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Avalanche Forecast

Archived

Jan 26th, 2023–Jan 27th, 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 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.

Regions

Purcells, Dogtooth, East Purcell, West Purcell.

Triggering a slab or getting even smaller amounts of loose snow moving could result in a widespread destructive avalanche if buried weak layers are affected by the increased load.

Be careful when transitioning into wind-affected terrain and keep the deep persistent problem at the forefront of your decision-making process.

Fresh tracks are best enjoyed on low-angle low consequent terrain today.

Confidence

Moderate

Avalanche Summary

Wednesday saw a range of avalanches that varied and size, characteristics, and what it was that triggered them. Wind slabs and deep persistent, with some avalanches having a combination of both, were the main culprits. They were between size 1 to 2.5. Most were natural avalanches but some explosive work on cornices that are beginning to be a problem also produced avalanches.

On Tuesday there was a report of a size 3.5 natural avalanche in the Dogtooth Range. This was likely a deep persistent avalanche with a wide propagation triggering near ridgetop at the upper end of treeline on an easterly aspect.

On Monday there was a size 3 skier triggered a deep persistent avalanche on an east aspect starting at 2300 m in the Terminator area in the Dogtooth Range. This MIN outlines the avalanche.

Skiers on Saturday were surprised by a size 2 avalanche that slid on deeply buried surface hoar. What was most surprising about this avalanche was how little of an angle it occurred on. Our persistent slab problem and deep persistent slab problem are still problems.

Snowpack Summary

New snow arriving Thursday night and into Friday will be surface hoar and/or wind slabs. The surface hoar can be found developing on most elevations and aspects in the western and northern parts of our region. While the wind slabs can be found in exposed areas at higher elevations.

The mid-snowpack contains a couple of layers of major concern. The first is down 30 to 50 cm and is a layer of surface hoar from early January. The second is a layer of surface hoar, facets, and crust created in December and found 40 to 60 cm down. The surface hoar is most prevalent in sheltered areas while crusts and any associated faceted snow are more widespread.

The bottom of the snowpack is weak and faceted and contains a weak layer from late November. The total depth of this weak and shallow snowpack ranges between 80 and 180 cm at treeline and is shallowest in the eastern part of our region.

Weather Summary

Thursday Night

Cloudy, up to 8 cm accumulation focused mainly in the west, wind southwest 13 km/h gusting to 40, treeline temperatures at -6 C with freezing level climbing to 1500 m.

Friday

Mostly cloudy, 4 to 12 cm accumulation, wind northwest 20 km/h, treeline temperatures -5 C.

Saturday

Mainly sunny, trace accumulation early in the morning, wind northest 15 km/h, treeline temperatures falling to -18 C.

Sunday

Sunny with cloudy periods, no accumulation, winds north 20 km/h, treeline temperatures -23 C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • 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.
  • Watch for changing conditions today, storm slabs may become increasingly reactive.
  • Uncertainty is best managed through conservative terrain choices at this time.
  • Avoid areas with overhead hazard.
  • Be carefull around freshly wind loaded features.
  • Choose slopes that are well supported and have limited consequence.

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.