Avalanche Forecast
Purcells
Previously cold snow below treeline will begin to warm Tuesday which may increase the likelihood of human triggered avalanches. Avoid steep open features, convexities and terrain traps at low elevation as temperatures warm in the afternoon.
Avalanche Problem 1: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Conservative terrain selection remains prudent in this time of change.
- Numerous large avalanches have run on these layers in well supported, treed terrain.
- Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.
Avalanche Summary
On Sunday a natural persistent slab avalanche cycle was observed on south and southeast facing terrain between 2100 m and 2500 m, producing avalanches to size 2.5. These were running on both the early January and mid-December weak layers. Loose wet avalanches were reported on southwest, south and southeast facing terrain between 1700 m and 2800 m to size 2.5.
On Saturday control work produced storm slab avalanches to size 2 on south and southeast facing alpine features. A cornice failure was also reported from a south/southeast facing alpine feature.
On Friday we received reports of a widespread natural avalanche cycle to size 3.0 that had occurred during the previous 48 hour period. It's suspected that these avalanches initially failed on the early January interface before stepping down to the mid-December and possibly late November weak layers.
Some themes that are emerging from recent activity in the Purcells and neighboring regions include accidental and remote triggering, 'step down' release types, releases on surprisingly low angle, supported terrain, as well as wide fracture propagations.
Snowpack Summary
The last series of storms left a variable 10 to 60 cm of snow in their wake. Moderate to strong southwest winds previously formed wind slabs in upper elevation terrain.
This snowpack is currently quite complex; there are three Persistent Weak Layers (PWL) that we are monitoring.
The first PWL is known as the early January interface, it is 10 to 60 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects. It is thought to be widespread at all elevations bands and has produced numerous recent large avalanches.
The next PWL is the mid-December facet/surface hoar/crust combination that is now buried 40 to 100 cm deep and is most problematic at and below treeline. This interface is not thought to be present in the alpine.
The most deeply buried PWL is the late November rain crust. It is found 90 to 150 cm below the surface. This layer is considered dormant but it may be triggerable in thin spots around variable snowpack areas in the alpine.
Weather Forecast
The ridge that setup the recent inversion will shift east as an offshore front makes landfall with the south coast Tuesday morning. The interior ranges will remain mostly dry with only some light precipitation on Tuesday. A stronger Pacific storm will hit the south coast on Wednesday morning with precipitation spilling into the interior on Wednesday afternoon. Light snowfall is expected to continue through Thursday.
TUESDAY: Increasing cloud cover, freezing level initially at valley bottom rising to around 1000 m in the afternoon. Light to moderate southwest wind, trace of snow in the afternoon, trace of snow possible Tuesday night.
WEDNESDAY: A few clouds in the morning, cloud cover increasing throughout the day, freezing level beginning at valley bottom rising to 2000 m in the afternoon, moderate southwest wind, trace of snow possible. 2 to 8 cm of snow possible at upper elevations Wednesday night.
THURSDAY: Overcast, freezing level beginning around 1900 m, lowering to around 1600 m in the afternoon, moderate southwest wind, 2 to 10 cm of snow possible at upper elevations.
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Alpine temperatures are expected to remain warm Monday night and above freezing temperatures are expected between valley bottom and 1000 m Tuesday, which may increase the likelihood of human triggered avalanches in unusual locations at low elevation.