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Avalanche Forecast

Archived

Apr 17th, 2023–Apr 18th, 2023

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

Regions

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

Numerous large persistent slab avalanches have been triggered by riders in the alpine during the past week.

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

Confidence

Low

Avalanche Summary

Numerous naturally triggered dry loose and wet loose avalanches up to size 2 occurred on steep, rocky slopes on all aspects on Sunday.

Two persistent slab avalanches were reported northwest of Terrace last Wednesday (12th). 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 last Tuesday (11th). 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.

Observations are limited at this time of year, please consider sharing any information or photos you have on the Mountain Information Network to help guide our forecasts.

Snowpack Summary

10-20 cm of recent snow and strong southeast wind formed wind slabs that may remain reactive to human triggers in isolated locations in the alpine.

The primary concern are two buried weak layers down 40-120 cm deep in most areas. They include facets and surface hoar in shaded areas, and a melt-freeze crust elsewhere. Numerous human triggered persistent slab avalanches have occurred on these layers during the past 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

Monday Night

Partly cloudy / 20 km/h east ridgetop wind / Temperature at treeline around -5 C / Freezing level valley bottom

Tuesday

Mix of sun and cloud with isolated flurries; 0-15 cm / 20 km/h east ridgetop wind / Temperature at treeline around -3 C / Freezing level 1200 m

Wednesday

Mostly sunny / 20 km/h southeast ridgetop wind / Temperature at treeline around -1 C / Freezing level 1400 m

Thursday

Mix of sun and cloud with isolated flurries; 3-10 cm / 20 km/h south 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.
  • Cornice failure may trigger large avalanches.
  • If triggered loose wet avalanches may step down to deeper layers resulting in larger avalanches.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger 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.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.