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Avalanche Forecast

Archived

Dec 25th, 2017–Dec 26th, 2017

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

Regions

Lizard-Flathead.

Frigid temperatures have preserved the low density snow, but be cautious in areas where the snow has settled into a slab.

Confidence

Moderate - Due to the number and quality of field observations

Weather Forecast

TUESDAY: Mostly sunny, moderate west winds, treeline temperatures around -20 C.WEDNESDAY: Mix of sun and cloud, moderate west winds, treeline temperatures around -17 C.THURSDAY: Cloudy with flurries starting in the afternoon, strong west winds, treeline temperatures around -15 C.

Avalanche Summary

Avalanche activity has quieted down as the cold weather sets in, although there have been some isolated reports of whumpfs at small avalanches failing on the mid-December surface hoar layer at lower elevations (e.g. see this Mountain Information Network report). The current forecasters' blog has additional advice on managing the current conditions (click here).

Snowpack Summary

Cold temperatures are preserving 50 to 70 cm of fresh snow from the past week. The snow has been deposited into harder wind slabs in exposed terrain, but remains low density in sheltered areas. The main question in the snowpack surrounds the mid-December layer sitting beneath the new snow. The layer includes crusts on south and west facing slopes and large feathery surface hoar crystals in sheltered terrain at and below treeline. The distribution of the layer, as well as the properties of the snow above it, is variable. The most suspect areas are where the wind has formed a stiff slab above this layer and on slopes with preserved surface hoar. The lower snowpack is generally well settled with a thick prominent crust that was buried near the end of November.

Problems

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.

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.