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Avalanche Forecast

Archived

Jan 18th, 2021–Jan 19th, 2021

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

Regions

Lizard-Flathead.

Winds will impact any remaining loose snow, be wary of wind slabs. Dig down and investigate deeper layers in the snowpack before committing to big lines.

Confidence

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

Weather Forecast

MONDAY NIGHT - Clear with patchy cloud / Northeast wind, 10-20 km/h / Alpine low temperature near -8

TUESDAY - Cloudy with isolated flurries / Southwest wind, 30-60 km/h and increasing through day / Alpine high temperature near -4

WEDNESDAY - Cloudy with isolated flurries / Southwest wind, 20-50 km/h / Alpine high temperature near -7

THURSDAY - Sunny / Northeast wind, 10-15 km/h / Alpine high temperature near -9

Avalanche Summary

On Wednesday and Thursday, explosives triggered numerous avalanches up to size 2.5, including a size 2.5 persistent slab avalanche each day.

Overnight Tuesday and during the day Wednesday, a widespread natural avalanche cycle up to size 3 was reported, one of which was a size 2.5 persistent slab avalanche.

Over the past week, there have been several natural and persistent slab avalanches between size 2.5-3 reported in the region. These avalanches are continued reminders of the "low probability; high consequence" scenario that persistent slab problems often create. 

Snowpack Summary

Strong winds have polished and pressed snow into hard slabs and sastrugi, and stripped windward slopes and exposed the previous surface crust. Soft pockets of redistributed snow may hide in sheltered areas. A melt-freeze crust covers surfaces up to 1800 m. Roughly 80-120 cm of snow is now sitting on (another) crust that extends up to 1900 m, at weak layer of surface hoar may persist at this interface.

The main feature we are monitoring in the snowpack is a layer of weak faceted snow over a hard melt-freeze crust found around 90-200 cm down. Recent sporadic, large, naturally triggered avalanches have occurred on this layer on large alpine slopes and were triggered by either smaller wind slab avalanches, or cornice falls. Dig down and investigate the deeper snowpack layers before committing to big terrain.

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • Be alert to conditions that change with elevation, aspect and exposure to wind.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

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.