The concept of “winter cloud seeding windows” is a familiar theme found in a number of earlier publications. More recent feasibility studies, physical observations and analyses of existing cloud seeding programs have indicated some of this earlier thinking has considerable merit. The concept that deep winter storm systems with cold cloud tops often appear to be naturally efficient with little or no supercooled liquid water content is especially important. It appears from a variety of earlier sources of information and more recent observations that shallow, orographically induced clouds often contain supercooled liquid water and therefore offer good cloud seeding potential. Several studies and observations suggest that shallow orographic clouds that contain supercooled liquid water frequently occur after the passage of a surface cold front and even after the passage of an upper level trough. If the occurrence of such clouds is viewed in the context of the orientation of the targeted mountain barriers, the question can be asked if mountain barrier orientations have any impact on the development of these types of “seedable” clouds? This is basically a question of the amount of up barrier flow that accompanies these shallow orographic clouds. North American Weather Consultants has developed a conceptual model that the barrier orientation that provides the best conditions for the formation of these kinds of clouds in the western United States (and perhaps elsewhere) are barriers with a north-south orientation since post-frontal or post upper trough passage conditions will produce considerable up barrier flow over these barriers. Fortunately, most mountain barriers in the western United States have such an orientation. North American Weather Consultants believes that recognition and verification of the above will be important in the conduct of future winter orographic cloud seeding programs. Placing “seedabilty” in the synoptic setting and relating “seedability” to barrier orientation will be important in estimating potential cloud seeding effects in different project areas in the future.
The San Luis Obispo County Flood Control and Water Conservation District (District) to conduct a feasibility/design study for a winter cloud seeding program to target the Lopez Lake and Salinas Reservoir (LLSR) drainage basins located in southern San Luis Obispo County, California contracted north American Weather Consultants (NAWC) in 2016. Recommendations from the American Society of Civil Engineers (ASCE 2016) publication entitled “Guidelines for Cloud Seeding to Augment Precipitation” were followed in determining that the proposed project was both technically and economically feasible. NAWC reviewed available information, compiled and analyzed data, and then developed a proposed program design. Regression relationships were established between November through April precipitation and annual runoff into the two target reservoirs. Estimated increases in precipitation of 9% or 17%, from an analysis of the long-term effects of an operational cloud seeding program conducted by the Santa Barbara County Water Agency in an adjacent drainage were inserted into the regression equations to estimate average increases in inflow to the two target drainages. The District provided NAWC with estimates of the value of this augmented runoff. The costs of conducting a seasonal cloud seeding program were estimated. These steps provided data that could be used to estimate benefit to cost ratios. NAWC concluded that the proposed program was both technically and economically feasible based upon ASCE 2016 criteria.
North American Weather Consultants, Inc. (NAWC) has conducted operational winter cloud seeding programs in the mountainous areas of Central and Southern Utah since 1974. Beginning in 1988, seeding has also been conducted in three additional mountainous target areas within the State. The goal of these programs has been to enhance winter snowpack accumulation in the target areas, which now include most of the mountainous areas of the State. Studies have demonstrated that a large majority of the annual runoff in Utah streams and rivers is derived from melting snowpack, which explains the focus on wintertime seeding (within the November – April period). Augmented water supplies are typically used for irrigated agriculture or municipal water supplies. Programs are typically funded at the county level with cost sharing grants from the Utah Division of Water Resources (UDWR) and the three Lower Colorado River Basin States of Arizona, California, and Nevada, since 2007. An earlier WMA paper (Griffith et al. 2009) provided a summary of seeding operations for the water years of 1974 through 2007 for the four target areas. This paper is focused on the Central and Southern Utah program which is both the largest target area and the longest running program in the state. It covers all but three water years from 1974 through 2021 and is one of the three or four longest operational winter cloud seeding programs that have been conducted in the United States. The target area encompasses several mountain ranges in Central and Southern Utah. NAWC has defined the target area boundaries as those locations that are above 7,000 feet MSL. This is a large area of approximately10,000 square miles. Cloud seeding is accomplished using networks of ground-based, manually operated silver iodide nuclei generators located in valley or foothill locations upwind of the intended target mountain barriers. As such, these programs are classified as orographic winter cloud seeding programs. Orographic winter cloud seeding programs are typically categorized as those with the highest level of scientific support based upon capability statements of such organizations as the American Meteorological Society and the Weather Modification Association. NAWC historical target/control evaluations of this program indicate an average increase in December-March target area precipitation of 12% or an average increase in precipitation of 1.3”. These results were significant at the 0.06 level from a one-tailed Student’s t-test. The UDWR has conducted periodic studies to estimate the increases in annual streamflow resulting from the estimated increases in April 1st snow water content produced by this seeding program. The most recent study (UDWR 2018) indicated an estimated average annual streamflow increase of about 84,000 acre-feet for the Central and Southern Utah target area. Factoring in the cost of conducting this program resulted in an estimate of the average cost of the augmented runoff to be $2.02 per acre-foot.
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