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Avalanche Forecast

Archived

Jan 16th, 2021–Jan 17th, 2021

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

Regions

Lizard-Flathead.

There have been a few large persistent slab avalanches recently, these are hard to predict and best managed with conservative terrain selection. You will likely find the safest and best riding (due to the recent winds!) on low-angled and sheltered slopes.

Confidence

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

Weather Forecast

SATURDAY NIGHT- Mainly cloudy with isolated flurries, up to 5 cm / Moderate southwest wind / Alpine high temperature near -8 

SUNDAY - Mainly cloudy and chance of flurries / Moderate west wind / Alpine high temperature near -4

MONDAY - Sunny breaks and scattered cloud / Light northwest wind / Alpine high temperature near -6

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

Avalanche Summary

On Thursday, explosives triggered numerous avalanches up to size 2.5, including one size 2.5 persistent slab avalanche.

Overnight Tuesday and during the day Wednesday, a a widespread natural avalanche cycle up to size 3 was reported. There were also reports of numerous explosives triggered avalanches up to size 2.5, one of which was a 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. Treeline wind slabs are bonding to previous surfaces. The storm earlier in the week brought rain to lower elevations, a melt-freeze crust covers surfaces up to 1850 m.

Roughly 80-120 cm of snow is now sitting on a crust that extends up to 1900 m. In isolated areas below treeline, this recent snow may be sitting on a weak layer of surface hoar.

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.

Terrain and Travel

  • Avoid freshly wind loaded terrain features.
  • 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.