Avalanche Forecast

Archived

Feb 21st, 2024–Feb 22nd, 2024

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Regions

Purcells, Dogtooth, East Purcell.

Before committing to a slope, carefully assess steep, rocky, and wind-affected areas where triggering slabs is more likely.

Confidence

Moderate

Avalanche Summary

On Wednesday a large (size 2) slab was triggered by sluffing from overhead rocky terrain.

On Tuesday, a large (size 2) cornice fall and a small (size 1) skier-controlled wind slab avalanche were reported.

Over the weekend, several wind slabs and loose avalanches were reported, up to size 1.5, in steep alpine terrain. Some where triggered by riders, and some by the sun.

Snowpack Summary

Up to 10 cm of recent storm snow overlies previously wind-affected surfaces, a sun crust, or surface hoar in sheltered areas.

In sheltered areas, 10 to 30 cm of settling snow sits over a frozen crust that formed in early February. This crust is generally supportive to the weight of a person below treeline, but becomes breakable around treeline, and disappears as you ascend higher into the alpine.

The midpack is generally faceted.

The bottom 15 to 30 cm of the snowpack consists of a deep persistent weak layer of facets, depth hoar, and crusts that continue to produce sporadic, large avalanches.

Weather Summary

Wednesday Night

Cloudy with a trace of snow, 5 km/h southwest ridgetop wind, treeline temperature around -6 °C, freezing level dropping to valley bottom.

Thursday

Cloudy with a trace of snow, 5 to 10 km/h west ridgetop wind, treeline temperature around -1 °C, freezing level rising to 1500 m.

Friday

Partly cloudy with trace amounts of snow, 10 to 20 km/h southwest ridgetop wind, treeline temperature around -1 °C, freezing level rising to 1500 m.

Saturday

Cloudy with 1 to 2 cm of snow, 25 km/h west ridgetop wind, treeline temperature around 0 °C, freezing level rising to 1700 m.

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.
In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.

Problems

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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.