Avalanche Forecast

Archived

Jan 31st, 2013–Feb 1st, 2013

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.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Northwest Inland.

Confidence

Poor - Due to limited field observations

Weather Forecast

The next frontal system looks to be aimed at the North Coast for Friday bringing generally light precipitation amounts and rising freezing levels. The brunt of the frontal system will take affect through the weekend with temperatures dropping Sunday night.Friday: Light precipitation, snow amounts 5 cm above 1000 m. Ridgetop winds light from the SW. Alpine temperatures -1.0 and freezing levels near 1100 m. Saturday: No significant precipitation expected. Cloudy skies with ridgetop winds moderate from the South. Alpine temperatures around zero degrees with freezing levels rising to 1800 m. Saturday night into Sunday: The next wave of precipitation bringing light-moderate amounts. Ridgetop winds moderate from the south. Alpine temperatures dropping to -5.0 and freezing levels near 900 m in the afternoon falling to valley bottom overnight.

Avalanche Summary

Most recent avalanche activity shows the most recent surface hoar/facet/ crust layer (down 30-50 cm) becoming reactive remotely. This remote triggered avalanche was a size 2.0 slab avalanche (initiated 50 m away from the skier) at treeline elevation. Another report of a size 2.0 slab avalanche (skier triggered) also occurred. Both incidents were at 1200-1300 m on North aspects with crowns 50-60 cm deep, 30-40 m wide, and running up to 150 m in length.

Snowpack Summary

Recent storm snow (30-50 cm) is building over a variety of old surfaces including old wind slabs, scoured thin slopes, crusts and surface hoar. Wind slabs have built on lee slopes and behind terrain features and may be touchy to rider triggers. Warmer temperatures have promoted some settlement and consolidated the recent snow into slab-like characteristics, and may have a poor bond. In some areas this layer has become reactive at treeline and below treeline elevations, under the weight of a person and remotely. With little observations from the field it's hard to get a good handle on how widespread this layer is. I stress the importance to dig down and become familiar with the snowpack in your neck of the woods. Look for, and test potential weak layers. Observe their reactivity before dropping into your run or line.The average snowpack depth at treeline is near 100 cm but remains quite inconsistent across the region. A strong mid-pack currently overlies a weak base layer of facets/depth hoar. Triggering of this basal weakness may still be possible from thin spots, rocky outcrops or under the weight of larger triggers such as cornice fall and larger amounts of new storm snow.

Problems

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.

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.