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Avalanche Forecast

Archived

Jan 25th, 2020–Jan 26th, 2020

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 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.

Regions

Sea To Sky.

Avalanche hazard will rise through the day, particularly in areas where heavy snow accumulates.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

SATURDAY NIGHT: Flurries, 5-10 cm. Alpine low temperature -4 C. Southeast wind 25-40 km/hr. Freezing level 1100 m.

SUNDAY: Snow, 15-25 cm. Alpine high temperature -1 C. South wind 35-55 km/hr. Freezing level 1400 m.

MONDAY: Scattered flurries, 5 cm. Alpine high temperature -3 C. South wind 20 gusting to 50 km/hr. Freezing level 1200 m.

TUESDAY: Sun, cloud, and isolated flurries. Alpine high temperature -2 C. Southeast wind 10-20 km/hr. Freezing level 1100 m.

Avalanche Summary

On Thursday and Friday, explosives easily triggered large (size 2) storm slab avalanches, producing 40-60 cm thick slabs with wide propagation. Large storm slab avalanches (size 2) failed naturally or were triggered by falling cornices.

Explosive control on Wednesday resulted in a very large (size 3.5) deep persistent slab on a west aspect the alpine, suggesting the deep persistent slab remained reactive and sensitive to new loads. With more loading on the way natural storm and deep persistent slab avalanches are serious concerns.

Snowpack Summary

10-25 mm precipitation in the last 24-36 hours fell mostly as rain below 1600 m. Above 1600 m, 30-100 cm recent snow sits over a rain crust from January 20 that was observed up to 2150 m. Wind over the past few days has been strong to extreme from the south, resulting in highly variable conditions in open terrain. Above 1800 m, strong winds continue to impact dry snow developing slabs and building cornices.

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.

Terrain and Travel

  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Extra caution is needed around cornices under the current conditions.
  • Be mindful that deep instabilities are still present and have produced recent large avalanches.
  • Dial back your terrain choices if you are seeing more than 25cm of new snow.

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.