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Avalanche Forecast

Archived

Dec 13th, 2019–Dec 14th, 2019

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

Regions

Lizard-Flathead.

Flurries have steadily accumulated new snow in the past 24 hours. This combined with moderate west winds have made wind slabs at upper elevations the primary concern.

Confidence

Moderate - Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

FRIDAY NIGHT: Scattered flurries, 30 km/h wind from the west, alpine high temperatures around -4 C.

SATURDAY: Mostly cloudy, light wind from the northwest, alpine high temperatures around -6 C.

SUNDAY: Mix of sun and cloud, light wind from the northwest, alpine high temperatures around -8 C.

MONDAY: Mix of sun and cloud, light wind from the west, alpine high temperature -8 C.

Avalanche Summary

Recent avalanche activity has primarily been limited to small wind slab avalanches (size 1) triggered with explosives. On Thursday there was a report of a size 2.5 storm slab avalanche on a northeast aspect in the alpine, which scoured down to a crust near the base of the snowpack. 

Snowpack Summary

New snow and wind has formed fresh wind slabs at upper elevations. In sheltered areas 30-40 cm of snow from last weekend is gradually settling. Crust layers from November and October can be found 40-100 cm below the surface. These layers produced large avalanches with explosive triggers last weekend, but since then have appeared to gain strength. Large avalanches on these layers may still be possible to trigger in steep rocky terrain. Snowpack depths range between 50-100 cm at higher elevations and taper rapidly below treeline.

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.