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Avalanche Forecast

Archived

Jan 19th, 2023–Jan 20th, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells, Esplanade, Dogtooth, East Purcell, West Purcell.

Snowpack structure in the region is just terrible. It's shallow, weak, and rife with persistent weak layers. Keep an eye open for wind slabs forming in unusual places as northwest winds pick up tomorrow, but keep the deeper problems front and centre in your terrain decisions.

Confidence

Moderate

Avalanche Summary

On Tuesday and Wednesday, operators across the region used explosives on a wide range of slopes (aspect and elevations). Explosives produced several smaller storm slab and wind slab avalanches, but more importantly numerous size 2.5- 3 deep persistent slabs that released at the base of the snowpack. This tells us large enough loads have strong potential to initiate one of these deep avalanches and that human triggering is a serious concern, particularly in thin or variable depth spots (rocks, thin to-thick 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 again ran on the facets near the bottom of the snowpack.

Snowpack Summary

15-25 cm of recent storm snow can be found in sheltered areas, while open terrain has been wind affected. The recent snow overlies a mid-January interface that includes small surface hoar all the way into the alpine in some parts of the region as well as a rain crust that can be found up to 2000 m in most parts of the region.

There are two additional concerning weak layers in the top metre of the snowpack. The first is a layer of surface hoar from early January - down 30-50 cm. The second is a layer of surface hoar, facets, and a crust from December, down 40-90. The surface hoar is most prevalent in sheltered areas while crusts and any associated faceted snow have more uniform distribution.

The bottom of the snowpack contains yet more weak, faceted snow that continues to produce large avalanches and will likely persist for a prolonged period of time.

All of these interfaces are at their shallowest in the east of the region and all of them have produced avalanches recently.

In general, even in the west of the region, the snowpack is shallow and weak.

Weather Summary

Thursday night

Clear. Light to moderate west winds.

Friday

Mainly sunny. Light southwest or northwest winds, more northerly with elevation. Treeline high temperatures around -5.

Saturday

Mainly cloudy with scattered flurries bringing less than 5 cm of new snow in the afternoon. Finishing quickly in the evening. Light variable winds, mainly southerly. Treeline high temperatures around -8

Sunday

Mainly sunny. Light to moderate northwest winds. Treeline high temperatures around -6.

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.
  • Wind slabs are most reactive during their formation.

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.

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.