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Avalanche Forecast

Archived

Dec 6th, 2022–Dec 7th, 2022

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

Regions

Purcells, Dogtooth, East Purcell.

Fresh and reactive wind slabs may exist at treeline and above. Watch and feel for signs of instability such as whumpfing, cracking, and recent avalanches.

Concern remains for the persistent weak layers near the base of the snowpack where a small avalanche could possibly 'step down' initiating a larger one. With uncertainty, the best approach is choosing conservative terrain.

Confidence

Low

Avalanche Summary

Explosive control on Sunday produced several small (size 1-1.5) persistent slab avalanches from North and Northwest facing treeline. These avalanches failed on the Mid-November surface hoar layer.

On Saturday, a small (size 1) skier triggered persistent slab avalanche occurred on a northwest aspect at treeline. A larger (size 2) natural slab avalanche was also observed from a distance on an east-facing alpine slope.

Last week, a small (size 1) natural persistent slab avalanche was observed at treeline on a steep, south-facing road bank. It failed on the sugary, faceted layer 25 cm deep.

We have very few eyes out there. If you do get out please consider reporting in the MIN.

Snowpack Summary

Up to 5 cm of new low-density snow fell on Tuesday. Consistent and strong westerly winds may have been enough to blow around the new and old snow, forming fresh wind slabs in the alpine and treeline. In exposed terrain, some slopes have been scoured back to rock and in sheltered areas, 20-60 cm overlies a generally weak and sugary (faceted) lower snowpack. The prolonged cold temperatures have weakened the generally thin snowpack, promoting faceting as a whole. A buried layer of surface hoar sits 25-60 cm deep and remains a layer of concern.

The overall height of snow is highly variable throughout the region with around 40-150 cm in the alpine.

Weather Summary

Tuesday Night

Cloudy with light snowfall, less than 5 cm of accumulation expected. Strong to extreme westerly winds at ridgetop. Temperatures steady at -15 C.

Wednesday

New snow up to 5 cm of accumulation is expected. Moderate to strong ridgetop wind from the southwest and temperatures slightly warmer near -12 C.

Thursday

Mix of sun and cloud and dribs and drabs for the remainder of the work week. Moderate ridgetop wind from the south-southwest and a daytime high of -10 C. Freezing levels 700 m.

Friday

Mix of sun and cloud. Southwesterly winds 30-60 km/h at ridgetop. Temperatures reach a daytime high of -8 C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

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.