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Avalanche Forecast

Archived

Feb 18th, 2021–Feb 19th, 2021

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

Regions

Lizard-Flathead.

Don't let your guard down just yet! Slabs on the late January persistent weak layer continue to be easily triggered by skiers and riders. Recently reported avalanches reach up to size 2.5 and are definitely large enough to bury, injure or kill a person. 

Confidence

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

Weather Forecast

THURSDAY NIGHT: Mostly cloudy, 5-10 cm new snow, moderate southwest wind, temperature low -12 C.

FRIDAY: Cloudy, up to 5 cm new snow, moderate southwest wind, temperature high -7 C.

SATURDAY: Mix of sun and cloud, trace of new snow, light to moderate southwest wind, temperature high -5 C.

SUNDAY: Mostly cloudy, 10-15 cm new snow, moderate southwest wind, temperature high -2 C, freezing level rising to 1500 m.

Avalanche Summary

More reports of easily triggered persistent slab avalanches came in on Wednesday, including these large avalanches at treeline on Mt. Fernie. These new reports highlight the persistent slab problem has not gone away yet. There were also several small loose dry avalanches which were triggered by skiers. 

On Tuesday, a natural cornice failure triggered a small avalanche on the slope below in the alpine. 

On Monday, skiers triggered size 1 avalanches at treeline and in the alpine on northeast aspects. One avalanche was triggered from a short distance away (see this MIN report). On Sunday skiers triggered avalanches up to size 2.5 on open features at the treeline/alpine interface in the Lizard Range and Tunnel Creek. These failed on the late January persistent surface hoar layer (see this MIN as an example). 

On Thursday and Friday there were human and remote triggered avalanches up to size 2 reported. Locations like Mount Fernie, 2000, Liverwurst and McDermid were a few location examples. Some avalanches were triggered from a distance away. 

Snowpack Summary

Surface faceting and surface hoar growth continues with clear nights and cold temperatures. Below 1600 m a hard melt-freeze crust is underneath 20-40 cm of recent snow. 

A persistent weak layer lurks 40-70 cm below the surface. In some places it consists of surface hoar, in other places just facets, or crust/facet combinations. This weak interface has been responsible for the majority of recent avalanches. 

A solid mid-pack sits above a deeply buried crust and facet layers near the bottom of the snowpack (150-200 cm deep), which is currently unreactive. 

Terrain and Travel

  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.

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.