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Avalanche Forecast

Archived

Dec 30th, 2022–Dec 31st, 2022

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells, Dogtooth, East Purcell.

Triggering large persistent slab avalanches is a serious concern this weekend. Conservative terrain choices are strongly recommended.

Confidence

Moderate

Avalanche Summary

We are coming out of a period of significant avalanche activity in the Purcells, with many notable large persistent slab avalanches. Buried weak layers have been very easily triggered with explosives, with several size 2 to 3 slabs reported on Tuesday, Wednesday, and Thursday. These avalanches have mostly been in alpine terrain and have propagated widely across terrain features. Over this period, humans testing smaller slopes have triggered several size 1 to 1.5 storm slabs in the top 40 cm of snow. There have also been some very large human triggered avalanches west of this region, where the weak layers are buried deeper.

Looking ahead to the weekend, triggering large persistent slab avalanches will remain a serious concern in this region.

Snowpack Summary

Roughly 40 cm of recent snow has settled into a slab. This snow may contain a rain crust at elevations below 2000 m. The additional load of this recent snowfall has stressed multiple weak layers including:

  • A facet and surface hoar layer directly beneath the recent snow

  • A surface hoar layer buried in mid-December that is 40 to 60 cm deep

  • Weak faceted snow and decomposing crusts near the bottom of the snowpack (ranging from 70 to 150 cm deep)

All of these layers have shown signs of instability over the past few days and need more time to strengthen.

Weather Summary

Friday night

Light snowfall with trace accumulations, 30 to 40 km/h wind from the southwest, treeline temperatures around -10 °C.

Saturday

Cloudy with very light snowfall, trace accumulations, 20 to 30 km/h wind from the west, treeline temperatures around -8 °C.

Sunday

Mix of sun and cloud, no precipitation, 20 km/h wind from the northwest, treeline temperatures around -8 °C.

Monday

Mostly sunny, no precipitation, light wind, treeline temperatures around -10 °C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • Remote triggering is a concern, watch out for adjacent and overhead slopes.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.