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Avalanche Forecast

Archived

Mar 21st, 2022–Mar 22nd, 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 possible, human triggered probable.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

Avalanche danger will increase as temperatures rise throughout the day. There is uncertainty around how the snowpack will handle this warm-up. 

Choose increasingly conservative terrain as temperatures rise.

Confidence

Moderate - Uncertainty is due to the fact that cornice falls are particularly difficult to forecast. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

MONDAY NIGHT: Cloudy with isolated flurries. Light westerly winds. Freezing level around 1000 m.

TUESDAY: Partially cloudy. Light to moderate westerly winds. Freezing level rising to 2000 m.

WEDNESDAY: Mainly cloudy, light flurries. Moderate to strong westerly winds. Freezing level around 2000 m.

THURSDAY: Mainly clear. Light northwesterly winds. Freezing level around 1000 m.

Avalanche Summary

Several natural and one human-triggered wind slabs occurred in the northwest of the region on Sunday in the alpine and treeline (size 1-2.5).

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

10-20 cm of recent snowfall overlies last week's 30-70 cm of settled storm snow. 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. The lower snowpack is generally well bonded.

Terrain and Travel

  • The more the snowpack warms-up and weakens, the more conservative you`ll want to be with your terrain selection.
  • Use ridges or ribs to avoid areas of wind loaded snow.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Cornice failure may trigger large avalanches.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • 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.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.