Avalanche Forecast

Archived

Jan 27th, 2020–Jan 28th, 2020

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 possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Sea To Sky.

New snow, strong winds and mild temperatures have been a recipe for reactive storms slabs and rapid cornice growth. As winds ease Tuesday, recently wind-loaded features will likely remain sensitive to human triggering.

Confidence

High - The number, quality, or consistency of field observations is good, and supports our confidence.

Weather Forecast

Monday night: Snow, 5-20 cm with Whistler on the low end of the range. Alpine low -3 C. Strong southeast to southwest winds. Freezing level 1000-1200 m.

Tuesday: Broken cloud with flurries bringing up to 5 cm. Alpine high -2 C. Light southwest winds. Freezing level 1200-1400 m.

Wednesday: Snow, 10-20 cm. Alpine high -1 C. Strong southerly winds, approaching extreme southwest at ridgetop. Freezing level 1000-1200 m.

Thursday: Mix of sun and cloud with flurries in the evening. Alpine high -2 C. Light to moderate southwest winds. Freezing level 700-1000 m.

Avalanche Summary

Explosive control work Sunday produced mostly size 1 storm slabs and up to size 2 cornice/slab avalanches with up to 75 m wide propagation. Natural windslab and cornice failures up to size 1.5 were observed in lee and cross-loaded alpine terrain.

A few large to very large (size 2-3) natural storm slab avalanches were observed in the Whistler backcountry on Saturday morning, and explosives triggered storm slabs and cornice-triggered avalanches up to size 2.

Snowpack Summary

Strong to extreme winds have left a variety of wind-affected surfaces in alpine and open treeline areas. Windward slopes are scoured down to the January 20 crust. On leeward terrain, moist surface snow has been pressed into slabs over the crust, and notable cornice growth has been observed at ridgetops. Overnight snowfall sits over a crust below 1700 m and on south facing slopes treeline and above.

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 careful with wind loaded pockets, especially near ridge crests and roll-overs.
Extra caution is needed around cornices under the current conditions.
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.

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.