Avalanche Forecast
Regions: Cascades - North East.
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.
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: Possible
Expected Size: 1 - 1