Avalanche Forecast

Archived

Dec 21st, 2018–Dec 22nd, 2018

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 possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

North Columbia.

As we come out of a very active avalanche period it's critical to make conservative terrain choices like avoiding steep slopes, convex rolls, and wind loaded terrain.

Confidence

Moderate.

Weather Forecast

FRIDAY NIGHT: Mostly clear skies, moderate wind from the northwest, alpine temperatures drop to -16°c. SATURDAY: Clear in the morning then increasing cloud throughout the day, light wind from the south, alpine high temperatures around -8°c. SUNDAY: Cloudy with scattered flurries and 4-8 cm of snow, moderate wind from the south, alpine high temperatures around -6°c. MONDAY: Mostly cloudy with isolated flurries, light wind from the south, alpine high temperatures around -6°c.

Avalanche Summary

Evidence of a natural avalanche cycle was reported on Friday in the wake of Thursday's storm. The cycle involved large storm slab avalanches on all aspects (size 2-3). Skiers and snowmobilers testing small terrain features were able to produce size 1 storm slab and wind slab avalanches, indicating human triggering would be likely on bigger slopes as well. None of the avalanches reported on Thursday or Friday involved deeper persistent slab problems, but earlier in the week there were several large persistent slab avalanches (size 2-2.5) that were triggered remotely from adjacent slopes (most recently on Tuesday). Many of these occurred on north and east aspects above 1900 m. We can't rule out the possibility of deeper layers still being reactive to human triggers, as these layers still have to adjust to the weight of the new snow.

Snowpack Summary

Strong wind from the southwest has formed fresh wind slabs in the alpine and around treeline, while 20-40 cm of recent storm snow continues to settle into a slab.A week of stormy weather has deposited 80-120 cm of snow above a weak layer of facets (sugary snow), surface hoar (feathery crystals), and a sun crust (on south aspects) that formed during the dry spell in early December. This layer has been responsible for large persistent slab avalanches over the past week, particularly on north and east facing slopes above 1900 m. The potential exists for smaller avalanches to step down and trigger this deeper layer, resulting in very large avalanches.Another weak layer from mid-November is now buried up to 150 cm, but has been less active recently. At the base of the snowpack is a crust that formed in late October. Concern for this layer is dwindling but it may still be worth considering in places such as steep, rocky, alpine terrain, especially where the snowpack is shallow.

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.