By Nata de Leeuw, Montana State University
and Karl Birkeland, National Avalanche Center
Many a meeting has run long when forecasters disagreed on which avalanche problem type is most appropriate for a given situation. I began my career at Castle Mountain Resort, known for its wind, so in our case, this debate was usually between wind slabs and storm slabs. Our team would spend the day on the same snow, see the same avalanches, and share our observations, yet during the PM meeting, we would disagree about whether to submit a storm slab problem or a wind slab problem to InfoEx. We eventually realized we were all talking about the same snow formation, and our argument was largely semantic. No one was wrong, we just interpreted the terms differently. As I ventured out into the wider avalanche world, I realized these debates were not isolated to our small corner of southern Alberta, and I became curious about the extent of this discrepancy within the industry.
Gallery: Getting blown away at Castle Mountain Resort.
BACKGROUND
Though the avalanche problem types in the Conceptual Model of Avalanche Hazard are explicitly defined (Statham, Haegeli, et al., 2018), inconsistency is observed in their practical use (Hordowick, 2022; Lazar et al., 2012; Statham, Holeczi, et al., 2018). Recently, researchers at Simon Fraser University found dissimilarity in the thresholds used by different public forecasters for adding, removing, or changing avalanche problems (Hordowick, 2022). While all forecasters interviewed stated their minimum threshold wind speed for adding a wind slab problem was in the moderate range (26-40 km/h), their maximum allowed wind speeds for a storm slab problem varied from 10–60 km/h. This large range of wind speeds highlights individual and operational inconsistency. Further, the conceptual model only considers wind in the definition of a wind slab, and not in the definition of a storm slab (Statham, Haegeli, et al., 2018). Consistent interpretation of avalanche problem type is important, as problem type largely dictates mitigation strategy (Atkins, 2004). Inconsistent interpretations could pose problems for operations that share information, or practitioners that move between multiple operations.
The Conceptual Model (Statham, Haegeli, et al., 2018) describes a storm slab as a “cohesive slab of soft new snow” (p. 674). It describes a wind slab as a “cohesive slab of locally deep, wind-deposited snow” formed by “wind transport of falling snow or soft surface snow” (p. 675). This explanation describes two different wind transport processes:
- “Wind transport of … surface snow” describes redeposition, which occurs when snow that has been on the ground for a period of time is entrained by the wind, transported, and deposited elsewhere (Fig. 1a).
- “Wind transport of falling snow” describes preferential deposition, which occurs when snow from the air column is deposited directly into a lee area without having previously touched the ground (Fig. 1b) (Lehning et al., 2008).
According to the definitions of the conceptual model, both redeposition and preferential deposition result in a wind slab avalanche problem
(Statham, Haegeli, et al., 2018). However, snowfall that is preferentially deposited during a storm is sometimes consciously included in a storm slab problem, as this may better describe the spatial distribution and simplify communication (Klassen et al., 2013). In practical situations, wind slabs and storms slabs exist on a continuum, which can make it difficult to distinguish between them. Public forecasters must consider risk communication when deciding which term to use, which may explain some of the noted inconsistency (Hordowick, 2022). Public forecasting has been the focus of previous terminology studies, but information is still missing on how professionals use the terms storm slab and wind slab when communicating with each other.
RESEARCH GOAL
The goal of this study was to assess the extent of inconsistency in the use of the terms storm slab and wind slab in professional communications. Inconsistencies exist in public bulletins (Hordowick, 2022; Statham, Holeczi, et al., 2018), but no one has investigated the extent of inconsistency in professional communication uninfluenced by public perception. In this study, I analyzed a sample of InfoEx submissions to determine the prevalence of operations using the term storm slab to describe an avalanche problem formed by preferential deposition, which may otherwise be described as a wind slab problem. The results provide information on professional use of these terms that is uninfluenced by the pressures of public communication. Understanding the extent of inconsistency is an important first step if we as an industry want to emphasize greater consistency.
METHODS
I reviewed all storm slab problems submitted to InfoEx in January 2022 and classified each as either aspect-dependent or all-aspect (Fig. 2). Aspect-dependent storm slabs were then labelled as preferential deposition only if the problem was located on aspects lee to the wind direction reported by that operation on that day. Comments associated with these storm slabs often emphasized the role of wind. Preferential deposition storm slabs were then grouped by operation for analysis. Additionally, many all-aspect storm slabs included a wind slab problem. This was determined from associated comments, which ranged from stating that the slab formed with wind, to explicitly stating that the problem included a wind slab. Storms slabs that included a wind slab were also grouped by operation.
RESULTS
Within the sample period, 133 operations submitted at least one storm slab problem. Of those 133 operations, 20% submitted at least one preferential deposition storm slab, and 26% percent submitted at least one storm slab that included a wind slab (Table 1). The second situation often occurred during storm cycles, and sometimes additionally encompassed a dry loose problem. Operations that used storm slab for preferential deposition are not confined to one geographic area (Fig. 3). However, based on the distribution of all operations in Western Canada, the proportion of operations that used storm slab to represent preferential deposition appears highest on the West Coast
and in the Alberta Rockies.
DISCUSSION
These results show inconsistency in how the term storm slab was used in InfoEx, and indicate inconsistency in how storm slab problems and wind slab problems are applied in the Canadian avalanche industry. At least one in five operations sometimes used the term storm slab to represent a slab formed through preferential deposition, which is not consistent with the definitions of wind slab and storm slab in the conceptual model. Additionally, at least one in four operations sometimes included a wind slab problem within a storm slab problem. These situations usually occurred during storms, or when uncertainty was high such as in a morning meeting with limited snowpack data. Results of this study demonstrate the complexity of avalanche forecasting, particularly the difficulty in applying a categorical classification to a situation that in reality exists as a continuum.
The root of this semantic discrepancy may lie with the two different processes by which wind can transport snow. Most would agree that an avalanche problem resulting from redeposition is a wind slab problem. Most would also agree that snow falling straight down results in a storm slab problem. However, problem type becomes less clear when snow falls sideways, as in the case of preferential deposition. Some practitioners may call this a storm slab and some may call it a wind slab. The distinction is further complicated when these transport processes happen simultaneously.
Local weather conditions could be a reason some operations were more likely to call preferential deposition a storm slab. The Alberta Rockies and the West Coast are both known for high winds, and these areas had the highest proportion of operations using storm slab for preferential deposition. It is possible forecasters in these windy areas are more likely to describe a slab deposited by relatively less wind as a storm slab rather than a wind slab. This may be due to the difference in how soft winds slabs and hard wind slabs behave, and the different mitigation strategies applied to each. Another reason could have been the desire to use explicit terminology to distinguish between an older buried wind slab and a newer surface slab associated with a windy storm. In these cases, applying the term storm slab to a softer or newer wind slab may have better supported operational communication.
If the goal is to reduce this inconsistency, a variety of options are possible. Some have proposed changes to avalanche problem types, which range from creating sub-problems describing each type of wind slab, to limiting the number of avalanche problems by creating an all-encompassing new-snow problem type. The Colorado Avalanche Information Center addresses inconsistencies with a flow chart created to provide guidance to forecasters (Lazar et al., 2012). Another approach could be to rely increasingly on proposed mitigation strategies when determining problem type (Statham, Haegeli, et al., 2018 p. 673-680). This is applicable since avalanche mitigation and terrain choice is the end goal of an avalanche forecast (Klassen et al., 2013). More options likely exist, and if we decide to address this inconsistency, there are a number of possibilities for moving forward.
CONCLUSIONS
Avalanche forecasters and researchers have previously noted inconsistent use of some avalanche problem types. This project highlights inconsistencies between storm slab problems and wind slab problems in professional communications. The conceptual model specifies that a wind slab problem results from both redeposition and preferential deposition, but during this study period many operations designated preferential deposition as a storm slab problem. Some operations also used storm slab to describe a new snow problem that included a wind slab. The inconsistencies in differentiating between storm slabs and wind slabs warrants further discussion in the Canadian avalanche community to assess if inconsistencies pose a problem that we should remedy.
ACKNOWLEDGEMENTS
Thank you to my committee members, Karl Birkeland, Jordy Hendrikx, and Eric Sproles, for supporting this research; and to Grant Statham and Ethan Greene for allowing me to interview them about terminology and the conceptual model. More thanks to the Castle Mountain patrol for inspiring my interest in wind slabs and for many productive forecast meeting debates. Finally, thank you to the InfoEx Advisory
Committee for approving this research.
REFERENCES
European Avalanche Warning Service. (2017). Typical avalanche problems. https://www.avalanches.org/wp-content/uploads/2019/05/Typical_avalanche_problems-EAWS.pdf
Hordowick, H. (2022). Understanding Avalanche Problem Assessments:A Concept Mapping Study with Public Avalanche Forecasters. Simon Fraser University.
Klassen, K., Haegeli, P., Consulting, A., Statham, G., & Canada, P. (2013). The role of avalanche character in public avalanche safety products. Proceedings of the International Snow Science Workshop, Grenoble, France
Lazar, B., Greene, E., & Birkeland, K. (2012). Avalanche problems and public advisories. The Avalanche Review, 30 (2). 14
Lehning, M., Löwe, H., Ryser, M., & Raderschall, N. (2008). Inhomogeneous precipitation distribution and snow transport in steep terrain. Water Resources Research, 44 (7).
Statham, G., Haegeli, P., Greene, E., Birkeland, K., Israelson, C., Tremper, B., Stethem, C., McMahon, B., White, B., & Kelly, J. (2018). A conceptual model of avalanche hazard. Natural Hazards, 90(2), 663–691.
Statham, G., Holeczi, S., & Shandro, B. (2018). Consistency and accuracy of public avalanche forecasts in western Canada. Proceedings of the International Snow Science Workshop, Innsbruck, Austria