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Archived

Avalanche Forecast

Dec 23rd, 2014–Dec 24th, 2014
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Cascades - East.

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.

Avalanche Problems

Persistent Slabs

Release of a cohesive layer of soft to hard 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 Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Likely

Expected Size: 1 - 2

Wind Slabs

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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1