Avalanche Forecast
Regions: Cascades - South East.
Recent N-NE winds have created some wind slab on a variety of mainly southerly aspects, especially near ridges, so make sure to evaluate wind loaded terrain. Moderate avalanche danger means human triggered avalanches are possible! The persistent slab varies in depth and likelihood of triggering across the east slopes, requiring careful snowpack evaluation and conservative decision making.
Detailed Forecast
Thursday should continue to be mostly sunny with light winds. Temperatures should remain warm. The main avalanche problem should be any areas of lingering wind slabs formed through Tuesday. More recent northerly winds likely redistributed snow on more southerly aspects above and near treeline. Warm temperatures and sunshine should again cause a chance of small wet loose slides on some steeper southerly facing slopes. good settled powder should persist on shaded slopes and wind protected slopes however.
Extra caution also remains warranted near avalanche terrain in these zones due to the potential for a persistent slab, this avalanche problem varies across the east slopes, may not be widespread but is noteworthy. Heavier triggers such as snowmachines may be needed, or finding a shallower spot in the snowpack to trigger.Â
Snowpack Discussion
A cold front moved over the Northwest on Saturday and Saturday night. West winds changed to northwest, with moderate snow east of the crest and a cooling trend. NWAC and Snotel sites east of the crest picked up about 3-10 inches of new snow by Monday morning.  Sites further from the crest such as Mission Ridge and Blewett Pass picked up most of their snowfall after the front Sunday night/Monday with easterly upslope.  New snowfall generally fell right side up but strong NE-E transport winds likely built new wind slabs on S-W aspects in the near and above treeline zonesÂ
Observations describing a persistent slab due to a facet/crust from 50-100 cm below the surface in the Washington Pass early this week have found less evidence of this layer over the past few days. Remotely triggered avalanches were reported last week, but no recent avalanches have been seen on these layers of late in the below and near treeline elevation bands.
However, in the central east zone above Lake Wenatchee on Dirty Face Mtn Wednesday, NWAC observer Tom Curtis found well developed depth hoar over a crust buried 95 cm below the surface. This was producing very easy, sudden planar test results. A strong 40-50 cm slab layer lies above this with teh recent storm snow above that. This may make triggering a slide on this layer difficult, however a snowmachine may be the right trigger, or finding a shallow trigger point, such as near rocks or a thin spot in the snowpack. This layer may not be widespread, but it definitely is worth investigating for in more exposed steeper terrain along the east slopes.Â
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Depth Hoar found above rain crust @ 95 cm below surface, Q1 Sudden planar, very easy results at Dirty Face above Lake Wenatchee on 12/31 by Tom Curtis.Â
At this time we believe the persistent weak layer may be more dangerous, with a greater ease of human triggering, in the central east Cascade zone.Â
No recent information is available from the south-east Cascade zone, so travelers are urged to perform numerous snowpack evaluations and make conservative decisions when planning routes.
Avalanche Problems
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: South East, South, South West, West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: Unlikely
Expected Size: 1 - 2