WMOD00301

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

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temperatures, ocean currents, winds and atmospheric blocking patterns are only a few of the features
that interact with weather and climate in Utah. These features provide guidance for storm systems
that enter the State and control the speed and general makeup of the system.

Storm Classification

We evaluated methods of classifying storm types by the California Institute of Technology (1943);
Williams and Peck (1962); the Western Region, National Weather Service (Rasch and MacDonald,
1975); and the Office of the State Climatologist, State of Colorado (1990).

Weather types shown in Elliott (1951) were developed after the California Institute of Technology
system. They portray ten different weather types that affect Utah and neighboring states. Type Bn-
c, which is included in our large-scale low pressure pattern shown in Figure 6, produces much below
average temperatures and copious precipitation amounts through Utah and Colorado. Other storm
types shown produce little to no precipitation for the Utah area, though their effect on temperature
is varied.

To evaluate terrain influences in Utah, Williams and Peck (1962) classified storms in the Utah area
as those having large-scale vertical motion ("cold lows"), cold frontal storms, storms with warm fronts
over-running, and all other storms. They found that precipitation produced in storms with large-scale
vertical motion has relatively little dependence on topographic lifting as compared to other storm
types. The Williams and Peck study addressed only the Wasatch Front area because it is orthogonal
to many general storm paths during the winter months. Storms classified by this system are included
in the classifications in our study.

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The State Climatologist for Colorado classified storms into seven types for the Western United
States. These storm types fit well within the classification techniques noted above.

The Rasch and MacDonald technique was developed to increase ability to forecast probabilities of
precipitation. This system classifies storms according to season and storm type. Ten storm types
were classified for the winter and ten were classified for the spring season.

Using data provided in the Rasch and MacDonald study, we classified storms into four major types.
The first type is a dry classification, placing the mean ridge position over the intermountain west as
shown in Figure 7. The other three classifications fit within the large-scale low pressure pattern
shown in Figure 6. These three general patterns are: 1) west through north flow; 2) southwest flow;
and 3) southeast flow.

Using precipitation data and upper level flow patterns shown in Rasch and MacDonald (1975), there
are 113 winter and 119 spring patterns that fit into the dry classification. Less than two percent of
these 232 patterns produced precipitation. One storm produced precipitation in the Uinta Basin and
4 of the storms produced precipitation along the Wasatch Front.

For the west through north flow, 766 winter patterns and 247 spring patterns were evaluated. The.
combined 1,013 patterns produced precipitation only 58 times (6 percent) in the Uinta Basin. The
Wasatch Front received precipitation 173 times (17 percent) from the storm patterns.

The southwest flow pattern, which included cut-off low pressure systems and split flow patterns,
occurred in 440 winter patterns and in 756 spring patterns. The 1,196 patterns produced

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