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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pattern at 0500 on 27 February. The advection of moisture had begun much earlier in the
southwesterly tlo\\." ahead of the trough. In contrast to much of storm period D, the 700 mb
temperatures preceding this trough were relatively mild at -4 to -50. The trough axis at 0500 was
o\c'er central ),ievada, \"lth the colder clouds in a \vide band centered near the Utah-Nevada border.
Figure 33 shows the RRS data, \Vith the SL W trace indicating that cloud development began
before midnight, and continued throughout the day. For most of the day the \vind was quite constant
from 240c at 6 m s-'. At 1630 the trough passed and the wind shifted quickly to 3200 and decreased
in speed to 3 m s<. \Vithin the first two hours after trough passage the temperature at RRS also
dropped about 3" to - TJ and SLW dropped off markedly. However, by 1900 SLW again increased
to depths exceeding 0.4 mm as the vapor depth remained steady at 0.6 em.
The 700 mb and satellite plots for 1700 on 27 February are shovYTI in Fig. 34. At this time the
trough axis was nearly over RRS as verified in Fig. 33, \vith RRS near the back edge of the main
cloud band. The dew point depression pattern in Fig. 34 also shows that significant moisture
remained upstream of RRS and accounts for the continued development of SL W after the trough
passage. The cloud seeding experiments during this storm period were a silver iodide release using
an MSC generator from AHS bet\veen 0855 and 1305, followed by a propane release from HAS
from 1325 to 1530. As in previous experiments, SF 6 was released simultaneously vvith the silver
iodide and propane. The research aircraft flew two missions: 0750 - 0920: and 1030 - 1420. As on
22 February there was an indication that SLW was minimized over RRS during the time that the
seeding plume \vas most strongly detected by the NCAR counter (Fig. 33). Unlike 22 February, the
temperature zone most likely encountered by the silver iodide on 27 February was not nearly as
conducive to nucleation, based on the CSU cloud chamber results. The apparent depletion of SL W
by seeding will require additional detailed study.
Some SLW remained over the plateau into the early morning hours of 28 February, but
significant drying occurred so that additional experiments were not feasible by daylight on the 28th.
This ended the last storm period of 1994 that produced suitable conditions for cloud seeding
experiments over the \Vasatch Plateau. A few days in March were used for the study of the transport
of radar chaff over the Platea~ but no further seeding experiments v.ith silver iodide or propane were
conducted.

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