Avalanche Forecast
Purcells
Fresh shallow wind slabs and sluffing should be manageable Saturday, but we can't forget about the potential for large human triggered persistent slab avalanches that seem to be most reactive on south facing terrain at upper elevation.
Avalanche Problem 1: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Pay attention to overhead hazards like cornices which could trigger persistent slab avalanches.
- Use caution around sheltered steep or convex slopes where buried surface hoar may be preserved.
- Minimize exposure to steep south-facing slopes where recent storm snow overlies a crust.
Avalanche Problem 2: Storm Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
5 to 10 cm of new snow is expected Saturday accompanied by southerly wind. Watch for changing conditions, widespread sluffing and the formation of fresh shallow slabs which will likely be most problematic immediately lee of ridge crest.
Travel and Terrain Advice
- Be cautious of sluffing in steep terrain.
- Saturday's storm snow will hide stubborn old wind slabs that may remain sensitive to triggering.
- Be especially careful with wind loaded pockets near ridge crests and roll-overs.
Avalanche Summary
On Thursday a skier accidently triggered a large persistent slab avalanche on a south facing feature at 2600 m. The size 2 avalanche had a crown 60 cm in depth and failed on the mid March Persistent Weak Layer (PWL). Explosive control work produced cornice failures and storm slabs to size 2.5 on a variety of aspects in the alpine.
Reported avalanche activity was limited on Wednesday; a cornice failure produced a size 2 soft slab in steep rocky terrain on an unknown aspect and sluffing was observed in steep terrain to size 1.
On Tuesday a small avalanche was triggered by a skier on a northeast facing slope at 2200 m. The size 1 avalanche failed on the late March interface. Control work produced avalanches from size 2 to 3 on north/northeast facing features above 2400 m that likely ran on the mid March interface as well.
Last Saturday a skier triggered a large slab avalanche (size 2.5) on a south aspect at 2600 m. The avalanche failed on a buried sun crust 150-200 cm below the surface. Another large (size 2.5) wind slab released naturally from a steep southeast-facing alpine slope in the St. Mary's area.
Snowpack Summary
Warming temperatures and time have helped to settle the upper 30 to 40 cm of ageing storm snow and have formed a weak surface crust on all aspects aside from true north below 2000 m. This crust extends up to around 2400 on south facing slopes. On north facing slopes above 2000 m, temperatures have remained cold and the snowpack is still quite "winter-like."
The main concern is the mid-March Persistent Weak Layer (PWL) which is now 40 to 90 cm below the surface. This layer is composed of a crust on all aspects at low elevations. At upper elevations the layer presents as a crust on solar aspects (those that face south and west) and buried surface hoar on polar aspects (those that face north and east). The reactivity of this interface has been steadily decreasing, but it continues to produce large avalanches and the potential for deep 'step down' avalanches cannot be dismissed. Solar aspects are expected to be the most suspect as the freezing level begins to rise over the next few days.
Deeper persistent weak layers from January and December are still being reported by professional observers, but are generally considered dormant. Facets also linger at the base of the snowpack.
Weather Forecast
The struggle between "spring-like" and "Arctic-like" will continue for a few more days. Saturday's storm should deliver a good shot of snow with the highest amounts concentrated in the southern portion of the region. A more spring-like pattern begins to take shape on Sunday with freezing levels creeping towards 2200 m by Monday.
SATURDAY: Overcast, freezing level beginning near valley bottom rising to around 1500 m, light to moderate south/southwest wind, 4 to 10 cm of snow possible.
SUNDAY: Broken cloud cover, freezing level beginning around 500 m rising to about 1800 m, light westerly wind, trace of snow possible.
MONDAY: Scattered cloud cover, freezing level beginning around 1500 m rising to about 2200 m, light to moderate west/southwest wind, trace of snow possible.
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Weak crust and surface hoar 60 to 100 cm below the surface has shown prolonged reactivity since the last storm. South facing features have been the most reactive as of late, and this trend is expected to continue as temperatures rise next week.