Hi there,
Before we get into this report, we wanted to share why we wrote it up, and what we hoped to achieve with it.
As you well know, backcountry conditions can be uncertain and you can always, always learn something from them. In writing this report, we hoped to do a few things:
Further our shared understanding of the NSW and Victorian backcountry by documenting and analysing it.
Share what we have learned so far, and most importantly ask you if you had seen this sort of thing before, or if you had any other ideas on what might have happened to cause this big event
We’ve kept the report itself fairly dry - we always try to be as objective as possible. A big thanks to all the people who have shared with us their thoughts.
If you have any questions or comments, especially if you have some ideas or have seen similar events around the mountains before, we’d be grateful to hear from you. You can do this in the comments section below or via email.
We hope you enjoy the read!
Craig Sheppard
Alex Sinickas
MSC Forecast Team
THE EVENT
Three large avalanches were reported to MSC on Tuesday 28 June, 2022. The avalanches occurred in the Main Range of NSW in the Sentinel Peak area. For the purpose of this article we have named the avalanches as Sentinel 1, Sentinel 2 & Sentinel 3.
As we discuss in the conclusion, rain with warmer (+0º) temperatures creates stress on the snowpack and means it’s time to avoid avalanche terrain.
Image provided by Rohan Kennedy
These avalanches were not witnessed directly but rather were noted by a party of two ski tourers in the area some days later. Although the precise timing of the occurrence(s) is unknown, it is estimated that the avalanches took place on Friday the 24th June during the rain event of that day.
LOCATION
The following Google Earth images show the location of the avalanches adjacent to Mount Sentinel.
Image provided by Jake Iskov
Screenshot from Google Earth: Mount Sentinel in Main Range of NSW
Screenshot from Google Earth: South Chutes next to Mount Sentinel
Screenshot of Google Earth: Looking west at the South Chutes, Sentinel slides 1 (Yellow), 2 (Orange) and 3 (Green). Pink line shows the possible extent of the avalanche flow, however the actual distance is unknown.
Avalanche Characteristics
Through speaking with the reporting party, analysing the avalanche photos, using Google Earth and estimating snow densities, given the nature of the snowpack, we were able to estimate the size of the three avalanches. Table 1 below contains the characteristics of the slides. Keep in mind that a Size 3 avalanche “Could bury and destroy a car, damage a truck, destroy a wood frame house, or break a few trees” (Canadian Avalanche Association Observation Guidelines and Recording Standards for Weather, Snowpack and Avalanches).
Table 1: Characteristics of Sentinel Slides 1, 2 and 3
(Note: these are best estimates that assume areas, depth and density based on Google Earth, field reports without analysis at the site).
WEATHER PRIOR TO THE EVENT
A summary of the weather details during the month of June can be found in Table 2 (see below).
On Friday the 24th of June the Main Range experienced the third rain event of the season. The three rain events were:
2022-06-16 (14mm precip, 0cm snow)
Break: Cold temps, no snow
2022-06-21 (33.8mm precip, 12cm snow)
2022-06-24 (11mm precip, 0cm snow)
Rain on snow events are one of the leading causes of winter avalanches in maritime snowpack climates (Stimberis & Rubin 2011). Although this fact is fairly well known, due to the nature of how water permeates the snowpack, and spatial variability, accurate forecasting is difficult. There is also a correlation of glide crack avalanches occurring with rain on snow events, but again, the timing of these events are hard to directly relate to the timing of the rain event.
A possible contributing factor to these natural avalanches was the rise in temperature that accompanied the rain. As noted on 24 June, the minimum temperature was above 0 degrees. As such, it is likely that the snow surface did not experience a refreeze and therefore the rain could penetrate further into the snowpack.
Table 2. Source: Bureau of Meteorology - Perisher AWS
WHAT LIKELY HAPPENED?
Hypothesis #1
These avalanches were glide crack releases. An important factor with glide crack releases is the amount of moisture and water at the ground layer (Mitterer, Christoph, and Schweizer 2012). The ground surface is moist or wet and often the ground isn’t frozen. With these conditions the amount of friction between the ground and the snowpack is decreased.
A contributing factor to the amount of ground friction is ground roughness. The ground surface where these avalanches occurred is predominantly grass. As such, the ground surface would have been more slippery and therefore less friction at the base.
Prior to the first snowfall this year it was quite a wet summer and autumn. Indeed, the region experienced 132% of its typical rainfall (BOM comparison of climate statistics and this year's measured amounts). Also, the first snowfall came early and deep thus insulating the ground and preventing it from freezing. Unfrozen warm ground likely leads to more ground moisture available to affect the snowpack/ground bond.
Another distinguishing factor of a glide crack release is that the snow is not necessarily isothermal, but as discussed above they are more a result of the ground snow interface characteristics. Snow profiles leading up to this event did not indicate an isothermal snowpack, thus supporting this hypothesis.
Hypothesis #2
These avalanches were wet slab releases. Wet slabs are often triggered by rain on snow events (Baggi & Schweizer 2009). With the rain, the snowpack loses strength as the water permeates through. Also, the rain is a significant stress to a snowpack that is already weakening. As the name suggests, a wet snow avalanche has loose water in the snowpack and therefore if not all, some of the snowpack is isothermal (all 0 degrees celsius).
LESSONS FROM THIS EVENT
Both wet slab and glide release avalanches are harder to predict than dry snow avalanches. However, regardless of the type, these were very large destructive avalanches!
What is the lesson here? Two things. First, rain on snow adds rapid heat and significant stress to the snowpack and generally speaking, snowpacks do not like rapid change. So when there is rain on snow with warm temperatures, it is time to avoid avalanche terrain. Secondly, when dealing with a phenomenon that is harder to predict (for example like glide cracks or rain on snow events), there is more uncertainty. When dealing with uncertainty, it is important to increase caution and again… practice avoidance!
REFERENCES
Baggi, S., Schweizer, J. Characteristics of wet-snow avalanche activity: 20 years of observations from a high alpine valley (Dischma, Switzerland). Nat Hazards 50, 97–108 (2009). https://doi.org/10.1007/s11069-008-9322-7
Mitterer, C., Schweizer, J. Towards a better understanding of glide-snow avalanche formation. International Snow Science Workshop ISSW (2012).
Stimberis, J., & Rubin, C. Glide avalanche response to an extreme rain-on-snow event, Snoqualmie Pass, Washington, USA. Journal of Glaciology, 57(203), 468-474 (2011). doi:10.3189/002214311796905686