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Avalanche Forecast

Archived

Dec 22nd, 2014–Dec 23rd, 2014

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.

Regions

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A dangerous persistent weak layer is reported in this area.

Detailed Forecast

Some light showers may move to the higher elevations east of the crest on Tuesday. This is not likely to create significant new layers by Tuesday afternoon. Note that a more significant increase in winds and rain or snow is expected in the northwest Cascades Tuesday afternoon.

Avoid avalanche terrain in this area due to the persistent slab layer. It is often possible to trigger persistent slab longer after the slab was formed and may not be possible to accurately predict. The avalanche danger will be listed at considerable and the size small to very large in all the elevation bands in this area until more is known about the extent of this layer.

Wind slab from the previous storm may also linger mainly on steep north to southeast slopes in the above treeline band.

Increasing winds and increasing rain or snow will likely begin to create new layers Tuesday night.

Snowpack Discussion

A strong warm storm moved over the Northwest Saturday and Sunday. Winds were seen to over 100 mph at some locations. Precipitation was generally heavy or very heavy near and west of the crest and moderate to heavy east of the crest. Precipitation fell as snow mainly in the above treeline zone in most areas but may have fallen lower in this area Saturday and Sunday.

Decreasing winds and a little cooling were seen at the tail end of the storm Sunday night and Monday with a little snow making it down to  the near or below treeline.

Our pro observers are reporting a persistent weak layer at 50-65 cm below the surface at the early December crust in the Washington Pass area on Monday. Skiers remotely triggered slab avalanches along with snowpack whoomping and cracking. Note that a persistent slab is one of the most dangerous types of slab layers.

Lingering wind slab may also linger mainly in the above treeline band on Monday.

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.