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6 Forecasting Avalanches Snow avalanche forecasting is the probabilistic assessment of both current and future snow stability. The philosophical purpose behind such forecasting is to provide information about current mountain conditions that helps people to avoid or to minimize exposure to avalanches. Forecasting is a formidable problem, as future stability assessments require consideration of additional loading in the form of anticipated precipitation and changes in snow strength resulting from temperature-controlled processes within the snowpack. Accurate avalanche forecasts are thus highly dependent on accurate weather forecasts, which are intrinsically difficult in mountainous areas. FORECASTING ORGANIZATIONS The oldest avalanche forecasting service is in Switzerland; it was established in 1945. This system, still the world standard, is centralized within a portion of the Federal Institute for Snow and Avalanche Research (FISAR) at Weissfluhjoch-Davos (Iaccard, 1986~. The relations between meteorology, snowpack information, and avalanche formation are trans- lated into avalanche hazard warnings that are distributed to the public. In addition, the avalanche warning service accumulates a data base of avalanche, snowpack, and weather information; provides information to the public on preventative and operational avalanche protection; and performs accident analyses for courts of law. A comprehensive view of avalanche conditions requires the daily acquisition and rapid transmission of observations from all Swiss mountain regions to the FISAR. Thus, about 70 observation stations are distributed throughout the Alps at altitudes of I,00~2,500 m. These stations are operated part time by local people of various occupations who have received special training; their daily reports reach TSAR each morning through the "Meteor" communication system of the Swiss Institute of Meteorology. Avalanche warnings are broadcast at noon several times a week by radio and can be retrieved by telephone; reports are also issued by television and the press. France operates a centralized avalanche forecasting service via Meteorologic Nationale and Centre d'Etudes de la Neige (de Crecy, 1980; LaFeuille et al., 1987~. The latter institute 47

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48 has developed systems to enable local agencies to conduct avalanche hazard forecasting (Navarre et al., 1987~. A diversified network of about seven forecast centers is maintained in the Alps and the Pyrenees, with each center issuing a forecast for its local area. In contrast, the forecast system in Austria is less centralized, and each province issues its own hazard bulletin (Bauer, 1972; Rink, 1987; Gallagher, 1981~. In Canada avalanche forecasting is carried out by a number of cooperating agencies, predominantly under the Atmospheric Environment Service of Canada (AESC). This agency receives weather data from its own weather ounces, public works departments, ski resorts, national parks, hydra companies, and private individuals. Two main area bulletins are issued by AESC, one for the coastal mountains and one for the interior mountains. More localized avalanche forecasting can be obtained from the individual agencies that relay weather data to the AESC, but only the park rangers in Alberta are currently engaged in public forecasting. These examples provide a comparative background for forecasting activities in the United States, where avalanche forecasting can generally be described as being carried out on both local and regional scales. In some locations forecasters are concerned with individual avalanche paths, perhaps as few as 5 to 50 in number, such as those located within ski areas, recreational land, or along specific sections of highways. Most highways do not have forecasting programs and wait for actual avalanche debris in the highway before ~ issuing c osures. Other forecasters have the responsibility to provide information covering entire regions without making reference to individual avalanche paths. Examples of regional forecasting, which may include thousands of square kilometers of avalanche terrain, include the orga- nized avalanche information centers in Colorado, Utah, and Washington. These centers derive their financial support from various agencies, which in turn receive the benefit of the mountain weather forecasts and avalanche information tU.S. Forest Service (USES), National Weather Service, National Park Service, state highway departments, s~-area orga- nizations, mountain clubs, etc.~. The USES administers and supports the Utah center and also administers the Northwest center with considerable financial support from additional organizations; the Colorado center is aciministered by the state but relies on numerous other organizations for its financial support. Housing for all centers is provided by National Weather Service Forecast Offices. Some centers claim financial problems; an operating center in Alaska was closed for financial reasons in 1986. The forecasters at the regional centers generally provide weather, snow stability, and avalanche hazard ratings within a given area, with specific information regarding the range of elevation, slope angle, and aspect. This information is produced at least once daily by the centers and is available to the public through recorded messages and through the media. For example, the Colorado Avalanche Information Center (CAlC) operates from November to April and monitors weather, snowpack, and avalanche data at 32 manned sites (22 ski areas; the remainder are highway and backcountry locations); provides twice-daily forecasts to the public via recorded telephone messages; issues avalanche warning bulletins via the National Oceanic and Atmospheric Administration's Colorado Weatherwire and the news media; and provides avalanche information to the public (Williams, 1986~. The CATC also maintains a computer data set of mountain weather and avalanche events from about 60 sites throughout the Western mountains, continuing the Westwide Avalanche and Mountain Weather Reporting Network originated by the USES in 1966. The Wes~wide network uses standardized instrumentation and data collection procedures and provides valuable statistics on avalanche occurrence and associated snowpack and weather

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49 conditions for several thousand avalanche paths in the United States. Modeled in some respects after the Swiss system, the Westw~de data set was expected to furnish the basic observations necessary for research efforts toward objective avalanche forecasting in the United States (Martinelli, 1973~. Research along these lines was in fact initiated (Judson et al., 1980) but was terminated with the closure of avalanche studies at Fort Collins. The network was also expected to provide the care of Tong-term avalanche occurrence records so essential for national land-use planning and zoning as well as the basic data for a conceivec! national avalanche warning system (Martinelli, 1973~. Apart from the Westw~de network responsibility, the Northwest center functions on a basis similar to Colorado. Emphasis is on highly detailed mountain weather forecasts (Marriott and Moore, 1984; Ferguson et al., 1989~. The Utah Avalanche Forecast Center conducts operations over a smaller region, but it services the most concentrated population of winter backcountry use in the country; its avalanche hotlines receive 50,000 calls per season (Tremper and Ream, 1988~. STATE OF THE FORECASTING ART To evaluate the probability of an avalanche release at some future time, the forecaster must have access to data that describe both the expected meteorological conditions and the anticipated strength conditions of the snow cover. Because useful snow strength data are extremely difficult to obtain, forecasting methods place greatest emphasis on meteorological variables (LaChapelle, 1980; Buser et al., 1985~. The collection of weather data has been aided by advances in digital recording systems and by the development of durable sensors adequate for winter usein mountain locations (Gubler, 1984; Marriott and Moore, 1984~. To some limited extent, weather data can be considered to represent the general conditions of the area of perhaps several square kilometers surrounding the measurement site. In the case of snowpack data, this same assumption cannot be made, since snow properties on a shallow, gently sloping, south-facing slope will differ greatly from those of a thick snow cover on a steep north-facing slope only a few meters away (Dexter, 1986; R. L. Armstrong, 1985~. While such snow properties as density, temperature, and crystal type can be measured by standardized methods, the correlation between these ant! representative snow strengths remains elusive, as does the measurement of snowpack strength. Existing models relating weather and snowpack parameters to snowpack strength are quite complex and thus far offer little practical assistance to the operational avalanche forecaster (Dexter, 1986; Judson et al., 1980; Anderson, 1976~. The essence of the problem in avalanche forecasting is to determine the amount of energy required to trigger an avalanche for a given set of strength conditions. The meteo- rological, snow structure, and snow mechanics data that actually become input variables for specific forecast methods are determined by both the requirements of the technique being used and the ability of the forecaster to obtain the data. Once the required information has been assembled, a detailed decision-making process is undertaken, whether by actual forecasters using conventional methods, by means of a numerical model, or, as is becoming more typical, by both methods. Widely practiced traditional methods of avalanche forecasting therefore require a blend of inductive logic and deterministic consideration of meteorological and snow physics pa- rameters to reach actual forecast decisions (LaChapelle et al., 1978; LaChapelle, 1980~. Conventional forecasting is thus an art baser! on experience, intuition, and process-oriented

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so reasoning that is difficult to learn, to teach, and to transfer from one region to another. This situation motivates the continuing search for objective computer-based procedures to aid in decision makingefforts currently concentrated in European institutions. Methods being examined include linear regression, multivariate discriminant analysis, time-series modeling involving nonparametric methods and pattern recognition, numerical deterministic mod- eling, nearest-neighbor methods, and artificial intelligence (Buser et al., 1987; Bakkehoi, 1987; LaFeuille et al., 1987; LaFeuille, 1989; Navarre et al., 19874. Although the emphasis on numerical and statistical modeling has focused almost ex- clusively on meteorological variables, the various methods in current use have produced reasonable results (Buser et al., 19SS, 1987~. Nevertheless, at present such forecasts achieve a score only comparable to the results obtained by conventional or intuitive methods, where the technique relies almost entirely on the experience of the forecaster (Armstrong and Ives, 1976; Fohn et al., 1977; Buser et al., 1987~. Computer techniques do not yet relieve the forecaster from making decisions but do provide a too} by which detailed specific infor- mation is made available as part of the basis for forecasting decisions. Such models offer promise, but they require an effective long-term data base of snowpack and meteorological information. In this respect the Westwide data network system is of crucial importance, an "invaluable treasure" in the words of Brugnot (1987~. COMMENTS I. In the United States no uniform policy exists regarding avalanche forecasting. The administration and funding of forecast centers are fragmented; several centers have financial problems and concern for survival; and the Alaskan center was closed due to lack of funcis. Because these centers provide a valuable service, an attempt should be made to find funding resources to ensure their continued operation. 2. Development of new forecasting methodologies for avalanche forecasting is now carried out mainly in Europe, where government-derived financial support is available for such activities. Additional funding would be required to enable forecast centers in the United States to develop, adapt, ant] test new technologies. 3. A data base essential to future computer-based forecasting in the United States is being maintained at a minimum level by the regional centers and by the Westwide data network. Because this data base is of crucial importance, it should at least be maintained and, if possible, upgraded. Additional funding is needed for equipment maintenance, repair, replacement, and modernization.