
RRS until about 0600 as winds gradually veered and continued to decrease. A seeding experiment
was conducted from about 1130 to 1445 with silver iodide releases from AHS and HAS, using the
MSC generators. Sulfur hexaflouride was released simultaneously, and a research flight occurred
from 1200 until 1500, to monitor conditions during the seeding experiment.
Cloud conditions near the end of the seeding experiment are sho\\'TI in Fig. 20, a GOES image
at 1400. Several southwest to northeast oriented bands were located over Utah, including one that
apparently had just passed RRS. The peak in SL \V just prior to 1400 (Fig. 19) was likely associated
\\ ith this band. The larger SL W maximum between 1500 and 1700 could have been associated with
bands visible to the \vest ofRRS in Fig. 20. The cloud and 700 mb patterns at 1700 are sho\\<TI in Fig.
21 \",here an enhanced region of colder cloud tops is seen just arriving at RRS. The sharp \'Yind shift
at 1600 \vas apparently the second trough passage noted above, and the drying pattern behind this
trough was reflected by the decrease in SL Wand vapor depth after 1700. Although Fig. 19 shows
much missing vapor data., the depth at 0000 on 19 February was 0.50 cm less than the depth at 1730
on 18 February. In contrast to earlier storm periods the liquid water development on 18 February was
considerably enhanced by convection in a post-trough environment. The cloud bands near the end
of the seeding experiment were particularly convective, with one being described as a squall line by
the research flight scientist (Heimbach, 19941.
The \\ide area of broken cloudiness seen over Nevada and Utah in Fig. 20 passed the Utah
research area during the night of 18 February with another in the series of short waves crossing Utah
early on the morning of 19 February. The temperature \vith the passage ofthis latest wave dropped
an additional 3" , to -13" at 700 mb by 0500. A much more vigorous trough off the California coast

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