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Avalanche Forecast

Archived

Feb 26th, 2021–Feb 27th, 2021

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

Regions

Yukon.

Continuous snowfall and strong to extreme winds will maintain dangerous avalanche conditions in the region over the weekend. Stick to low angle, low consequence slopes and avoid exposure to overhead avalanche terrain.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

FRIDAY NIGHT: Becoming cloudy with isolated flurries and a trace of new snow, increasing in the morning. Winds becoming strong southwest.

SATURDAY: Cloudy with continuing flurries bringing up to 5 cm of new snow, continuing overnight. Strong to extreme southwest winds, continuing overnight. Alpine high temperatures around -7.

SUNDAY: Cloudy with continuing flurries bringing 5-10 cm of new and 2-day snow totals to 10-20 cm, continuing but easing overnight. Strong southwest winds, easing over the day. Alpine high temperatures around -10.

MONDAY: Cloudy with continuing flurries bringing about another 5 cm of new snow and 3-day totals to 15-25 cm, continuing again overnight. Strong to extreme southwest winds. Alpine high temperatures around -11.

Avalanche Summary

Evidence of a recent storm slab avalanche cycle to size 2 has been observed in the days since last weekend's storm. Heightened avalanche danger has been maintained through the week by subsequent bouts of elevated southwest wind. Slopes that have not avalanched remain a serious concern for human triggering potential, particularly where wind loaded.

This MIN report outlines conditions at the end of last weekend. 

Snowpack Summary

60+ cm of snow from last weekend's storm rests on heavily wind affected and faceted snow in exposed terrain and perhaps a weak layer of surface hoar in sheltered areas, as reported here and here. 

Although our best weather stations in the region are currently down, we have observations from Wednesday and again on Thursday night that describe a mix of strong southwest and northeast winds have been redistributing our aging storm snow into new wind slabs in exposed terrain. More strong to extreme southwest winds are forecast for Saturday.

The lower snowpack is strong around White Pass, but inland areas like the Wheaton Valley likely have a thinner and weaker structure.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.

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.