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Avalanche Forecast

Archived

Mar 5th, 2017–Mar 6th, 2017

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, human triggered possible.

Regions

Lizard-Flathead.

A week of heavy snowfall and strong winds have left large storm slabs and cornices primed for human triggering. Avoid overhead hazards and stick to mellow terrain.

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain

Weather Forecast

We're looking at scattered flurries and cool temperatures throughout the forecast period. MONDAY: Cloudy with scattered flurries and 5-15 cm of new snow, light gusting strong south winds, alpine temperatures around -7 C. TUESDAY: Isolated flurries with up to 5cm of new snow, 20-40 km/h south winds, alpine temperatures around -8 C. WEDNESDAY: Light flurries with 5-10cm possible, 20-30 km/h south winds, alpine temperatures around -9 C.

Avalanche Summary

Cornices are large and remain easy to trigger. See here for a great MIN post from Sunday illustrating just how surprising the results can be. Several size 1.5 to 3 natural and explosives triggered avalanches were reported on Saturday. Aspects were east through northwest, from 1500m to ridgetop.A widespread natural avalanche cycle occurred on Friday night, with many size 2-3 cornice triggered avalanches on north and east aspects. Natural activity is expected to taper off, but storm slabs will remain primed for human triggering.

Snowpack Summary

Another 20 cm of snow fell on Friday, bringing the three day storm total to roughly 80 cm and the weekly total to over 120 cm. Strong southwest winds have extensively redistributed the snow and formed large brittle cornices. The new snow came in "upside down" (heavier, denser snow over lighter snow) and is rapidly settling into a slab which sits over a variety of old surfaces including surface hoar, facets, stiff wind slabs, sun crusts, and a rain crust below 1900 m. Isolated basal weaknesses may still exist in shallow snowpack areas and appear to isolated to north-northeast aspects in the alpine.

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.