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Avalanche Forecast

Archived

Dec 15th, 2011–Dec 16th, 2011

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

Regions

Northwest Inland.

Confidence

Fair - Forecast snowfall amounts are uncertain on Saturday

Weather Forecast

Friday: Snow amounts amounts 20-30cm, and rain at lower elevations. Freezing levels are expected to rise to 1600m. Ridgetop winds will be blowing strong from the south then switching south west. Saturday: Light-moderate snow amounts, rising freezing levels during the day, then falling back to valley bottom at night. Sunday: Looks like a break in the systems. Mix of sun and cloud, freezing levels back to valley bottom.

Avalanche Summary

Several size 2 natural avalanches have been reported. These occurred on northerly aspects failing on the SH/CR/FC combo layer. I suspect we will see a natural cycle through the early part of the weekend with more snow and wind forecast.

Snowpack Summary

New surface hoar crystals have formed up to 5mm in size and sits on the recent 20cm of storm snow which is slowly settling. The storm snow we received earlier this week sits on a variety of old weak snow surfaces including surface hoar, melt-freeze crusts and facetted snow (SH/CR/FC). For the most part all aspects between 900m-2000m have a hard melt freeze crust. This crust is widespread and is lingering in most start zones. This is the layer of concern as we receive more snow and wind Friday and Saturday. Above this crust surface hoar (feathery crystals) and facets (sugary crystals) are preserved. This exists at treeline and below treeline elevations. The recent winds in the alpine have destroyed the surface hoar eliminating one problem but creating a wind slab problem. These wind slabs are in the immediate lee of features, sitting on the SH/CR/FC combo. They are reactive and rider triggered avalanches are likely. The midpack seems to be well settled and strong. Thanks to those who have sent in field observations. If you have any info from the field, we would love to hear from you. Write to us at [email protected]

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.