Avalanche Forecast

Archived

Dec 25th, 2022–Dec 26th, 2022

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

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 avalanches possible, human triggered probable.

Regions

Purcells, Dogtooth, East Purcell.

Watch for wind effect in recent storm snow. Expect wind loading around ridgelines and in mid slope terrain features.

Strong winds have transported snow lower on slopes than you expect.

Deeper weak layers remain a concern for step down avalanches in wind loaded terrain.

Confidence

Moderate

Avalanche Summary

Activity from Friday was limited to small loose dry avalanches in steep terrain features.

On Tuesday and Wednesday explosive control in the region produced small avalanches with a depth of 10cm.

If you are headed into the backcountry please consider filling out a Mountain Information Network report.

Snowpack Summary

Sustained light to moderate southwest winds have continued to move snow, creating wind slabs on north and east facing terrain features. Wind slabs sit over hard wind affected surfaces, a layer of facets or surface hoar increasing reactivity.

The main concern is a layer of surface hoar, facets, or crusts buried 40 to 80 cm deep from mid November. Avalanche activity on this layer has tapered off but it could still be possible to trigger this layer in isolated terrain features near treeline. Triggering becomes more likely as temperatures rise.

Alpine snowpack distribution is variable, with depths ranging from 70 to 140 cm.

Weather Summary

Sunday Night

Cloudy with flurries bringing trace amounts of new snow. Moderate southwest winds and a high of -6°C .

Monday

A mix of sun and cloud with 5 to 15 cm of snow possible. Moderate southwest winds. Freezing levels rise to 1000 m. High temperatures of -5°C expected.

Tuesday

Mostly cloudy with snowfall 5 to 10 cm. Moderate to strong southwest winds. Freezing levels remain below 1000 m with alpine highs of -4°C.

Wednesday

Mostly cloudy with light snowfall. Moderate to strong southwest winds. Freezing levels remain below 1000 m with alpine highs of -4°C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.

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.