Avalanche Forecast

Archived

Dec 19th, 2020–Dec 20th, 2020

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

Alpine

Widespread avalanches certain.

Treeline

Widespread avalanches certain.

Below Treeline

Widespread avalanches certain.

Regions

Glacier.

Natural triggering has slowed, but with strong winds forecast for this afternoon (and heavy snowfall tonight) another natural cycle will begin later today. Avalanche terrain that has not had recent activity should be avoided.

Weather Forecast

A brief respite from a weak ridge today, then a couple more frontal systems give moderate-heavy snowfall tonight and early next week.

Today: Sunny periods. Alpine High -7 °C. Ridge wind moderate (gusting strong) SW.

Tonight: Snow (20 cm). Low -8 °C. Strong (gusting Extreme) SW wind.

Sunday: Flurries (9 cm). High -6 °C. Mod-Extreme SW wind.

Snowpack Summary

20+ cm of new snow, strong SW winds and warm temps continue to build a sensitive slab at all elevations. Weak layers of concern are Dec 13 surface hoar/facets down 60cm+ and Dec 7 crust/surface hoar layer down 75cm+. The Nov 5 crust lingers near the base of the snowpack but has been unreactive recently.

Avalanche Summary

A widespread cycle of natural and artillery controlled avalanches up to size 3.5 occurred in the highway corridor (and likely in the backcountry) overnight.

There have been several reports of near misses from both skier triggered and natural avalanches in the park over the past few days (see MIN).

Confidence

Timing of incoming weather systems is uncertain

Problems

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.

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.