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Avalanche Forecast

Archived

Apr 14th, 2023–Apr 15th, 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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Coastal, Kitimat, Nass, Rupert, Seven Sisters, Shames, Stewart, Howson, Ningunsaw.

Numerous large persistent slab avalanches have been triggered by riders this past week. Additional snow and wind Friday night will increase the likelihood of triggering large slab avalanches.

Conservative terrain selection is critical, choose only well-supported, low-consequence lines.

Confidence

Moderate

Avalanche Summary

No new avalanches were reported on Thursday.

Two persistent slab avalanches were reported northwest of Terrace on Wednesday. A skier triggered size 2 on a north aspect in the alpine and a size 2.5 on an east aspect in the alpine that was triggered by solar radiation. Both failed on a layer of weak facets and/or surface hoar down 40-50 cm.

A fatal avalanche occurred north of Stewart on Tuesday. It was a skier-triggered size 3 persistent slab avalanche that occurred on a northeast aspect in the alpine.

Two additional skier-triggered size 2-2.5 persistent slab avalanches were also reported on northerly aspects at treeline and above on Tuesday. The depth of the weak layer was around 60-80 cm.

Snowpack Summary

10-20 cm of forecast snow and strong south wind Friday night are expected to form fresh storm slabs and increase the likelihood of triggering large persistent slab avalanches at treeline and above.

A weak layer buried at the end of March sits 50-90 cm deep in most areas. It includes facets and surface hoar in shaded areas, and a melt-freeze crust elsewhere. Numerous human triggered persistent slab avalanches have occurred on this layer this week.

The mid and lower snowpack are considered generally strong and well-bonded. In the far northern reaches of the region, basal facets may exist which are currently considered inactive. This layer may become active with abrupt changes to the snowpack, such as rapid loading (heavy snowfall or rain) or prolonged warming.

Weather Summary

Friday Night

Snow; 5-20 cm / 60 km/h south ridgetop wind / Temperature at treeline around -4 C / Freezing level 900 m

Saturday

Mostly cloudy with flurries; 3-15 cm / 30 km/h south ridgetop wind / Temperature at treeline around -4 C / Freezing level 1100 m

Sunday

Cloudy with flurries; 3-10 cm / 25 km/h south ridgetop wind / Temperature at treeline around -4 C / Freezing level 1100 m

Monday

Mostly sunny / 20 km/h southeast ridgetop wind / Temperature at treeline around -3 C / Freezing level 1200 m

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • If triggered, storm slabs in-motion may step down to deeper layers and result in very large avalanches.

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.

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.