Avalanche Forecast
Regions: Cascades - North East.
Avoid slopes with shallow snow and without terrain anchors due to uncertainty about persistent slab layers. Continue to watch for wind slab formed over the weekend and found mainly on northwest to southeast slopes near and above treeline. Light E-NE winds may begin to redistribute snow to more westerly aspects near and above treeline by Tuesday afternoon.
Detailed Forecast
Only light new snow accumulations are expected Monday night through Tuesday afternoon along the east slopes. A clearing trend is likely for the northeast Cascades by Tuesday afternoon but mostly cloudy conditions and scattered snow showers are likely to linger for the central-east and southeast Cascades through the day. A cooling trend that begins Monday night will continue during the day Tuesday.Â
More information is needed about several potential persistent slab threats, particularly in the northeast and central-east zones. Due to the lack of direct avalanche activity on any PWL, the avalanche danger will be maintained at Moderate. Evaluate snow and terrain carefully, erring on the side of caution if you are unsure about the snowpack structure in your area and realizing large avalanches are possible in isolated areas under Moderate danger. 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.
Continue to watch for wind slab formed over the weekend and found mainly on northwest to southeast slopes near and above treeline. Light E-NE winds may begin to redistribute snow to more westerly aspects near and above treeline by Tuesday afternoon.Â
Snowpack Discussion
Weather and Snowpack
An atmospheric river arrived over the Northwest Tuesday 1/17 causing heavy snow in the northeast zone with about 2 feet of snow accumulating at Washington Pass. A mix of snow, sleet and freezing rain was likely seen in the central east zone and also in some lower elevation areas east of the crest. The Lyman Lake Snotel, Holden Village and Berne Camp east of Stevens saw about a foot or more of new snow while Mission Ridge and Lake Wenatchee saw about 6 inches. 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 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.
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 on north slopes in the Cutthroat drainage on Saturday. They found non-reactive storm layers in the top 20 cm of storm snow. New wind slab and loose wet snow on solar slopes were considered sensitive to triggering.
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. You will need to avoid steep slopes with shallow snow and especially those without terrain anchors.
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.Â
South
No observations.Â
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: Unlikely
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1