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Avalanche Forecast

Archived

Mar 16th, 2014–Mar 17th, 2014

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

Regions

Northwest Coastal.

Confidence

Fair - Due to the number and quality of field observations

Weather Forecast

Synopsis: A low pressure system in the Gulf of Alaska will feed varying amounts of moisture onto the coast for the next 4 or 5 days.Tonight: Clearing as precipitation tapers off, freezing level at valley bottom, winds from the north west up to 50 km/hr.Monday: Cloudy, 5 cm of precipitation in the forecast, freezing level around 600 metres, winds from the south west, 50 Km/hr.Tuesday: Cloudy with flurries, possible 5 to 10 cm of precipitation,, freezing level around 800 metres winds from the south west light occasionally gusting to moderate.Wednesday: Sunny with cloudy periods and isolated flurries, not much precipitation expected, freezing level around 700 metres, winds light from the west, gusting moderate to strong.

Avalanche Summary

Observations during the storm have been limited but we have received reports of wind slab activity and explosive triggered slabs releasing on the early-March layer. Widespread natural activity is expected for Saturday night and Sunday.

Snowpack Summary

Storm slabs continue to build. S-SW winds are building wind slabs in lee features. A widespread crust is expected below the storm snow at lower elevations.Before this weekends storm, the early March persistent weak layer was down up to 1m in the north of the region. The south of the region had less snowfall and the slab was down roughly 60cm. This weak layer consists of any of the following: hard wind slabs or wind-scoured slopes in exposed terrain, a thick layer of faceted snow on sheltered/shady slopes, isolated surface hoar on sheltered/shady slopes, and sun crust on steep solar aspects. The early-February weak layer of surface hoar or a crust/facet combo is typically buried over 1.5m deep. This layer continues to react in snowpack tests, primarily on sheltered north aspects around tree line. The weight of the new snow and rain has the potential to reactivate this layer resulting in large, destructive avalanches. Basal facets and/or depth hoar remain an isolated concern in shallow snowpack areas in the northern part of the region.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.