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Avalanche Forecast

Archived

Jan 19th, 2023–Jan 20th, 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 unlikely, human triggered possible.
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.

Regions

Kootenay Boundary, Bonnington, Kootenay Pass, Norns, Rossland, South Okanagan, Ymir, Moyie, St. Mary.

Keep an eye open for wind slabs forming in unusual places as northwest winds pick up tomorrow, but don't lose sight of the deeper issues. The snowpack contains deep, human-triggerable weak layers that have steadily been producing large, destructive avalanches. Keep up the conservative terrain selection regardless of what surface conditions tell you.

Confidence

Moderate

Avalanche Summary

Wednesday's reports continue to show a general slowing down of avalanche activity with a few notable occurrences, like an anomalous natural size 2 near Ymir peak that may have failed on the early January surface hoar, considering its distance from the natural cycle on the weekend. A vehicle in the southern Valhallas triggered a size 1.5 on this layer as well.

On Tuesday, improved weather allowed for better visibility noting the widespread natural avalanche activity that occurred last weekend. Many of these avalanches failed on the early January surface hoar.

Explosive control results since this cycle have included large deep persistent slab avalanches up to size 3 and storm slabs to size 2. Human triggering and remote triggering remain possible, with 1700-2100 m and east-to-northeast aspects being the critical elevation band and aspects for the early January layer.

Snowpack Summary

Up to about 10cm of new snow has accumulated on a thin rime crust throughout the region on all aspects and elevations. This will be an interface to watch with the next snowfall and/or wind loading.

Including the new snow, about 30-40 cm now has been settling and bonding to a mix of wind-affected and sheltered low-density storm snow above about 1700 m and a rain crust at lower elevations.

A weak layer of surface hoar now sits preserved 50 to 70 cm deep, especially prevalent in sheltered terrain at treeline. This layer has been identified as the failure plane in many recent avalanches in the region.

A facet/crust layer is down 60-90 cm (and 2-10 cm thick at TL elevation). Below this, the mid-pack is settled and consolidated.

A concerning and weak layer of facets and crust 20-50 cm thick make up the basal snowpack.

Weather Summary

Thursday nightSome clearing. Light west or southwest winds increasing a bit and shifting decisively to northwest in the morning.

Friday

A mix of sun and cloud. Light to moderate northwest winds, potentially strong in the alpine. Treeline high temperatures around -4.

Saturday

Increasing cloud with a short period of snowfall at the end of the day bringing up to 5 cm of new snow, ending quickly in the evening. Moderate to strong south winds, increasing over the day. Treeline high temperatures around -5

Sunday

Mainly sunny. Moderate to strong northwest winds easing over the day. Treeline high temperatures around -4.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Be aware of the potential for surprisingly large avalanches due to deeply buried weak layers.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.
  • 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.