WMOD00377

• Contract/Permit #: 14-06-D-5640
• Project Name: Park Range Atmospheric Water Resources Program
• Title: Park Range Atmospheric Water Resources Program - Phase II - Part A
• Date: 11/15/1966
• Weather Modification - Doc Type: Report

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which was frequently in a precipitation shadow from the White River Plateau,
the San Juans and the Uncompahgre Plateau located southwest of the target
area. There were one hundred days with dry air under 500 mb ridging condi-
tions over, or to the west of, the Park Range. Most of the precipitation was
quite showery and of short duration, with only four long-period, moist, west-
northwe sterly orographic flow patterns occurring during the entire season.

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Early in the season, we suspected that a precipitation shadow effect
from the high White River Plateau southwest of the Park Range was minimiz-
ing precipitation on the target area under conditions of southwesterly upper-
level flow. This suspicion grew throughout the season as every case of south-
westerly moist flow produced much less precipitation over the Park Range
than would have been forecast in the absence of upstream terrain barriers.

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To gain a rough idea of the quantitative effect of the precipitation shad-
ow, the following sch~me was used:

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For the 1964-65 and 1965-66 snow seasons (November-April),
days on which the temperature-dewpoint spread was s..50C at
both 700 mb and 500 mb over the Park Range were tabulated.
The se days were broken into two categories according to (a) e s-
timated average 700 mb wind direction more southerly than 2400
and (b) 700 mb wind direction greater than 2400 over the Park
Range.
o Average 24-hour precipitation amounts for various stations in the

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western half of Colorado were also computed according to the

wind categorization.

The ratio:

Avg. 24 hr. precip. for Park Range 700 mb wind < 2400
Avg. 24 hr. precip. for Park Range 700 mb wind~ 2400

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was computed for the various stations, and results plotted in
Figure 26.
The precipitation shadow appears to be strong in a forty-mile wide zone

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