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Avalanche Forecast

Archived

Dec 23rd, 2014–Dec 24th, 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 potentially dangerous persistent weak layer is reported in this area.

Detailed Forecast

Northwest winds, decreasing snow showers heaviest at higher elevations and much lower snow levels will be seen on Wednesday. About 5-10 inches of new snow seems likely mainly in the near and above treeline east of the crest by Wednesday morning.

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 on all aspects in all the elevation bands in this area until more is known about the extent of this layer.

New wind slab and possible new storm slab should also be seen Wednesday. These new layers should be seen mainly in the near and above treeline. Avoid travel on steep lee slopes with firmer wind transported snow that is a sign of wind slab. Storm slab should be limited to steep slopes in areas that receive more than an inch of snow an hour for at least several hours.

Have a plan in place before you get to the trailhead. Confidence is fair for this forecast.

Snowpack Discussion

A strong warm storm moved over the Northwest Saturday and Sunday. Decreasing winds, a little cooling and snow was seen at the tail end of the storm Sunday night. A relative break was generally seen Monday.

Important observations via our pro observers and the NWAC Recent Observations are available from Monday. A potential persistent weak layer is reported at 50-65 cm below the surface at a facet/crust combination from early or mid December in the Washington Pass area. This was seen on southwest to southeast facing slopes. Skiers remotely triggered slab avalanches along with snowpackwhoomping and cracking on Monday.

Remotely triggered slab avalanche near Washington Pass on 22 December by CB Thomas.

Widespread natural wind and storm slab releases were also reported on north to east facing slopes below ridges on Mt Cashmere on Monday.

A moist cold front is moving over the Northwest Tuesday to Wednesday morning. This will be accompanied by strong southwest winds, moderate to heavy rain changing to snow, and lowering snow levels.

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.

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.