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Avalanche Forecast

Archived

Nov 28th, 2018–Nov 29th, 2018

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

Purcells.

With few field observations, uncertainty exists around the buried, weak surface hoar layer. We suspect that the potential for human triggered avalanches remain high. Please post your observations to the Mountain Information Network.

Confidence

Low - Due to the number of field observations

Weather Forecast

The weather trend will start to stabilize as a high pressure system sets up through the forecast period. THURSDAY: Mostly cloudy with new snow 5-10 cm/  alpine temperatures near -7/  generally light winds from the southeast/ freezing level 900mFRIDAY: Cloudy with a trace of new snow/  alpine temperatures near -9/  ridgetop winds light from the southeast/  freezing levels 1400 mSATURDAY: Mix of sun and cloud/  alpine temperatures -11/  ridgetop winds light from the northeast/  Freezing levels near 800 m

Avalanche Summary

Numerous natural loose dry avalanches up to size 1 from steeper terrain features were reported on Tuesday with less action Wednesday. With little avalanche observations and low confidence of the surface hoar distribution, I suspect that human triggered avalanches remain possible, especially where deeper deposits of storm snow sit above the buried weak layer.Last Saturday, a large human triggered avalanche was reported in the South Columbia forecast region. The size 3 avalanche occurred on a steep, north aspect in the alpine and failed to ground, likely on the October crust. This avalanche is notable for the Purcells where we have a similar, but shallower snowpack than in the South Columbia, resulting in a higher possibility of impacting a weak layer near the ground.  Check out  the MIN report here.

Snowpack Summary

The early season snowpack is highly variable in this region. Approximately 20 cm new snow fell in this area during the most recent storm, which may have been blown into wind slabs on exposed northerly and northeasterly slopes. In most places, there are two layers of feathery surface hoar being reported, one around 20 cm below the surface and one down 35 cm. One or both of these may be associated with a crust on south aspects. The most likely places to trigger one of these layers is in deeper snow areas in shady spots on smooth slopes. A prominent feature of the snowpack is a combination of a crust and underlying sugary, faceted snow found near the base of the snowpack. It may still be possible to release an avalanche on smooth slopes in the alpine.

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.

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.