Avalanche Forecast
Regions: Cascades - South West.
Another round of storm slab and wind slab are likely Wednesday, with new wind slab mainly forming on lee slopes ranging from N-SE facing, especially by afternoon as wind speeds increase. Loose dry avalanches in non-wind affected terrain will be difficult to manage on steeper slopes, so use extra caution around terrain traps.
Detailed Forecast
Another Pacific frontal system, in the long series of recent storms, is moving into the Cascades Tuesday evening. This next system will cause renewed light to moderate snow and light to moderate westerly winds Tuesday night. This should begin to build new areas of wind slab on lee slopes below ridges and create some isolated storm slabs. Moderate to heavy showers should persist with increasing westerly ridgetop winds through the day Wednesday.Â
Dangerous avalanche conditions should persist Wednesday in all elevations bands and especially in the near and above treeline zones.Â
Expected avalanches should remain within the near surface storm related weak layers. Watch for evidence of wind deposited snow or pillows. Test for inverted of strong over weak storm snow and give cornices a wide margin.
Loose dry avalanches in non-wind affected terrain will be difficult to manage on steeper slopes. Even a small loose dry avalanche could have unintended consequences such as knocking you off your feet and into a terrain trap. The recent unconsolidated storm snow is now very deep, two to over three feet in places! There was a snow immersion fatality in a tree well in bounds at Snoqualmie on Saturday, and a close call at Crystal Mtn Tuesday, so always travel with a partner.Â
Wind slab, storm slab and recent cornices and deep unconsolidated snow are all the key features to plan to avoid in your route-finding and descent routes in the coming days.Â
Snowpack Discussion
The Christmas snow globe keeps giving here in the PNW! It's gotten very deep out there. Most west slope areas received 1-2 feet in 24 hours ending Tuesday morning! Slightly less in the Mt Baker area, with over 2 feet at Snoqualmie and Crystal areas. Up to 40 inches have accumulated in the past 2 days and 4-6 FEET have fallen in the past five days!Â
The most recent observations from professional backcountry observations or professional ski patrol performing avalanche control over the past few days, all indicate the extensive avalanches and avalanche potential are being confined to the most recent storm snow layers. A natural avalanche cycle was reported early Tuesday morning at Alpental, likely a response to heavy showers in the waning convergence zone over Snoqualmie Pass Tuesday morning. These very soft slab avalanches were extensive in number, ranging mostly from 8-12" and did not propagate significantly. Likely releasing down to short lived storm layer weaknesses produced often during variations in snowfall intensities and wind speeds or snow crystal type.Â
In the Crystal backcountry Tuesday, observer Dallas Glass made similar observations, noting most avalanche problems were within the shallow storm slab or wind slab formed during Monday's storm. The greatest wind loading and cornice formation was noted on generally east facing terrain near ridges.Â
Crystal backcountry near Bullion Basin, shallow (10") skier triggered soft slab, SW aspect, 5800 ft. Released near noon Tuesday, December 22. Photo: Dallas Glass  Â
The Crystal pro patrol reported similar conditions within the area Tuesday, with numerous storm related soft slabs or wind slabs releasing mostly from 6-12" with a few isolated larger pockets.Â
NWAC observer, Matt Schonwald, found increasing wind and storm slab layers forming through the day Monday in the Crystal Mountain backcountry. Multiple test pits on the wind loaded west facing slopes at about 6800 feet showed propagation in ECT tests. Failures ranged from 16-30 inches down on or near the rain event crust formed December 17th. Wind and storm slabs were noted developing at lower elevations on cross loaded terrain features, well below ridgeline.
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Avalanche Problems
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.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1