Avalanche Forecast

Archived

Dec 15th, 2022–Dec 16th, 2022

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 unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead, Flathead, Lizard.

Watch for fresh wind slabs forming on atypical aspects at ridgetop as upper level wind picks up from the northwest.

We have uncertainty around the reactivity of a buried weak layer that produced large rider-triggered avalanches last weekend. Read our latest blog for advice on how to manage a persistent slab problem.

Confidence

Low

Avalanche Summary

Earlier this week, avalanche observations were mostly limited to small, explosive-triggered wind slabs.

Last weekend, there were several alarming reports of large (size 2-3) persistent slab avalanches. They were remotely triggered by skiers or snowmobiles at treeline elevations. Check out these MIN reports here and here. A huge thank you to the groups who shared their observations to the Mountain Information Network!

Snowpack Summary

A thin surface crust may be found on steep south aspects. 5-10 cm of recent snow rests over a layer of surface hoar at treeline and below. Near ridgetop, the recent snow is likely being reverse-loaded into atypical terrain features as upper level northwest winds pick up on Friday.

A well settled upper snowpack may overlie a layer of surface hoar formed in early December, now buried 30-50 cm deep.

A concerning weak layer consisting of facets and surface hoar formed in November is now buried 80-90 cm deep. At the bottom of the snowpack, a thick crust sits on the ground.

At treeline, snowpack depths vary from 120 to 200 cm.

Weather Summary

Thursday night

Partly cloudy. Ridgetop wind light to moderate west. Alpine low -14 C.

Friday

Mix of sun and cloud. Ridgetop wind moderate northwest. Alpine high -11 C.

Saturday

Around 5 cm new snow. Light southwest wind. Alpine high -9 C.

Sunday

Mix of sun and cloud with isolated flurries. Light to moderate southwest wind. Alpine high -20 C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
Recent wind has varied in direction so watch for wind slabs on all aspects.
Avoid open slopes and convex rolls at and below treeline where weak layers may be preserved.
In times of uncertainty conservative terrain choices are our best defense.

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.