Large Avalanche a Serious Reminder for Backcountry Users

23rd August 2023|1min

Yesterday, New Zealand Avalanche Advisory (NZAA) forecasters witnessed a very large size 5 avalanche up the Murchison Valley in Aoraki/Mt Cook National park.  

The avalanche released on Mt Phyllis and across the Aida Glacier, generating a huge cloud of snow and ice. The crown wall was estimated to be about 1.5km long and debris ran about 2km onto the flats.  

A size 5 avalanche is the largest classification of an avalanche size, with the potential to devastate the landscape and cause catastrophic destruction to anything in its path.  

While the exact trigger of the avalanche is unknown, NZAA forecasters who witnessed it say it could have been the collapse of an ice cliff. The avalanche ran on the present Persistent Weak Layer (PWL) which is prevalent throughout much of the Southern Alps.  

A PWL is a weak layer in the snowpack that resists forming a strong bond to neighbouring grains in the snowpack over an extended time period. 

The PWL continues to be a real concern in the South Island backcountry, with NZ Mountain Safety Council (MSC) and NZAA reporting on the issue in many of the thirteen avalanche forecasting regions since the start of the season.  

This large avalanche is a serious reminder for backcountry users to not underestimate the PWL and snowpack conditions, says MSC Chief Executive Mike Daisley.  

“Conditions this winter are highly unusual, and with the multiple large avalanches we have seen this season it shows the PWL warrants significant respect and requires a very high degree of caution.” 

“Avalanches may be larger, and travel further than you expect and can also be triggered from below. Needless to say, a very cautious, calculated approach is vital when in the backcountry.” 

“Our advice is to pay very close attention to the avalanche forecasts, follow the terrain and travel advice provided and be very cautious and calculated in your movements. Triggering the PWL to fail will likely generate a very large avalanche.”  

The NZAA forecasters will update the 13 regional forecasts at on a daily basis, or as conditions change and new information becomes available.     

Are you avalanche ready? If you’re heading into the New Zealand back country this winter, be sure to:   

Submit a public observation here.   

Hikers and trampers can find the avalanche forecast under alerts on Plan My Walk.