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Avalanche Forecast

Archived

Feb 5th, 2013–Feb 6th, 2013

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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
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, human triggered possible.

Regions

Northwest Inland.

Confidence

Fair - Due to limited field observations

Weather Forecast

Wednesday: Expect mostly cloudy skies with a chance of flurries. Winds should be light southerly and alpine temperatures should reach -7.Thursday: We should see a mix of sun and cloud with winds turning westerly then northwesterly and strengthening. Very isolated flurries are possible. Alpine temperatures should reach around -8.Friday: Clouds return with a slight chance of flurries. Winds settle back to westerly and increase to moderate values. Expect temperatures around -7.

Avalanche Summary

A natural cycle has passed with windslab avalanches at treeline and in the alpine up to size 2.5 and loose wet sluffing at lower elevations.

Snowpack Summary

Up to 40cm of recent storm snow has fallen in the past 72 hrs. Consistent southwesterly winds have maintained snow transport, and many lee zones show layers of windslab over windslab. The deepest storm interface (from the very beginning of this storm cycle) includes a huge variety of old surfaces from facets to crusts and isolated pockets of surface hoar (sheltered treeline and below treeline). There is very limited information with regards to this interface, with the only results suggesting reactivity in sheltered, shady treeline and below treeline slopes (likely associated with the preserved surface hoar pockets). I would stress the importance of digging down to find and test weak layers. A cautious and curious approach is critical.At low elevations (below 800m) the recent storm snow is still moist below the surface. Wet slabs at low elevations may still be possible in steep convex features.The strong mid-pack 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.

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.