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Avalanche Forecast

Archived

Feb 1st, 2020–Feb 2nd, 2020

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

Regions

Sea To Sky.

Start with conservative terrain and make observations to inform your decisions. Natural avalanches are still possible, and human triggered storm slabs are likely.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

Saturday night: Clear periods. Moderate southwest wind. Freezing level valley bottom.

Sunday: Mostly cloudy with flurries bringing 10-15 cm new snow. Moderate southwest wind. Alpine high -8 C. Freezing level 300 m.

Monday: Sunny. Light north wind. Alpine high -6 C. Freezing level valley bottom.

Tuesday: Mostly cloudy. Moderate southwest wind. Alpine high -6 C. Freezing level valley bottom.

Avalanche Summary

On Saturday, reports came in from Friday's widespread storm cycle, of up to size 3 storm slabs in the alpine and size 2 loose wet below 2100 m prior to the cooling trend. Extensive explosive work conducted Saturday morning produced mostly size 2 avalanches with storm slabs up to size 3 and cornices up to size 2.5.

The deep persistent problem most recently reared its head on Monday with large explosive loads producing a size 2.5. in shallow, rocky terrain.

Snowpack Summary

Above 1900 m, storm totals are in the range of 50-70 cm. Extreme southwest wind during the storm has scoured windward aspects, formed loaded pockets up to 150 cm deep in alpine lees and contributed to rapid cornice growth. Below 1800 m, snow overlies a crust and tapers with elevation.

Weak faceted grains and crusts near the base of the snowpack continue to be a concern in inland parts of the region such as the Spearhead Range. With the recent new loads, we are watching these layers closely.

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Be alert to conditions that change with elevation.
  • Extra caution is needed around cornices under the current conditions.
  • Be aware of the potential for surprisingly large avalanches due to deeply buried weak layers.

Problems

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.