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Avalanche Forecast

Archived

Jan 29th, 2020–Jan 30th, 2020

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

Regions

Sea To Sky.

New snow and cornice growth receiving direct sun on Thursday morning will be particularly sensitive to triggering. In your route selection, account for the potential for human triggered storm slabs as well as the low probability/high consequence deep persistent slab problem.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

Wednesday night: Clear skies. Light to moderate west to southwest wind. Alpine low -8 C. Freezing level 800 m, valley bottom further inland.

Thursday: Mix of sun and cloud with flurries starting in the afternoon bringing 5-15 cm new snow. Southwest winds increasing to strong. Alpine high -4 C. Freezing level 1100 m.

Friday: 20-50 cm new snow with rain below 1300 m. Strong southwest winds, approaching extreme at ridgetop. Alpine high 0 C. Freezing level rising 1300-1900 m.

Saturday: 40-100 cm of new snow in the alpine. 60-160 mm water equivalent in precipitation will start as rain below 1600 m, turning to snow through the day for elevations above 500 m. Strong to extreme southwest winds easing to light. Freezing level peaking at 1900 m in the early morning, dropping to valley bottom by Sunday morning.

Avalanche Summary

Explosive control work Sunday through Tuesday produced mostly size 1-1.5 storm slabs and size 2 cornices with up to 75 m wide propagation. Natural windslab and cornice failures up to size 2.5 were observed in the Whistler backcountry on lee and cross-loaded alpine terrain.

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

Snowpack Summary

20-30 cm of new snow has been highly wind affected in the alpine and open areas at treeline. Suspect wind slabs in leeward terrain and cornice growth at ridgetops. Below treeline, fluctuating freezing levels have left a combination of moist and crusty surfaces.

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

  • Expect slab conditions to change drastically as you move into wind exposed terrain.
  • Be carefull around freshly wind loaded features.
  • Brief periods of sun could quickly initiate natural avalanche activity.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.
  • Be mindful that deep instabilities are still present and have produced recent large avalanches.

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.