Indications of Downwind Cloud Seeding Effects in Utah

Abstract

Estimation of effects on precipitation downwind (or “extra-area effects”) of a long-standing winter operational snowpack augmentation project in central and southern Utah mountainous areas was originally conducted in a 2003 study.  The study utilized a target/control linear regression technique, which has been used to estimate the seasonal effects of cloud seeding within the program’s intended target area. The results of the original study have been updated through the 2018 water year.   Seeded target area analyses of December-March high elevation (NRCS SNOTEL) precipitation data for this program indicate an overall season-average increase of about 12% for 41 seeded seasons.  Estimations of downwind seeding effects were made for individual stations and various distance bands downwind as far as 100+ miles.  The analyses suggest increases of similar percentages to those for the target, expressed as ratio values to the natural precipitation amounts, extending as far as 100 miles downwind (approximately the Utah/Colorado border area).  At approximately 100 miles downwind, the area-averaged ratio values (observed divided by predicted values) approach 1.0, suggesting a lack of any significant seeding effect at these distances.  Expressed as average-depth precipitation amounts, the target area precipitation increase from seeding is likely about 1.3” of additional water, with much lesser total precipitation increases in the much drier downwind area within 100 miles, trending to 0 beyond this distance.  Although variability of the individual downwind station results remains high, the updated results are in good overall agreement with those of the original (2003) study.