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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Utah to the northwest, brought dry air into the area and ended this second storm period.
Each of the two upper level troughs had an accompanying surface cold front. By 1700 MST
on 7 February one surface low was located in northern California with a cold front arcing
southeastward into Kevada then back southwest\vard through southern California and offshore. The
trough to the north began pushing an east-west oriented cold front southward, so that it was
positioned across southern OreQ"on and Idaho bv 1700 on 7 Februarv. Bv 0500 on 8 Februarv the
- . - .. ..

southern system had developed a low over central ),jevada with its cold front crossing the southwest
comer of Ctah_ and extending southward along the western border of Arizona. The northern cold
front stalled but an elongated surtace low developed across northern Utah and Colorado, merging
with the low over ~eYada. This complex low pressure area moved east\vard directly across Utah, so
that by 1700 on 8 February it \vas on the Utah-Colorado border. A cold front from the southern low
pressure area may have passed the research area about midmorning on 8 February.
The seeding experiment times and the evolution of the storm over the Wasatch Plateau are
sho\\n in the data from RRS plotted in Figs. 9 and 10. The arrival of moist air is seen in the rapid
increase in both vapor and liquid depth beginning at 0500 on 7 February. Over the first 10 hours of
the storm liquid depths often exceeded 1 mm. This occurred during a period of cloud seeding \vith
AgI from a set of four valley-based generators. Icing at RRS was consistent and vvinds at mountain
top level were 230 - 2400 and increased in speed to about 9 m s-: by 1200. The cloud field during one
research aircraft night from 1045 to 1410 is shown in Fig. 11, indicating the research area remained
under a region of relatively warm-topped clouds with a colder cloud shield located to the south. This
indicates that the early part of the storm lacked a seeder cloud system, and likely accounts for the
high SL \V amounts in the feeder cloud.
A slight suggestion of the first short wave passage is seen in Fig. 9 at about 2200 when the
\\ ind speed increased markedly, the temperature dropped about 2 ~ and "vind direction veered briefly
to about 2600. This was preceded by a well defined maximum in SL W depth and the highest icing
rates of the storm. Supercooled liquid \\c'ater depths were periodically high throughout the night of
7 February and into the early morning of8 February. The periodicity noted in Figs. 9 and 10 appears
to be on a scale of 2-3 hours between maxima, and suggests persistent mesoscale influences. The
,,\ind pattern. particularly speed, on 8 February also showed relatively large fluctuations, indicative

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