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Avalanche Forecast

Archived

Jan 24th, 2019–Jan 25th, 2019

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

Regions

Purcells.

Continue to select conservative terrain to avoid being caught in a deep persistent slab avalanche. Expected warming this weekend will increase the chance of a large avalanche.

Confidence

Moderate - Freezing levels are uncertain

Weather Forecast

THURSDAY NIGHT: Dry. Moderate northwesterly winds.FRIDAY: Dry with clear spells. Rapidly warming temperatures with freezing levels rising to 1800 m by the afternoon. Light northwesterly winds. SATURDAY: Dry with a mix of sun and cloud. Freezing levels continuing to climb as high as 2500 m. Light northwesterly winds.SUNDAY: Light precipitation falling as rain up to around 1500 m and snow above that. 2-4 mm/cm expected. Strong westerly winds.

Avalanche Summary

One skier-triggered avalanche was reported on Wednesday in storm snow at approximately 2000 m that released 40 cm below the surface. There is a MIN report that shows a deep persistent natural slab avalanche size 3.5 that happened last Saturday in International Basin. It may have been triggered by a wind event. Dealing with the deep persistent basal facet/crust problem requires avoiding terrain where this kind of avalanche is possible.Check out the MIN here.

Snowpack Summary

Up to 20 cm of recent snow has buried large surface hoar crystals and/or sun crusts. This will likely develop into a touchy problem in areas where a cohesive slab sits above the weak interface. The most suspect terrain features are steep slopes and rolls between 1500 m- 2000 m (where the largest surface hoar exists) and steep south-facing slopes in the alpine (where sun crust exists). Reactive wind slabs have also been noted at upper elevations on lee (N-NE) slopes. The snowpack also has a significant weekness at depth. The base of the snowpack is composed of weak faceted grains and a crust in many parts of the region. There have been sporadic reports of people triggering this layer in areas where the snowpack is shallow. The most likely trigger points include ridgelines and large open slopes and bowls at upper elevations, particularly those that have rocky or variable (thin to thick) features. If you trigger a deep persistent slab it will go big and be a highly destructive avalanche.

Problems

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.

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.