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Avalanche Forecast

Archived

Feb 7th, 2021–Feb 8th, 2021

Alpine
Natural avalanches possible, human triggered probable.
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 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, human triggered possible.

Regions

Lizard-Flathead.

A weak layer of surface hoar lucks in the snowpack, problems like this are best managed with conservative terrain choices.

Confidence

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

Weather Forecast

SUNDAY NIGHT - Partly cloudy and isolated flurries / moderate to light northwest wind / alpine low temperature near -24

MONDAY - Cloudy with sunny periods and isolated flurries / light north wind / alpine high temperature near -18

TUESDAY - Sunny / light northwest wind / alpine high temperature near -20

WEDNESDAY - A mix of sun and cloud / light northeast wind / alpine high temperature near -26

Avalanche Summary

As natural avalanche activity tapers, wind slabs and persistent slabs may still be primed for triggering. These may propagate widely and can catch you by surprise even in low angle terrain. Moderate northerly winds are likely building fresh and reactive wind slabs which may be easy to trigger.

On Saturday riders continued to trigger slab avalanches 20-60 cm deep, documented in a handful of MIN reports (Trespass, Mammoth, Spicy, Different Day).

On Friday, there were numerous reports of natural and human triggered persistent slab avalanches up to size 2.5. Some of these were triggered remotely, as outlined in a MIN report that can be viewed here. There were also numerous natural and explosives triggered dry loose, and storm slab avalanches reported up to size 2. 

It has been a busy week for avalanche activity, with reports of natural, human, and explosives triggered storm and/or persistent slab avalanches up to size 2 every day for the past week. On Wednesday, persistent slab avalanche activity really picked up and has been a daily occurrence ever since.

Many thanks for all of the great MIN reports over the past week!

Snowpack Summary

20-65 cm of recent snow has formed a reactive slab on top of a persistent weak layer that consists of surface hoar, facets, and a crust that was buried in late January. This sits on top of a plethora of old snow surfaces: hard wind slab, wind-scoured areas, sastrugi, and isolated pockets of soft snow. Below 1800 m a hard melt-freeze crust is underneath the new snow.  

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

  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Be aware of highly variable recent wind loading patterns.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

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.