Avalanche Forecast
Purcells
New storm snow will add to the complexity of an already tricky snowpack.
Avalanche Problem 1: Wind Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- If triggered the wind slabs may step down to deeper layers resulting in large avalanches.
- Be cautious as you transition into wind affected terrain.
- Give cornices a wide berth when travelling on or below ridges.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
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.
Travel and Terrain Advice
- Avoid the runout zones of avalanche paths. Very large avalanches have been running full path.
- Avoid steep convexities or areas with a thin or variable snowpack.
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, explosives control continued to trigger persistent slab avalanches to size 3.5 in mainly north 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.
Looking forward, expect a new round of wind slab activity in response to new snow and wind on Tuesday night. A surface avalanche in motion has the potential to re-activate deeper, more destructive persistent weak layers.
Snowpack Summary
By Wednesday morning I would expect up to 15cm of new snow. Forecast strong ridgetop winds are expected to redistribute the new snow into much deeper, reactive slabs in wind-exposed terrain. The new snow will overlie older wind slabs in higher elevation terrain, and a melt-freeze crust below 1700m.
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
Tuesday Night: 5-15cm of new snow / Strong southwest winds / Alpine temperature of -9
Wednesday: Generally overcast skies / Moderate northwest winds / Alpine temperature of -13
Thursday: Mix of sun and cloud with scattered flurries / Light and variable winds / Alpine temperature of -17
Friday: Overcast skies / Moderate northwest winds / Alpine temperature of -10
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New snow and wind will add to recently formed wind slabs and cornices on lee slopes. On solar aspects and at lower elevations the new snow will cover an old crust and may be touchy.