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Avalanche Forecast

Archived

Nov 23rd, 2012–Nov 24th, 2012

Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.
Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.
Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.

Regions

Purcells.

This bulletin is based on limited data. Local variations in conditions are likely to exist. Check out the forecaster blog for further details on interpreting early season bulletins.

Confidence

Poor - Due to limited field observations for the entire period

Weather Forecast

A Pacific frontal system is moving through the interior regions Friday and extending into Saturday. This will bring strong ridgetop winds and moderate amounts of snow, especially in upslope areas. After the front passes, a weak ridge of high pressure will push into the region bringing dryer, cooler, and clearing conditions that will last through the weekend. Freezing levels will stay in the valley bottom through the period.

Avalanche Summary

Several size 1.0 natural avalanches were reported near the central part of the region on North aspects @ 2300m. Control work in the Dogtooth Range produced Size 2-2.5, on NE through SE aspects above 2100 m. The failure plane being the Nov 17th surface hoar layer. This layer is a concern of mine heading into the weekend, it may be easily triggered by the load of a sled, or skiers hitting the sweat spot and initiating a large avalanche.

Snowpack Summary

Storm slabs and wind slabs are reported to be widespread at treeline and in the alpine. Most suspect slopes that host touchy wind slabs are leeward (North-North East) and behind ridge crests and terrain features. A couple early season persistent layers also exist. The most reactive being a surface hoar layer that was buried on November 17th. This is buried down 40-80 cm and may be easily triggered under he weight of skier or sledder. The early November crust also exists deeper in the snowpack has been reported as widespread at all elevations and on all aspects. Below this crust large faceted crystals are forming. Alpine snow depths are 150-200cm, and treeline is roughly sitting around 50-100 cm in depth.

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.