The Boise River Drainage, located in Central Idaho, is productive in terms of annual streamflow, a large majority of which is derived from accumulated winter snow pack. There are three dams in the upper river: Anderson Ranch, Arrowrock and Lucky Peak. Capacities of the three reservoirs are: 413,000, 272,000 and 306,000 acre feet, respectively. Both Anderson Ranch and Lucky Peak have hydroelectric production capabilities. Lucky Peak is located below the first two dams. North American Weather Consultants, Inc. conducted winter cloud seeding programs over the Boise River Drainage above Lucky Peak Reservoir during the water years of 1993-1996. A target/control analysis of these four seasons of seeding indicated an average increase in target area April 1st snow water content of 12% (an average additional 2.50" of snow water content per season). Additional analyses were performed to estimate the potential economic benefit that might be derived from the seeding program based upon the value of the estimated increased hydro-power production from Lucky Peak Dam. Lucky Peak has an installed turbine capacity of 100 MW. It was estimated that a 12% increase in April 1st snow water content would result in an average 16,409 MWh of additional electricity production per year. This amount of additional electricity was estimated to have a value of $820,182. The average annual cost of the cloud seeding program during the four seasons of operations was $85,000. These values result in an average estimated benefit/cost ratio of 9.7/1. This analysis does not consider the value of the additional electricity produced from the Anderson Ranch Dam, which is a Bureau of Reclamation facility, or the value of the enhanced streamflow to irrigation interests downstream of the Lucky Peak Dam.

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 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 that 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 (NAWC) has developed a conceptual model in which 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 accompanied by lowering and warming cloud tops. Fortunately, most mountain barriers in the western United States have such an orientation. NAWC believes that recognition and verification of the above will be important in the design and 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. WINTER “CLOUD SEEDING WINDOWS” AND POTENTIAL INFLUENCES OF TARGETED MOUNTAIN BARRIERS Don Griffith, David Yorty, Warren Weston and Mark Solak North American Weather Consultants, Sandy, Utah Corresponding Author: Don Griffith (dgriffith@nawcinc.com) less real-time measurements of SLW (e.g., microwave radiometers or ground-based icing rate meters) are available during the conduct of an operational winter orographic cloud seeding program, then some type of conceptual model that indicates the likely presence of “seedable” conditions can be quite useful in determining when cloud seeding operations should begin and end. Consequentially this paper takes another look at the concept of developing “cloud seeding windows” for use in real-time decision-making.

Low-level thermodynamic stability, an important consideration for winter season ground-based cloud seeding site selection and conduct of operations, varies considerably in and near mountainous terrain. Rawinsonde observations may sometimes be poorly representative of nearby areas, with significant lower-level atmospheric temperature variability common between basins and sometimes within a basin. Operational forecast models may lack the ability to resolve or properly initialize low-level stability or “cold pooling” specific to a given basin or portion of a basin. Analyses of surface observations, as well as plume dispersion modeling with HYSPLIT, can be used both for real-time operations and in post-hoc examinations of low-level stability specific to storm periods with cloud seeding potential. Analyses of this type have been performed in various portions of Utah, and a recent analysis in the Uinta Range and Uinta Basin area of northeastern Utah highlights both the geographic variability and the seasonality of basin cold pooling. The Uinta Basin analyses have shown much greater low-level thermodynamic stability during relevant winter storm periods than other portions of Utah. Strong correlation of stability to site elevation within the basin, as well as distinct seasonality, are observed.

A quote from Silverman states “Silverman (2007) showed that it is imperative to use as a control or controls, to the extent that available data permits, the streamflow station or stations that yield the most precise results.” He showed the control or combination of controls that had the highest correlation with the target and, especially, the lowest standard deviation of the residuals (differences between the observed and predicted values) will yield the most precise evaluation results. NAWC readily agrees with that statement, however, examination of the correlations obtained in this study as provided in Table 4 in The Vail paper (Silverman 2009a) suggests this ideal was not obtained. The r2 values obtained were significantly lower than those previously obtained by Silverman in the analyses he has performed on longterm Sierra Nevada programs (Silverman, 2007, 2008, 2009b). Values from Table 2 from Silverman (2009) indicate the correlation coefficients in his Vail analysis ranged from 0.775 to 0.918 or r2 values of 0.60 to 0.84; values considerably lower than he established for the three California programs where the r2 values ranged from 0.96 to 0.98.

In mountainous regions where winter season cloud seeding is conducted for the purpose of higher-elevation snowpack augmentation, the frequency and character of low-level atmospheric stability can significantly impact transport of cloud-seeding material released from valley and foothill locations over higher elevation target areas. A two-surface-site (2SS) method was developed to estimate stability in the layer from the valley/foothill surface to mountain-top height (approximately 700 mb) in Utah, using available surface temperature and dew point data. The method yields approximations of integrated stability in the layer, which were classified according to their likely impact on operational seeding, and can be expressed in terms of the low-level warming, or upperlevel cooling, required to yield a neutral lapse rate (well-mixed environment). The stability estimation method was applied to stormy periods during three winter seasons when mountain-top icing was documented via ground-based high elevation icing rate sensors, and when temperatures were adequately cold for activation of silver iodide particles as ice-forming nuclei. That partitioning method identifies periods when silver iodide seeding potential likely exists. The indications of the 2SS analysis method are that seeding material releases from most valley/foothill locations are likely to undergo timely and effective dispersion to mountain barrier crest height during a large percentage (~75%) of icing periods exhibiting apparent silver iodide seeding potential. Comparisons of the 2SS method stability estimates to similar rawinsonde-derived estimates showed good correspondence in over 80% of the cases analyzed, providing some confidence in the utility of the 2SS method in the absence of available rawinsonde data. Comparisons were also made between 2SS stability estimates and modeled seeding plume behavior using the NOAA HYSPLIT (Hybrid Single Particle Lagrangian Intregrated Trajectory) model with NAM (North American Model) meteorological input data during icing periods. Agreement between modeled plume behavior and stability indications of the 2SS method was found in over 80% of the modeled periods. Results of these comparisons provide confidence in the overall stability climatology for icing periods as presented in this paper, as well as the real-time operational utility of the 2SS method in areas where other data (e.g., rawinsonde) are not available. The analyses presented here comprise a portion of a more comprehensive study, based on data from several ice detector sites in Utah. Support for the establishment of these sites, and for analysis of the data, was provided by a consortium of Lower Colorado River Basin States.

North American Weather Consultants (NAWC) performed a feasibility/preliminary design study of potential means of augmenting an existing operational winter cloud seeding program in the Upper Boise River Basin (UBRB) program in Idaho by extending the base project period of November through March by one month (April) and possibly adding remote generators and seeding aircraft to the existing lower elevation manual generator network. This study was performed for the Idaho Water Resources Board (IWRB). The IWRB noted that the upper Boise, including the North Middle and South Forks, and Mores Creek supplies 90% of the water for the lower Boise Basin. The UBRB ranges in elevation from approximately 915 m (3,000 feet) MSL at Lucky Peak Dam to crest elevations of approximately 2590 – 2700 m (8,500 to 9,500 feet) MSL between the Boise River and Big Wood River Basins. NAWC recommended that the intended target area for the UBRB program be defined as those regions in the basin that are above 1524 m (5,000 feet) MSL. The primary program goal would be to increase winter snowpack in the target area through operational cloud seeding. The resulting augmented spring and summer stream flow would be used in a number of ways including augmented hydroelectric power production and agricultural irrigation. Average increases of 4.7% in April 1st snow water contents from cloud seeding were estimated through transference of the indicated results from the Climax I and II research programs. Simulations using empirically derived snowpack-stream flow relations yielded estimated average increases in March-July stream flow from three seeding modes totaling approximately 1.004 x 108 m3 (81,425 acre-feet). The costs of the estimated increases in March-July stream flow range from $0.003 to $0.01 per cubic meter ($3.29 to $12.45 per acre-foot) of additional water in an average water year. A preliminary design for an augmented operational winter cloud seeding program is described.

An icing rate detector originally designed for aircraft is used to measure supercooled liquid water in winter orographic clouds at a fixed mountain-top site in the central Sierra Nevada of California. Supercooled liquid water concentrations have been determined using continuous records of rime ice accretion and windspeed. It is shown that supercooled liquid water is occurring within larger portions of storms than prior airborne observations over the operational area have indicated, particularly during pre-frontal periods. In many instances, supercooled liquid water occurs below safe aircraft operational altitudes over the mountainous project area. The ground-based measurementsystem provides an effective tool for semi-quan- titative determination of supercooled liquid water within specific cloud volumes heretofore unmeasured.

A winter orographic cloud seeding program has been conducted in the Gunnison, Colorado region for the past eight winter seasons. The intended target area is elevations above 9,000 feet MSL that provide streamflow to Blue Mesa Resevoir located in western Gunnison County. The goal of this operational program has been to augment higher elevation winter snowpack, which subsequently contributes to spring and summer sreamflow. This program has operated under permits granted by a number of local entities and it has also recieved some funding support from the Colorado Water Conservation Board. The program is supported by a number of local entities and it has also received some funding support from the Colorado Water Conservation Board and the three Lower Colorado river basin States (Arizona, California and Nevada). A network of 20-25 ground based silver iodide generators has been used to seed all storm periods thought to represent good seeding opportunities based upon targeting considerations and the likely presence of supercooled liquid water. An historical target/control evaluation technique, was developed, based upon NRCS SNOTEL April 1st snow water content observations, to provide estimates of the potential effects of cloud seeding. These estimates indicate average seasonal increases in the 10-15% range. Calculations were made of increases in April through July sreamflow based upon the indicated increases in April 1st snow water contents. Increases in the range of 79,600 to 96,200 acre-feet in an average April - July runoff were indicated based upon a 10% increase in April 1st snow water content for an average winter season. Costs of producing the augmented runoff based upon these calculated increases in streamflow ranged from $0.94 to $1.13 per acre-foot.

North American Weather Consultants performed a feasibility/preliminary design study for a potential operational winter cloud seeding program in the Eastern Snake River Basin Program (ESRBP) in Idaho. Two potential target areas were identified. One area was located along the south slopes of the Centennial Mountains and the Lion Head and Henrys Lake Mountains in northeastern Idaho. This area is denoted as the North Target Area. The other area encompasses all or portions of the Big Hole Range, the Snake Range, the Grays Lake Mountains, and the Aspen Range in eastern Idaho. This area is denoted as the East Target area. The primary program goal would be to increase winter snowpack in the target areas through operational cloud seeding. Average increases of 5.5% in April 1st snow water contents for the North Target area and 7.6% for the East Target Area via cloud seeding were estimated through transference of the indicated results from the Climax I and II research programs. Simulations using empirically derived snowpack streamflow relations yielded estimated average increases in March-July streamflow from two seeding modes totaling approximately 149,350 acre-feet (1.84 x 108 m3) for the combination of the two areas. The costs per acre-foot for the estimated increases in March-July combined area streamflow range from $2.95 to $4.51 per acre-foot of additional water in an average water year. A preliminary design for an operational winter cloud seeding program is described. One preliminary winter season of supercooled liquid water and lower-level temperature and wind observations is recommended to determine the presence of supercooled liquid water and low-level temperature inversions.

This is an interesting paper that was published by Silverman. The length of the Vail seeding program lends itself to detailed analysis. North American Weather Consultants (NAWC) does have a few comments as well as concerns regarding this paper. These comments and concerns are addressed in the following.

North American Weather Consultants (NAWC) published a peer-reviewed paper in the WMA 2009 Journal of Weather Modification entitled 30+ Winter Seasons of Operational Cloud Seeding in Utah, Griffith et al. 2009. That overview paper describes several operational winter cloud seeding programs being conducted in Utah (Griffith et al. 2009), hereafter referred to as Griffith. The paper included estimations of seeding effects using an historical target/control method to assess the ongoing nonrandomized seeding projects. Silverman (2010) has submitted comments to the Editor of the WMA Journal of Weather Modification questioning the basis and accuracy of estimates of seeding effectiveness summarized in the Griffith paper.

North American Weather Consultants (NAWC) has conducted operational winter cloud seeding programs in many of the mountainous areas of Utah since 1974. The goal of these programs has been to enhance winter snowpack accumulation in several mountainous target areas throughout the State. Studies have demonstrated that a large majority of the annual runoff in Utah streams and rivers is derived from melting snowpacks, which explains the focus on wintertime seeding. 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. Cloud seeding is accomplished using networks of ground-based, manually operated silver iodide 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, the World Meteorological Organization, and the Weather Modification Association. NAWC historical target/control evaluations of these Utah programs based upon high elevation precipitation and snow water content observations indicate a range of apparent increases in target area average precipitation or April 1st snow water content of 3-21%. The Utah Division of Water Resources conducted an independent assessment of the seeding programs in 2000. That assessment confirmed the NAWC indicated increases in snow water content, and then took the additional step of estimating the increases in annual streamflow resulting from the estimated increases in snow water content. Average annual increases from four seeded areas were estimated to total 249,600 acre-feet. Factoring in the cost of conducting these programs resulted in an estimate of the average cost of the augmented runoff to be $1.02 per acre-foot.

North American Weather Consultants performed a feasibility/preliminary design study for a potential operational winter cloud seeding program for the Salt River/Wyoming Ranges in Wyoming, under contract to the Wyoming Water Development Commission. The Desert Research Institute (DRI) conducted atmospheric modeling as part of this study. The primary project goal is to increase winter snowpack in the target area through operational cloud seeding. An average increase of 10% in November through March precipitation via cloud seeding was calculated, using results from the Climax I and II research programs. Simulations using empirically derived snowpack-streamflow relations yielded increases in streamflow from three seeding modes totaling approximately 109,500 acre feet (1.35 x 10^8 m^3) on average. The costs per acre foot for the estimated increases in streamflow range from $1.91 to $7.13 per acre foot of additional water and associated benefit/cost ratios range from 5.8/1 to 1.6/1, depending on the seeding mode(s). A preliminary design for an operational winter cloud seeding program is described. One preliminary winter season of supercooled liquid water and lower-level temperature and wind observations is recommended to determine the frequency of low-level temperature inversions during seedable periods. The DRI case study modeling results indicated that such inversions could inhibit the effectiveness of low elevation ground based seeding releases.

Interest in weather modification in Santa Barbara County (County), California, USA dates back to 1950. This interest developed shortly after the discoveries of Drs. Schaefer and Vonnegut in the late 1940’s that established a scientific basis for weather modification (commonly referred to as cloud seeding). Two weather modification research programs have been conducted in the County. Numerous winter season operational cloud seeding projects have been conducted in the County dating back to 1950.

Winter precipitation data from selected locations within the States of Utah and Nevada were analyzed to determine if there were any indications of reductions in mountainous precipitation when compared with upwind valley precipitation. This work followed the approached utilized in a comprehensive study of precipitation in Israel and California that indicated the orographic component of precipitation was declining at precipitation stations at mountain locations which were downwind of major cities. The authors of that study theorized that these reductions were due to the transport of air pollution from the cities into winter storms at these downwind mountainous locations leading to microphysical changes within the affected clouds resulting in reductions in observed precipitation. The work which we performed in Utah and Nevada indicated similar reductions in mountainous precipitation downwind of the Salt Lake City/Provo metropolitan complex. These indicated reductions in precipitation extended downwind of the first mountain barrier into a mountain valley location and into the upwind slope of a second mountain barrier some 80 km downwind. Reductions in precipitation at other mountain stations in Utah and Nevada were not indicated. These stations were located in more rural settings which may suggest that air pollution from major cities may in fact be related to the reductions in precipitation that are indicated downwind of the Salt Lake City/Provo metropolitan complex.

Mountain-top measurements of rime ice accumulations during the winter season of 2003-2004 in the Wasatch Range southeast of Salt Lake City, Utah, were analyzed to estimate and characterize the seasonal occurrence of supercooled liquid water during more than twenty storms, specifically toward assessment of winter cloud seeding opportunities in the region. The data indicated substantial periods of supercooled liquid water occurrence and colder than anticipated temperatures overall during riming periods. Using precipitation measurements at a nearby site, the apparent relative precipitation efficiency of storms and periods of storms was estimated. In many cases, rather orderly transitions in apparent precipitation efficiency have been documented and many periods of sustained inefficient precipitation production were noted. These and other findings suggest substantial cloud seeding opportunity for snowpack augmentation and provide useful insights regarding seeding opportunity recognition. Descriptive statistics and storm case examples are summarized.

North American Weather Consultants (NAWC) conducted a winter cloud seeding project for the Boise Project Board of Control in the mid1990's. The project was discontinued when some wetter winters impacted Idaho in the latter 1990's. In discussions with their Board of Directors in 2000 a question arose as to whether increases in streamflow that might be produced by a winter cloud seeding project would be lost to hydro generation at Lucky Peak Dam in the high runoff periods resulting from spring and early summer snow melt. Some board members indicated that the turbine capacities at Lucky Peak could be exceeded in some situations. This would obviously affect the value of the additional water from cloud seeding. We conducted an in-house study to determine if this might in fact be the case. We (NAWC) subsequently published a paper entitled “Economic Feasibility Assessment of Winter Cloud Seeding in the Boise River Drainage, Idaho” in the 2002 in the reviewed section of the edition of the WMA Journal of Weather Modification. That paper by Griffith and Solak will herein be referred to as GS. The intent GS was to explore the concerns of these board members and to produce some estimates of the potential economic benefits of the cloud seeding project, based upon estimates of increases in snowpack water content values on April 1. Although the method used to produce the estimates of seeding effects was described briefly in GS as background material, the intent of that paper was not to present a definitive explanation of the historical target/control analyses that were used to establish the estimates of increases in April 1 water content. More comprehensive discussions on the target/control evaluations were provided in our annual reports to our client, but have not been formally published. The title of GS indicated that it was an economic feasibility assessment. We used the term estimate (or estimates) in discussions of the potential increases in April 1 snow water content. These numbers were never cited as exact, nor could they have been, based upon the type of analyses that were performed in the absence of a randomized data set.

Interest in weather modification in Santa Barbara County (the County), California, USA dates back to 1950. This interest developed shortly after the discoveries of Drs. Schaefer and Vonnegut in the late 1940’s that established a scientific basis for weather modification (commonly referred to as cloud seeding). Two weather modification research programs have been conducted in the County. Numerous winter season operational cloud seeding projects have been conducted in the County dating back to 1950. This interest in cloud seeding has been driven by two primary water needs in the County: drinking water supplies and agriculture. Water is a valuable commodity to both groups due to the amounts and seasonality of rainfall in the County. The County enjoys a Mediterranean climate. The months of November through March typically produce most of the rainfall in the County. Operational winter seeding projects were conducted in the County from 1950-1955 utilizing ground based silver iodide generators. Analyses of the effectiveness of these non-randomized projects indicated an average 16% increase in precipitation. Figure 1 provides the location of Santa Barbara County relative to the State of California.