WMOD00301

• Title: Precipitation Pattern Analysis - Uinta Baisn - Wasatch Front
• Date: 10/1/1990
• State: UT
• Weather Modification - Doc Type: Report

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Orographic Precipitation Model Results

The Rhea model, using only the orographic component, was calibrated using upper air data and
precipitation amounts from winter and spring storms of 1984. During this very wet period, the level
of the Great Salt Lake was rising and all forms of weather modification in the Utah mountains had
ceased. As far as we could determine, the storms were uncontaminated by man's intervention.

To show the effects of topography, various wind directions and resulting precipitation patterns, all
parameters of the calibrated model were held constant and the model wind component was varied.
This study produced 36 precipitation patterns for successive 10 degree increments. Orographic
precipitation patterns for wind trajectories from the northwest and southwest are shown in Figure
12. Resulting calculated 12-hour precipitation amounts for several stations are also included in Table
7 for quantitative comparison. Patterns for the north, northeast, east, southeast and south are shown
in Appendix B. An overlay with State and County boundaries is attached to the back cover so that
precipitation patterns in Appendix B may be properly oriented.

Evaluation of the precipitation patterns and the point precipitation amounts verify that the Uinta
Basin receives most of its topography related precipitation from storms from the southwest through
southeast. Evaluations also verify that storms from other directions, particularly the west and
northwest, result in little precipitation in the Basin.

Summary and Conclusions

There has been concern that winter precipitation augmentation projects along the Wasatch Front
have depleted the amount of precipitation in the Uinta Basin. In agreement with the Division of
Water Resources, State of Utah, the climatology of winter storms in the Uinta Basin was evaluated.
Several winter storm types were studied. It was shown that winter and spring storms that produce
precipitation in the Uinta Basin and on the southern slopes of the Uinta Mountains have southerly
wind trajectories. Precipitation along the Wasatch Front is generally produced by storms w~~h a
westerly or northwesterly wind flow pattern. It was also shown that large-scale atmospheric
conditions are inter-related with wet and dry periods and are very significant in determining wet and
dry periods in the Western United States.

Correlations of precipitation stations and snow courses in the Uinta Basin and in other areas of Utah
showed that when stations in southern Utah received precipitation, the Uinta Basin was also likely
to receive precipitation. Storms from the southwest through southeast are therefore likely to
produce precipitation for the Basin. There was no correlation between the stations along the
Wasatch Front and those in the Uinta Basin. Storms from the west or northwest are not as likely
to produce precipitation for the Basin.

The lifting caused by mountains and subsequent subsidence on the lee side is responsible for
precipitation distribution patterns. The west through northwest storm trajectories favor precipitation
along the Wasatch Front. Southerly storm trajectories favor precipitation in the Uinta Mountains.

Cyclonic, large-scale storms provide the lifting mechanism that produces general precipitation
regardless of terrain features. These storms tend to diminish in intensity as they move from the
north and northwest toward the southeast. Storms which move from the southwest and south toward
the northeast tend to intensify.

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