Avalanche Forecast

Archived

Mar 22nd, 2021–Mar 24th, 2021

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural and human triggered avalanches likely.

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

North Rockies.

Recently formed slabs may remain triggerable by riders on Tuesday. New slabs will likely rapidly form during Wednesday's storm.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

MONDAY NIGHT: Mostly cloudy with isolated flurries, accumulation 1 to 3 cm, 30 km/h northwest wind, alpine temperature -9 C.

TUESDAY: Cloudy with snowfall, accumulation 5 to 10 cm, 30 km/h west wind, alpine temperature -8 C.

WEDNESDAY: Cloudy with snowfall, accumulation 20 to 40 cm, 50 km/h northwest wind, alpine temperature -5 C.

THURSDAY: Partly cloudy, 20 km/h northwest wind, alpine temperature -7 C.

Avalanche Summary

The recent snow was reported as being reactive to rider traffic over the weekend, particularly in steep wind-affected terrain. Looking forward, storm and wind slabs could continue to be triggered by riders as the wind speed remains elevated. New slabs are likely to develop on Wednesday.

Snowpack Summary

Around 20 to 30 cm of snow accumulated over the weekend for most riding areas. Southwest wind has likely formed wind slabs up high, as the snow is blown around at treeline and alpine elevations. The snow may also have settled into storm slabs, particularly in steep terrain sheltered from the wind. This snow overlies a hard melt-freeze crust up to treeline and to mountain tops on sun-exposed slopes. Another 20 to 40 cm of snow may accumulated by Wednesday afternoon, forming new storm and wind slabs. Cornices are very large along ridgelines and always have the possibility of failing naturally or from the weight of a human.

A weak layer of facets buried mid-February may be found around 100 to 120 cm deep, or shallower in the east of the region and in thin snowpack areas. The most likely place to trigger this layer is where the snowpack is shallow in alpine terrain. Large loads, such a cornice falls or multiple sleds on the same slope, often trigger slopes that single rider couldn't trigger.

Terrain and Travel

Watch for changing conditions today, storm slabs may become increasingly reactive.
Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
Avoid exposure to slopes that have cornices overhead.
Be aware of the potential for large avalanches due to the presence of a persistent slab.
Avoid areas with a thin or variable snowpack.

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.