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Avalanche Forecast

Archived

Mar 20th, 2023–Mar 21st, 2023

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells, Dogtooth, East Purcell, West Purcell.

Wind slabs may still be reactive in exposed terrain at higher elevations.

Human triggering of the lingering persistent and deep persistent layers remains a concern and very large avalanches remain possible.

Confidence

Moderate

Avalanche Summary

On Sunday, several natural size 3 solar-triggered deep persistent slab avalanches were reported from rocky alpine features between 2200 and 2700 m over the weekend. A remotely triggered size 2 persistent slab avalanche was reported on a NW aspect at 2400 m which failed down 70 cm on facets and was triggered from 30 m away. A human-triggered size 2 wind slab avalanche also occurred on a SE aspect at 2400 m which was estimated to have failed down around 80 cm. A variety of solar-triggered loose wet avalanches up to size 1.5 were also observed from steep, rocky slopes.

On Saturday, a natural size 2 cornice failure was reported from a NW aspect in the alpine which did not trigger a slab on the slope below. This MIN post describes an older natural size 4 near Golden and a more recent natural size 3. This is a good reminder of the scale and destructive potential of avalanches which are still possible when the deep weak layer is triggered.

Snowpack Summary

The snow surface includes a sun crust on solar aspects, surface hoar up to 10 mm in shady and wind-sheltered areas, small facets on polar aspects, and wind-affected surfaces in exposed terrain. Until recently, the wind was blowing from the southwest but it has now switched to the northeast and therefore wind slabs could exist on all aspects in exposed terrain.

A weak layer buried around March 12 is now down around 20-40 cm and typically consists of wind-affected snow, facetted snow, and a thin sun crust.

The middle of the snowpack includes at least a couple of lingering persistent weak layers down 50-90 cm including surface hoar in wind-sheltered terrain and a sun crust on south aspects. Activity on these layers has tapered off but still remains a concern in isolated areas.

The lower snowpack is made up of a widespread layer of large, weak facets, and/or depth hoar crystals. This weak layer has been responsible for a number of recent very large, destructive avalanches and will likely continue to be a concern for the rest of the season.

Weather Summary

Monday Night

Clear, winds northeast 20-30 km/h, treeline low around -10 °C.

Tuesday

Sunny, winds northeast 10-20 km/h, freezing level reaching around 1800 m.

Wednesday

Sunny, winds northeast 15-25 km/h, freezing level reaching around 2000 m.

Thursday

A mix of sun and cloud, winds southwest 15-25 km/h, freezing level reaching around 1700 m.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Avoid slopes when the solar radiation is strong, especially if they have large cornices overhead.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

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.