BOARD00961

• Board Meeting Date: 9/22/2003
• Description: Flood Section - Probable Maximum Precipitation Site-Specific Study for the Cherry Creek Reservoir-Study Findings and Recommendations
• Board Meetings - Doc Type: Memo

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Figure 2 depicts the results of evaluating the relationship of cloud \vind fields to
the resulting stoml rainfall pattern in Colorado's most extreme precipitation events. The
black arrow represents the winds steering the motion of the storm cloud while the red
arrow represents the direction of most sub-cloud air feeding the stoml's development.
The resulting rainfall pattern is depicted with green ellipses. This pattern shows the
meteorologically consistent and physically possible scenario.

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Figure 2 Observed relationships of cloud layer and sub-cloud layer winds 10 the rainfall pattern

Since the Cherry Creek basin is oriented roughly north-south as shown in Figure
2. the extreme stonn events that align \l,:ith the basin orientation have southerly cloud
layer winds and southeast to east sub.c1oud layer winds. By comparison the ~WS study
specifies northerly sub-cloud winds. For northerly sub-cloud wind directions. Figure 2
should be rotated .1.5.90 degrees to the left to provide a "physically possible" scenario.
This rotates the rainfall pattern from its most stressing position alibTfled with the basin to a
cross-basin pattern that reduces the volume of rain within the basin boundaries
dramatically. The combination of the ~\VS study rainfall pattern aligned with the basin
with northerly sub.cloud winds is not consistent \\:ith Colorado extreme stornlS. The
A \V A team study also aligns the rainfall pattern with the basin but relies on southeast to
east sub.c1oud wind.

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