WMOD00011

• Applicant: Don Griffith North American Weather Consultants, Inc.
• Sponsor Name: City and County of Gunnison
• Project Name: Cloud Seeding Evaluation
• Title: The Development, Conduct and Evaluation of a Cloud Seeding Program for the Gunnison River Basin, Colorado During the 2003-2004 Season
• Prepared For: City and County of Gunnison
• Prepared By: North American Weather Consultants
• Date: 8/1/2004
• County: Gunnison
• Weather Modification - Doc Type: Report

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100 miles downwind of Cllrrent or historic cloud seeding programs in Colorado may also be
cOntaminated. limiting their usefulness as control areas. For example. from Fig. 4.1. there is a
gap between the winter 2002-03 cloud seeding projects in southwestern Colorado and the
Gunnison project. This would be a tempting area to look for control sites since they would be
close to the target area and would probably be well correlated. Hov.:cvcr. since winds during
storms that impact the target areas in southwestern Colorado are frequently blowing from the
southwest. the region of central Colorado between primary' target areas is likely to be subject to
dov,.:nwind cloud seeding effects. This is a problem for both this past seeded season and also
historically. since there have been a number of cloud seeding projects conducted in southw"est
Colorado in the past (refer to Appendix B). As a consequence. we did not consider any
precipitation sites in this particular area as control sites. Likewise. we did not consider some of
the precipitation sites in southeastern Utah as control sites due to concerns about possible
dO\\.'nwind effects from the central and southern Utah seeding program.

Considering all of the foregoing information. NA WC in the preparation of a final report
for the first season of the Gunnison seeding program conducted a search for possible
precipitation control sites in extreme eastern Utah and parts of southwestern Colomdo. as \,"ell as
portions of western and northwestern Colorado. One site in northern New ~1exico. near the
Colorado border. was also considered. Our normal approach in selecting control sites for a new
project is to look for sites that will geographically bracket the intended target area. The reason
for this approach is that we have observed that. while some winter seasons may be dominated by
a particular upper air flow pattern. other seasons can be dominated by other flow patterns. The
result of different upper air flow patterns often results in heavier precipitation in one area versus
the other. For example. a strong El :\Iino pattern may favor the production of heavy winter
precipitation in the southwestern United States while a strong La :--;ina pattern may favor the
production of below normal precipitation in the southwestern United States. Having control sites
either side of the target area can improve the prediction of target area precipitation under these
variable upper air flow patterns.

One final consideration in the selection of control
historical target/control relationship is one of data quality.

sites for the development of an
A potential control site may be

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