WMOD00301

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

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Table 6. Precipitation Necessary at the end of September 1990, to Bring Palmer Drought Severity
Index Near Normal

Precipitation needed to reduce
PDSI to near normal in
Division 1 month 2 months 3 months 6 months 1 year
Western 4.92 5.11 5.33 6.96 11.16
Dixie 5.82 6.69 7.43 11.04 14.90
No. Central 10.06 10.49 11.49 14.98 21.45
So. Central 7.23 7.65 8.22 10.97 16.23
No. Mountains 5.28 6.57 8.47 13.76 20.42
Uinta Basin 4.48 4.42 4.79 5.70 10.01
Southeast 5.58 5.54 5.85 7.03 11.29

Normal precipitation (1951-1980) for the Basin is 7.83 inches and the 12-month requirement to
return the Basin to normal is 10.01 inches. The amount of precipitation necessary to fill the drought
requirement is the difference, or 2.18 inches for the year. This can be compared to the deficit of
1.86 inches during the recent drought in 1977, the 2.61 inch deficit in 1935 and the 2.90 inch deficit
in 1900.

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Figures 9 and 10 show the areal extent of drought in the Western United States. It is obvious that
drought conditions are wide-spread and reflect the large-scale weather pattern that has dominated
the West for the past two years.

In the Uinta Basin, drought conditions are forestalled by two factors. The first is the amount of
snow received in the mountains during winter months and available as snowmelt runoff during the
Spring and early Summer months. In this particular drought, even the mountains are suffering from
severe to extreme drought conditions, as are other areas in the State.

The second is the extensive reservoir and water distribution system in the Basin that collect water
during periods of high flow and, through a complex management system, augment low flows.
Through these processes the severity of drought is mitigated for at least a year, and with careful
management, additional years. The PDSI considers only meteorological conditions and not reservoir
or management decisions.

Orographic Precipitation Model Verification Study

One objective of this study is to show the effect the topography of the Uinta Basin on storm systems
that approach the Basin from several different directions. In addition to empirical studies of storm
systems, the orographic precipitation model developed by J. Owen Rhea (1978) was used to
determine precipitation patterns from various wind trajectories. The resulting precipitation patterns

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