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Avalanche Forecast

Archived

Feb 19th, 2023–Feb 20th, 2023

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 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 unlikely, human triggered possible.

Regions

Purcells, Dogtooth, East Purcell, St. Mary, Bull.

The current snowpack is very complex and requires careful terrain selection, and extremely diligent group management. Seek out low-angle, low-consequence terrain, and avoid areas where an avalanche could propagate widely. 

Very large, full depth avalanches remain possible. This layer resulted in a fatal avalanche that occurred west of Golden on Thursday. Details can be found in this report.

Confidence

Moderate

Avalanche Summary

On Saturday a large (size 2.5), natural avalanche was reported from the far north of the region. It was initiated as a wind slab near ridgetop, before stepping down to basal facets and ground.

A couple of wind slab avalanches were also reported Saturday throughout the region, in steep alpine terrain.

On Friday, our field team observed a size 3.5 avalanche in the Bull River area (South Rockies) on an east through southeast aspect that started high and ran nearly 2000 m to the river. They estimated this avalanche to likely have occurred on or around February 15. Given its size, this avalanche likely failed on a deep persistent weakness in the snowpack.

On Thursday, a size 3.5 avalanche failed on a deep persistent weakness, resulting in 2 fatalities west of Golden. More details regarding this avalanche can be found in this report.

Snowpack Summary

New snow over the long weekend has buried a layer of surface hoar that formed in open areas and a sun crust that exists on steep south aspects. In exposed terrain, recently formed wind slabs exist in leeward features.

A variety of persistent layers still exists in the middle snowpack and continue to see avalanches occur on them periodically. Don't let these layers surprise you.

The lower snowpack contains a widespread layer of large, weak facets that is typically 80 to 150 cm deep. This layer continues to periodically produce very large and destructive avalanches.

Snowpack depths at treeline range between 80 and 200 cm, with the shallowest snowpacks found on the eastern edge of the Purcells.

Weather Summary

Sunday night

Cloudy with trace amounts of snow. Moderate to strong west to northwest alpine winds. Treeline temperatures -5 to -10 C.

Monday

Cloudy with flurries, 0 to 5 cm. Moderate to strong west to northwest alpine winds. Treeline temperatures -5 to -10 C.

Tuesday

Cloudy with snowfall beginning Monday night, 10 to 20 cm by end of the day. Moderate to strong southwest alpine wind, switching to the northeast by the end of the day. Treeline temperature -10 to -15 C.

Wednesday

A mix of sun and cloud, with flurries and trace amounts of snow. Moderate northerly alpine winds. Treeline temperatures -15 to -20 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.
  • Use careful route-finding and stick to moderate slope angles with low consequences.
  • Avoid rock outcroppings, convexities, and anywhere the snowpack is thin and/or variable.

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.

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.

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.