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Avalanche Forecast

Archived

Feb 18th, 2023–Feb 19th, 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 and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

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

The snowpack in this region is very complex and requires sophisticated snowpack analysis, careful terrain selection, and extremely diligent group management. The only way to minimize exposure is to select low-angle, low-consequence terrain, and avoid areas where an avalanche could propagate widely. 

A weak layer at the base of the snowpack means 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 Friday, natural wind slabs and loose dry avalanches up to size 2 were reported as well as a few natural cornice releases. Some of these were triggered by solar and some were triggered by the wind. Skiers also triggered some small wind slabs as well as a couple of small deep persistent avalanches in small alpine features.

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

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.

Snowpack Summary

Any new snow that falls on Sunday will bury surface hoar which has formed in open areas and a sun crust that exists on steep south aspects. In exposed terrain, wind slabs of all ages still exist in leeward features while windward features have been scoured.

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 depths found on the eastern edge of the Purcells.

Weather Summary

Saturday night

Partly cloudy, possibly clearing by morning for the south of the region, light to moderate W-NW wind, treeline low around -12 ˚C.

Sunday

A mix of sun and cloud in the morning, cloudy in the afternoon with light flurries, moderate to strong SW-W wind, treeline high around -5 ˚C.

Monday

Snowfall beginning overnight 10-20 cm, moderate to strong W-NW wind, easing in the afternoon, treeline high around -6 ˚C.

Tuesday

Snowfall 5-15 cm overnight with another 5-15 cm during the day, moderate to strong SW wind switching to NE by the end of the day, treeline temperature dropping to around -16 ˚C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Use careful route-finding and stick to moderate slope angles with low consequences.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid rock outcroppings, convexities, and anywhere the snowpack is thin and/or variable.
  • Expect slab conditions to change drastically as you move into wind exposed terrain.

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.