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Avalanche Forecast

Archived

Jan 6th, 2022–Jan 7th, 2022

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

Regions

Lizard-Flathead.

New snow and wind are forming deep and reactive slabs. Potential exists for storm slab avalanches to step down to deeply buried weak layers, resulting in very large avalanches. Avoid avalanche terrain and overhead hazard.

Confidence

Low - Forecast snowfall amounts are uncertain.

Weather Forecast

Thursday night: Snowfall starting 10-20 cm. Moderate to strong SW wind. Treeline temperature around -10 °C. 

Friday: Snowfall 20-40 cm. Strong SW wind. Treeline high around -4 °C.

Saturday: 10-15 cm overnight then mainly cloudy. Moderate to strong SW wind. Treeline high around -8 °C.

Sunday: A mix of sun and cloud. Moderate W wind. Treeline high around -2 °C.

Avalanche Summary

Evidence of a natural storm slab avalanche cycle size 2-2.5 was observed on NE aspects Wednesday, likely having run during the storm earlier in the week. On Monday and Tuesday, natural storm slabs were reactive to explosive up to size 2 and ski cuts produced storm slabs and loose dry avalanches up to size 1. By Wednesday, explosive results were limited to size 1 loose dry.

On Sunday, a size 2.5 persistent slab avalanche was triggered on an E aspect at treeline. This avalanche is detailed in a MIN post and features in our latest blog post, Photos of recent persistent slab avalanches in the southern interior.

Snowpack Summary

20-40 cm of new snow is forecast to fall by the end of Friday. Strong ridgetop winds will likely load new snow into leeward terrain features in the alpine. An accumulated total of 40-70 cm of new and recent snow now sits over variable and potentially weak snow surfaces including widespread facets, wind affected snow, and/or surface hoar in sheltered areas, which may make for a weak bond at this interface.

The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 90-200 cm. Activity on this layer has been sporadic, the most recent persistent slab avalanche in the region was on January 2. This layer has created a low likelihood, high consequence scenario which is best managed through conservative terrain choices and disciplined backcountry travel techniques.

Terrain and Travel

  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Travel in alpine terrain is not recommended.
  • Seek out sheltered terrain where new snow hasn't been wind-affected.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.