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Avalanche Forecast

Archived

Feb 9th, 2023–Feb 10th, 2023

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

Regions

Purcells, Dogtooth, East Purcell.

Persistent slab avalanches continue to be reported in this region. The message remains the same – a complex, and generally weak snowpack is best managed by continuing to stick to lower angled, low consequence terrain.

Confidence

Moderate

Avalanche Summary

On Wednesday there were two reports of natural deep persistent slab or persistent slab avalanches to size 3. These were viewed in the central part of the region and in the Dogtooth range. It seems both were triggered by heavy loads (cornice fall and wind slab avalanche stepping down) on north east and east aspects at 2500m.

There were also reports of wind slab avalanches to size 1.5 and 2.5 on north and east aspects in the alpine.

The possibility remains of triggering facets near the base of the snowpack, particularly on shallow, rocky slopes.

Snowpack Summary

20-50 cm of snow since late January is settling, or has been redistributed by moderate southwest wind. This overlies another layer of windslab, or surface hoar sitting on facets or a thin sun crust.

50-70 cm below the surface, the mid-pack contains a layer buried in early to mid January that remains a concern. It mostly exists as surface hoar, but facets or a crust may also be present.

A layer from late November is found 75 to 150 cm deep and is part of a generally weak, faceted snowpack.

Snowpack depths range between 80 and 180 cm at treeline, with the shallowest depths found on the eastern edge of the region.

Weather Summary

Thursday Night

Mainly cloudy with isolated flurries, accumulation 1-3cm. Moderate southwest ridgetop wind. Treeline temperature around -7 °C.

Friday

Mainly cloudy with scattered flurries, accumulation 3-5cm. Moderate to strong southwest ridgetop wind. Treeline temperature around -4 ºC. Freezing level rising to 1300m.

Saturday

Mainly cloudy with isolated flurries. Light to moderate west ridgetop wind. Treeline temperature around -5 ºC. Freezing level rising to 1200m.

Sunday

Mostly cloudy. Light to moderate west and southwest ridgetop wind. Treeline temperature around -6 °C. Freezing level around 1100m.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • Approach lee and cross-loaded slopes with caution.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Uncertainty is best managed through conservative terrain choices at this time.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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.