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Avalanche Forecast

Archived

Mar 20th, 2022–Mar 21st, 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, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

Crusts on solar aspect are becoming more the norm. Dry snow will exist on the more shaded aspects but watch for wind slabs at higher elevations.

Danger ratings are a step higher in the western part of the region around the Bugaboos where concern for persistent slabs is greatest.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

SUNDAY NIGHT: Mainly cloudy, light flurries. Light to moderate westerly winds. Freezing level dropping to 500 m.

MONDAY: Mainly cloudy, isolated flurries. Light to moderate westerly winds. Freezing level rising to 1500 m in the afternoon.

TUESDAY: Partially cloudy. Light to moderate southwesterly winds. Freezing level rising to 2000 m in the afternoon.

WEDNESDAY: Mainly cloudy, isolated flurries. Light to moderate southwest winds. Freezing level rising to 1500 m in the afternoon. 

Avalanche Summary

We have heightened concern about a developing persistent slab problem in some parts of the Purcells (read more in this blog).

Several notable human-triggered slab avalanches occurred last week, suggesting that the recent snow is bonding poorly to the underlying layers. These occurred on a range of aspects above 2000 m, with the most notable reactivity observed on south-facing aspects. Most slabs were small in the top 20 to 30 cm of snow (size 1), but one larger 70 cm thick slab was remotely triggered from below by a group of skiers (size 2). This avalanche occurred on a south aspect at 2250 m.

Snowpack Summary

Last week's 30-70 cm of settled storm snow has been redistributed by westerly winds into wind slabs in lee areas. Snow depths taper significantly with elevation, with moist and crusty surfaces below 1800 m and on south aspects into the alpine.

In some areas, last week's snow is well bonded to old crust layers, while in other areas the snow has settled into a reactive slab above weak layers around the crusts. At this point, it appears persistent slab problems could be developing in the deeper snowpack areas along the western side of the range, while the eastern side more likely has thinner wind slabs. The lower snowpack is generally well bonded.

Terrain and Travel

  • Carefully evaluate steep lines for wind slabs.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Uncertainty is best managed through conservative terrain choices at this time.
  • Cornice failure may trigger large 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.