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Avalanche Forecast

Archived

Mar 22nd, 2023–Mar 23rd, 2023

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

Regions

Purcells, Dogtooth, East Purcell, West Purcell.

Danger ratings have declined but the inherently weak and dangerous snowpack structure remains.

A moderate rating reflects the possibility of large, high-consequence human-triggered avalanches. Remain diligent in your terrain selection by avoiding steep, rocky, wind-affected terrain and choosing small, low-consequence features.

Confidence

Moderate

Avalanche Summary

Strong solar radiation in the past few days has triggered several natural size 3 deep persistent slabs, mainly on south aspects. These avalanches were reported from rocky alpine and treeline features between 1900 and 2700 m. A variety of solar-triggered loose wet avalanches up to size 1.5 were also observed from steep, rocky slopes.

On Sunday, 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.

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 crust on solar aspects, surface hoar up to 10 mm in shady and wind-sheltered areas, faceting snow on polar aspects, and wind-affected surfaces 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

Wednesday night

Clear with cloudy periods. Alpine temperatures drop to a low of -5 °C. Ridge wind light from the southwest. Freezing level at valley bottom.

Thursday

A mix of sun and cloud with isolated flurries. Alpine temperatures reach a high of 3 °C. Ridge wind 10 to 30 km/h from the southwest. Freezing level rises to 2000 metres.

Friday

Mainly cloudy with scattered flurries, up to 5 cm of accumulation. Alpine temperatures reach a high of -5 °C. Ridge wind 10 to 35 km/h from the southwest. Freezing level rises to 1400 metres.

Saturday

Cloudy with sunny periods and isolated flurries. Alpine temperatures reach a high of -6 °C. Ridge wind 10 to 30 km/h from the southwest. Freezing level rises to 1200 metres.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Be mindful that deep instabilities are still present and have produced recent large avalanches.
  • Avoid rock outcroppings, convexities, and anywhere the snowpack is thin and/or variable.
  • Avoid slopes when the solar radiation is strong, especially if they have large cornices overhead.

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.

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.