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Avalanche Forecast

Archived

Jan 18th, 2023–Jan 19th, 2023

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

Regions

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

Continue to make conservative terrain choices and consider the consequences of an avalanche on a chosen slope. Regroup in safe locations, space out, and avoid shallow rocky start zones.

Confidence

Moderate

Avalanche Summary

On Tuesday, operators across the region used explosives on a wide range of slopes (aspect and elevations). Explosives produced several smaller storm slabs avalanches, however, the most concerning events were the numerous size 2.5- 3 deep persistent slabs that released down deep on the basal facets to the ground. This suggests that a large enough load could possibly initiate one of these deep avalanches or even a lighter load from a thin spot (rocks, thin to-think areas) in the snowpack.

On Monday, A few small (size 1 to 1.5) human-triggered slabs were reported, mostly failing on a 30 cm deep surface hoar layer. A few larger (size 2) human-triggered slabs were reported in the Esplanade range. Explosive avalanche control produced multiple large (size 2) and one very large (size 3) deep persistent slab.

Last Saturday, a skier triggered a size 2.5 avalanche on a low-angle alpine slope near Golden. This avalanche had a 1 m crown and ran on the facets near the bottom of the snowpack.

Snowpack Summary

10 to 20 cm of settled storm snow can be found in sheltered areas, while open terrain has been wind affected. A rain crust can be found up to 2000 m in most parts of the region.

There are two concerning weak layers in the top meter of the snowpack. The first is a layer of surface hoar from early January and the second is a layer of surface hoar, facets, and a crust from December. Both these layers are most concerning at treeline and above. The bottom of the snowpack sees weak facets that continue to produce large avalanches and will likely persist for a prolonged period of time.

In general, the snowpack is shallow and weak.

Weather Summary

Wednesday Night

Cloudy with a trace of new snow. Light to moderate southwest wind and a low of -9 C. Freezing level valley bottom.

Thursday

A mix of sun and cloud. Light to moderate northwest winds and a high of -8 C at treeline. Freezing levels at valley bottom.

Friday

A mix of sun and cloud. Ridge wind light from the West. Treeline temperatures near -6 C.

Saturday

Cloudy with some sunny periods. Flurries 5-8 cm. Ridge wind strong from the southwest with a treeline high temperature of -4 C.

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 steep, rocky, and wind effected areas where triggering slabs is more likely.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Uncertainty is best managed through conservative terrain choices at this time.

Problems

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.

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.