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Avalanche Forecast

Archived

Jan 31st, 2022–Feb 1st, 2022

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

Regions

Purcells.

Recently formed wind slabs are expected to remain touchy on Tuesday, especially where they overlie surface hoar or a crust. Loose dry avalanches should be expected on steep slopes.

Confidence

Low - Uncertainty is due to the limited number of field observations.

Weather Forecast

Unsettled conditions are expected on Tuesday as Arctic air shifts southward. 

Monday Night: Partly cloudy, light to moderate NW wind, treeline temperature dropping to around -12 C°.

Tuesday: Periods of snow flurries up to 5 cm and sunny breaks both expected, light to moderate NW wind, treeline high around -8 C°.

Wednesday: A mix of sun and cloud, light variable wind, treeline high around -14 C°.

Thursday: A mix of sun and cloud, light to moderate variable wind, treeline high around -10 C°.

Avalanche Summary

On Sunday, small loose dry avalanches were reported from very steep terrain. On Saturday, a skier triggered a size 1.5 wind slab on an E aspect in the alpine which failed down 5-20 cm. 

Avalanche activity on the early-December weak layer described in the snowpack summary has been sporadic over the past few weeks. Prior to this most recent drought, we were tending to see reports of avalanches on this layer every few days. Most of the avalanches occurred at elevations around 2000 m. Possible natural triggers include daytime warming, warming from the sun, cornice falls, or smaller avalanches stepping down. Human triggering is most likely in steep, rocky slopes with a shallow or thin-to-thick snowpack.

Snowpack Summary

Up to 20 cm of new snow has buried a highly variable snow surface which consists of wind affected surfaces in exposed high elevation terrain, a melt freeze crust at lower elevations and on solar aspects extending into the alpine, and/or widespread surface hoar in sheltered areas which was typically 5-15 mm. 

A layer of surface hoar buried around January 24 is now down 30-40 cm. While this layer was not creating a problem prior to storm, it is yet to be determined how it will react to the additional snowfall. A crust and surface hoar interface buried around January 11 is now down around 50-60 cm. 

The most notable layer of concern in the snowpack is a facet/crust layer that formed in early December and ranges in depth between 80 cm along the shallower eastern Purcells and up to 150 cm in the western Purcells. This recent blog post goes into more details on this interface and why it is expected to continue to create problems for the rest of the season. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Be aware of the potential for loose avalanches in steep terrain where snow hasn't formed a slab.
  • Conditions may have improved, but be mindful that deep instabilities are still present.

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.

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.