Introduction
As the popularity of backcountry skiing continues to grow, particularly in the early winter months, we often hear questions about how and when we transition from General Avalanche Information to a five-scale daily avalanche forecast. At MWAC, we believe transparency in our forecasting process is important to serving the backcountry community. Our goal is to provide clear, actionable information to help users understand the risks and make informed decisions. It’s important to emphasize that no avalanche information product can replace careful decision making, experience and personal responsibility in avalanche terrain. This document aims to explain the factors that influence our approach to early season avalanche forecasting and why we rely on the General Avalanche Information product during this period.

Sparse Snow Coverage
During November and often well into December, a building snowpack within the MWAC forecast area is typically limited and localized, accumulating primarily in wind-loaded or sheltered terrain features, such as the Cutler River Drainage. Tuckerman Ravine, covering just 0.25 square miles, often sees greater snow depth due to consistent west winds and an ideal fetch above, which efficiently transports and deposits snow into the ravine. While this can make Tuckerman Ravine develop a deeper snowpack faster and potentially hazardous for avalanches, the broader forecast area of 106 square miles generally experiences sparse and isolated snow distribution. The North American Public Avalanche Danger Scale considers the distribution of avalanche problems within the forecast area as an integral part of assigning danger ratings. Sparse snow coverage often results in avalanche problems that are isolated and confined to a few terrain features rather than widespread across the forecast area. Assigning a higher avalanche danger rating for such isolated problems would misrepresent the overall risk across the broader forecast area. The North American Public Avalanche Danger Scale is designed to communicate risk across a large forecast area, not for single slopes or small terrain features. Applying it to isolated problems within a forecast area would constitute a misapplication of this messaging tool, potentially leading to confusion or misunderstanding among backcountry users.

In these conditions, offering General Avalanche Information is a more effective tool. It allows forecasters to highlight specific localized hazards, such as those found in Tuckerman Ravine, without overstating the risk for the entire zone. The General Avalanche Information’s broader temporal coverage and less rigid structure make it better suited for addressing early season variability and uncertainty.

Variable Weather Conditions
Early in the season, weather patterns tend to fluctuate significantly and often, with freezing levels rising and falling. These fluctuations, common during early season transitions between autumn and winter as atmospheric systems shift and stabilize, greatly influence total snowfall and snowpack development. The inconsistent freeze elevation across the forecast area introduces high levels of uncertainty for avalanche forecasting, making it challenging to accurately assess and communicate potential hazards. Since the snowpack remains thin overall, early season rain events can further reduce the size and connectivity of potential avalanche paths, limiting the overall potential for more widespread avalanche hazard. This combination of variability and uncertainty highlights the difficulty of producing more specific and accurate avalanche forecasts during this period.

Limited Observations
In early season conditions, when the snowpack is thin, fewer people venture into backcountry areas outside of Tuckerman Ravine, leading to a reduction in reports of avalanches or instability from those locations. This decrease in observations, combined with the logistical challenges of visiting multiple areas frequently to verify conditions, makes it difficult to sustain an accurate five-scale daily avalanche forecast. By design, the five-scale avalanche forecast (Low, Moderate, Considerable, High, Extreme) is intended to provide hazard assessments for a specific 24 hour period, requiring consistent updates to remain reliable. Given the limitations in early season data and observation frequency, the broader temporal scope and slightly less specific nature of the General Avalanche Information offers the most accurate and practical depiction of current conditions. This approach ensures avalanche forecasters can maintain accuracy and provide actionable guidance despite the inherent challenges of early season forecasting.

Historical Context
The decision to transition from General Avalanche Information to a five-scale avalanche forecast is driven by snowpack development rather than a specific calendar date. Historically, this change occurs when the snowpack becomes sufficiently developed across the forecast area to warrant more detailed hazard assessments. For reference, recent transitions include:

  • January 4 during the 2019/20 season
  • January 16 in 2020/21
  • December 18 in 2022/23 *
  • January 12 in 2023/24

An exception occurred in the 2022/23 season when a very large early season storm followed by sustained cold temperatures led to the transition on December 18.

It is important to note that before 2018, the MWAC forecast area was limited to just the Cutler River Drainage. While this reduced the complexity of distribution factors when applying the North American Public Avalanche Danger Scale, a decision was made to expand the forecast area to better serve a growing backcountry skiing community venturing into more diverse terrain. This expansion reflects a commitment to providing relevant information across a wider geographic area, but also shifts some of the responsibility to users for making slope specific avalanche hazard assessments. This approach aligns MWAC’s operations with those of other North American avalanche centers, emphasizing the importance of individual decision making in avalanche terrain.

Applying General Avalanche Information
The General Avalanche Information product is particularly valuable early in the season because it provides actionable information without overstating the hazard level. Like the five-scale avalanche forecast, it addresses the key questions: “What is it? Where is it? What can readers do about it?” However, the General Avalanche Information also acknowledges that much of the forecast area lacks sufficient snow for widespread avalanche concerns and avoids misapplying the North American Public Avalanche Danger Scale to conditions that are highly variable and localized. When there is a potential for avalanches in the coming days, this information is communicated clearly and explicitly. However, the absence of a daily overall danger rating prevents confusion that could arise from the isolated and uneven nature of early season conditions across the forecast area. The General Avalanche Information strikes an effective balance, offering actionable guidance while accounting for the inherent uncertainties and challenges of forecasting during the early season with an underdeveloped snowpack.

 

References

Conceptual Model of Avalanche Hazard
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.

  • This framework provides a probability-consequence approach to assessing avalanche hazards, forming the foundation for modern avalanche forecasting.

North American Public Avalanche Danger Scale
Statham, G., Haegeli, P., Birkeland, K., Greene, E., Israelson, C., Tremper, B., Stethem, C., McMahon, B., White, B., & Kelly, J. (2010). The North American Public Avalanche Danger Scale. Proceedings of the 2010 International Snow Science Workshop.

  • Defines the five-scale avalanche danger scale (Low, Moderate, Considerable, High, Extreme) used across North America for public avalanche forecasts.

General Avalanche Information Guidelines
USDA Forest Service. National Avalanche Center (Internal Guidance Document).

  • Describes the rationale and best practices for issuing General Avalanche Information bulletins, emphasizing their value during early-season conditions.

Historical Transitions in MWAC Operations
MWAC Internal Reports. (2010–2024).

  • A summary of MWAC’s operational changes, including the expansion of the forecast area and historical transitions between General Avalanche Information and five-scale avalanche forecasts.

Rain-on-Snow Events Research on Mount Washington
Mount Washington Observatory. Overview of Rain-on-Snow Research.

  • Examines the frequency and impact of rain-on-snow events, which are closely linked to fluctuating freezing levels.