Avalanche Forecast
Regions: Cascades - East.
Exercise extreme caution near avalanche terrain due to both expected new snow layers, and the difficult to manage persistent slab problem. Stick with lower angled terrain not connected to large avalanche paths.
Detailed Forecast
Increasing storm conditions Saturday. Light rain and snow should increase early Saturday through the day, becoming moderate with increasing westerly crest level winds. This should build unstable storm and wind slab layers through the day, with wind slabs spreading to NE-SE facing exposed terrain, especially below ridges by afternoon.Â
Expect dangerous avalanche conditions near and above treeline, especially by late Saturday afternoon. Careful snowpack evaluations, cautious routefinding and conservative decision-making will be essential Saturday.Â
Exercise extreme caution near avalanche terrain in this area due to the persistent slab identified in the snowpack discussion. Subsequent snowfalls may make this layer tougher to trigger... But the increasing slab depth, especially on wind loaded slopes translates to larger avalanches. Slides beginning in new storm layers may possibly step down to this more deeply buried layer, where present. Â
Professional observers and mountain guides are treading carefully with this snowpack structure and gravitating toward lower angled (but still enjoyable)Â terrain not connected to large avalanche paths. Â Â
Snowpack Discussion
The most recent snowfall occurred in the NE and Central WA Cascade east slopes Tuesday and Tuesday night when about 10 inches accumulated (Harts and Washington Passes).  This snowfall began relatively warm and cooled through the deposition, hopefully forming a desirable new snow profile. Moderate NW crest level winds accompanied snowfall and likely created wind slab conditions on the favorable SE lee facing terrain below ridges. No first hand observations have been received since that snowfall in the NE zone.
A warm and moist storm system preceded that, over last weekend with moderate snow with a change to rain at lower elevations before cooling. A natural avalanche cycle likely peaked late Saturday night through Sunday morning, (Dec. 20-21) during the height of the warming (Hart's Pass Snotel peaked at 31 deg F Sat night).Â
Important observations arrived via our pro-observers, North Cascade Mountain Guides and the NWAC Recent Observations page describing a persistent weak layer 50-100 cm below the surface. The PWL consists of a facet/crust combination formed earlier in December. Remotely triggered avalanches were reported in the Washington Pass area Monday, as well as a large natural avalanche on an E aspect in the Cutthroat drainage that likely ran during the weekend cycle. The PWL was generally found on south-east aspects near and above treeline, but we cannot rule out other aspects at this time. Â
Pro-observer Tom Curtis was in the Lake Wentachee area Wednesday. He found rounding facets closer to the surface in the shallower snowpack of Dirtyface Mtn near treeline on a southeast aspect. The fracture character of this layer showed sudden collapses in column tests. While the slabs are shallower in the East Slopes Central zone... they would be easier for a human to trigger. Â
Remotely triggered slab avalanche near Washington Pass, Dec 22nd. Photo by CB Thomas.
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: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 2
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: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
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, South.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1