Avalanche Forecast
Regions: Cascades - North East.
Northeast or east winds will begin to redistribute new snow to lee westerly aspects Wednesday. New wind slab may become particularly touchy in areas where it bonds poorly to an underlying sun crust or builds over recently buried surface hoar. The wind slab hazard may actually be the greatest in the near-treeline zone Wednesday where the combination of building wind slab over recently buried surface hoar come together to create dangerous avalanche conditions. Wind slab should primarily be found near and above treeline but look for firmer wind transported snow down into the upper portion of the below treeline band.
Detailed Forecast
Northeast or east winds will begin to redistribute new snow to lee westerly aspects Wednesday. New wind slab may become particularly touchy in areas where it bonds poorly to an underlying sun crust or builds over recently buried surface hoar. Wind slab should primarily be found near and above treeline but look for firmer wind transported snow down into the upper portion of the below treeline band. All aspects are listed to cover older wind slab formed over the weekend. Â
The wind slab hazard may actually be the greatest in the near-treeline zone Wednesday where the combination of building wind slab over recently buried surface hoar come together to create dangerous avalanche conditions.Â
More information is needed about several deeper potential persistent slab threats, particularly in the northeast and central-east zones. Dig a snow pit to check for weak layers in areas with shallow snow and avoid steep slopes with shallow snow and especially slopes without terrain anchors.
Snowpack Discussion
Weather and Snowpack
An atmospheric river arrived over the Northwest 1/17 causing heavy snow in the northeast zone, a mix of snow, sleet and freezing rain in the central-east zone and also in some lower elevation areas east of the crest. Rain was likely seen up to about 5-6000 feet in the southeast zone during this stretch. The 1/17 crust is very thin in the northeast zone and more notable in areas further south.  Â
A large upper ridge and warm air mass built over the Northwest last Thursday to Saturday. By Saturday temperatures warmed into the 30s at higher elevations along the east slopes but stayed cold at lower elevations. Moderate west winds were seen at higher elevations.
A front crossed the Northwest Sunday afternoon causing continued west-southwest winds at higher elevations along with slight cooling and little to no measurable precipitation. Light snow fell Monday night through Tuesday morning with generally 2-6 inches recorded along the east slopes, with the most at the Berne Camp east of Stevens. An observation from Mt. Cashmere Tuesday reported locally up to 10 inches of low density snow between 4000-5700 feet. Â
Recent Observations
North
Reports from the NCH Barron Yurt last week indicated no direct signs of instability, but avalanche professionals did find unstable results in snowpack tests around faceted grains just below the 1/17 interface in isolated areas on cold northerly aspects. Persistent slab will be re-listed in the northeast zone to deal with this uncertainty.Â
The NCMG were in the Hairpin Valley on Sunday and noted strong W-SW winds along the ridge crests with colder temperatures at lower elevations. Compression tests gave no results on the 1/17 layer at 45-50 cm.
Central
Tom Curtis was at Dirty Face above Lake Wenatchee Thursday, 1/26 and found a persistent weak layer of buried surface hoar, consistently buried about 2 feet (60cm). Test results were inconsistent upon multiple column tests. No avalanches were seen on this layer.
Stevens DOT personnel were on Tumwater Mountain on Friday and found a continental-like profile with 77 cm total snow, 20 cm of basal facets, foot penetration to the ground.Â
Jeff Ward was at Cannon Mountain off of Icicle Creek Monday. He found a variable and shallow snowpack that ranged from 1 to 2 m, depending on elevation and wind affect. Many windward slopes had been scoured to the 1/17 crust with thin wind slab present on lee slopes. The 1/17 crust was down 20-40 cm but no avalanche activity was observed on this layer. Large surface hoar was found at all elevations in non-wind affected terrain.Â
South -Â No observationsÂ
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
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 - 1