Avalanche Forecast
Purcells
Wind slabs, persistent slabs and weak cornices make for a complex snowpack. I'd continue to choose conservative terrain and limit my exposure to overhead hazards.
Avalanche Problem 1: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Avoid steep convexities or areas with a thin or variable snowpack.
- Avoid the runout zones of avalanche paths. Very large avalanches have been running full path.
- Pay attention to overhead hazards like cornices which could easily trigger persistent slabs.
Avalanche Problem 2: Wind Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
New wind slabs may especially reactive and slow to gain strength due to a mix of underlying weak surfaces. Watch for new wind slab development and reverse loading due to forecast strong northwest winds on Thursday.
Travel and Terrain Advice
- Be cautious as you transition into wind affected terrain.
- If triggered the wind slabs may step down to deeper layers resulting in large avalanches.
- Avoid travelling in areas that have been reverse loaded by winds.
Avalanche Summary
On Sunday there were a few reports of naturally triggered size 3 persistent slab avalanches on south to northwest facing alpine slopes. These avalanches are suspected to have failed due to wind loading and/or solar inputs. Also, explosive control work continued to produce large, deep avalanches up to size 3 on north to southeast aspects above 2200m. On the same day, a skier is believed to have remotely (from a distance) triggered a size 1.5 avalanche on a northwest aspect at 2800m that failed on the early January layer.
On Monday and Tuesday, explosives control continued to trigger persistent slab avalanches to size 3.5 in mainly north to west facing alpine terrain. Although deeper, persistent avalanche activity is becoming less frequent, these avalanches point to the continued reactivity and destructive potential of these layers.
At the time of publishing this bulletin there were no new observations from Wednesday, although I'm sure there was evidence of a natural wind slab cycle in response to new snow and wind on Tuesday night.
Snowpack Summary
By Wednesday morning up to 15cm of new snow had fallen. I suspect moderate southwest winds shifted these accumulations into deeper, reactive slabs in wind-exposed terrain. These accumulations overlie a mix of older wind slabs in exposed higher elevation terrain, a sun crust on steep solar aspects and surface hoar on sheltered slopes.
Below the snow surface several persistent weak layers make up a troublesome snowpack which is not tolerating recent storm loads. In the top 1-2 m of the snowpack, two surface hoar/ crust layers buried in January can be found. Expect to find one or other of these on all aspects and elevations.
Deeper in the snowpack (around 200 cm deep) is a facet/crust/surface hoar layer from December, most prevalent at and below treeline.
Near the base of the snowpack is a crust/facet combo, most likely to be triggered from thin spots in the alpine.
All of these layers have produced large avalanches recently. The wide distribution and ongoing reactivity of these layers suggests that avoidance through choosing simple terrain is the best strategy.
Weather Forecast
Thursday: Mainly clear skies / Moderate to strong northwest winds / Alpine temperature of -18
Friday: Mix of sun and cloud / Moderate to strong westerly winds / Alpine temperature of -12
Saturday: 3-8cm of new snow / Light and variable winds / Alpine temperature of -14
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Several troublesome layers exist in the snowpack and may be reactive to large triggers such as a cornice or storm slab release. Human triggering may also be possible in shallow or thin, rocky areas.