Avalanche Forecast

Archived

Jan 17th, 2021–Jan 18th, 2021

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

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

Lizard-Flathead.

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

SUNDAY NIGHT- Cloudy with starry breaks / Moderate northwest wind / Alpine high temperature near -8

MONDAY - Sunny with patchy cloud / Light northwest wind / Alpine high temperature near -6

TUESDAY - Cloudy with isolated flurries / Increasingly strong southwest wind / Alpine high temperature near -9

WEDNESDAY - Cloudy with isolated flurries / Moderate southwest wind / Alpine high temperature near -7

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. 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, a weak layer of surface hoar may persist in isolated areas at treeline.

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. Weak layers are best managed with conservative terrain selection and careful snowpack analysis. Dig down and investigate the deeper snowpack layers.

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.