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Avalanche Forecast

Archived

Dec 26th, 2017–Dec 27th, 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 possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead.

Winds are expected to increase, so watch for slab development on lee slopes.

Confidence

Moderate - Wind speed and direction is uncertain

Weather Forecast

WEDNESDAY: Mostly cloudy, strong west winds, treeline temperatures around -17 C.THURSDAY: Cloudy with flurries starting in the afternoon, strong west winds, treeline temperatures around -15 C.FRIDAY: 10-20 cm of low-density snow, strong west winds, treeline temperatures around -15 C.

Avalanche Summary

On Monday, one large (size 2) naturally-triggered avalanche was reported on a north-facing alpine feature that likely failed on the mid-December surface hoar layer. Otherwise, avalanche activity has generally quieted down as the cold weather sets in. There have been some isolated reports of whumpfs at small avalanches failing on the mid-December surface hoar layer, but in most areas the snow above this layer has not settled into a slab yet.The current forecasters' blog has additional advice on the developing persistent slab problem (click here).

Snowpack Summary

Cold temperatures are preserving roughly 50 cm of low density snow from last week's storms. The snow is being 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 areas 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 stiff slabs above the 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

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.