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Avalanche Forecast

Archived

Mar 4th, 2021–Mar 5th, 2021

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

Regions

North Columbia.

Cool cloudy weather has likely reduced the avalanche danger, but heightened conditions still exist on steep and wind loaded terrain.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

Cloudy, windy, and some light snowfall on the way.

THURSDAY NIGHT: Cloudy, moderate southwest wind, freezing level drops to 1200 m.

FRIDAY: Cloudy with some isolated flurries, strong south wind, freezing level climbs to 1800 m with treeline temperatures around -1 C.

SATURDAY: 5-10 cm of new snow by the morning then some sunny breaks in the afternoon, light southwest wind, freezing level climbs to 1300 m with treeline temperatures around -4 C.

SUNDAY: Cloudy with some isolated flurries, light south wind, freezing level around 1200 m with treeline temperatures near -5 C.

Avalanche Summary

Preliminary reports suggest warm weather on Thursday resulted in many small wet loose avalanches on steep south-facing terrain. On Wednesday, in addition to wet loose avalanches, there were a few human triggered slab avalanches showing the impacts of warming on slab avalanche conditions. Two notable incidents were on south-facing slopes in Glacier National Park, including a size 3 wind slab avalanche in a steep alpine couloir (MIN report) and a size 1.5 slab avalanche in below treeline terrain (MIN report).

Persistent slab avalanche activity has dramatically declined in the past week, with the last reports on Feb 24 (two size 3 explosive triggered avalanches on the late January weak layer) and Feb 26 (a size 2 human triggered avalanche in a cutblock on the February facet layer). At this point we are uncertain whether the recent warming has irritated the lingering persistent weak layers or whether they have gone dormant.

Snowpack Summary

Cool cloudy weather will likely form a surface crust on slopes that warmed up on Thursday, most likely all aspects below 2000 m as well as south-facing slopes at higher elevations. Lower elevations will likely have moist surfaces and high shaded terrain will have a mix of soft and hard wind slabs. The lower snowpack has been strengthening with reactivity on persistent weak layers on the decline. The two layers of concern have been a layer of facets that was buried in mid-February (60-100 cm deep) and a layer of surface hoar and/or crusts that was buried in late January (80-120 cm deep).

Terrain and Travel

  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • If you are increasing your exposure to avalanche terrain, do it gradually as you gather information.

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.