Avalanche Forecast
Regions: Cascades - East.
Back country travel is still not recommended in the northeast zone on Wednesday. Conditions are also still very uncertain in the central east zone on Wednesday.
Detailed Forecast
The strong warm front that has caused heavy rain most of Tuesday is expected to pass by early Wednesday with cooling and a period of light to moderate precipitation along with continued strong winds.
Further cooling and lighter showers are expected later Wednesday with strong southwest winds shifting to strong westerly winds.
Update Wednesday morning: In most of the Cascades this weather will cause a transition in the snowpack from wet and saturated surface layers to a draining and gradually refreezing upper snowpack.
But cooler conditions in the central east zone and especially the northeast zone are likely to maintain previous layers and persistent weak layers in the snowpack!
Do not assume that these layers have stabilized in the central east and northeast zones!
The total snow gage at Washington Pass shows an additional 14 inches of snow in the past 24 hours ending Wednesday morning. This snow will be loading previous layers and persistent weak layers in the snowpack. This is still a potentially very dangerous snowpack in the central east zone and the northeast zone. Back country travel is still not recommended in the northeast zone on Wednesday. Conditions are also still very uncertain in the central east zone on Wednesday.
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Snowpack Discussion
Weather and Snowpack:The northeast and north central Cascade zones have the deepest snowdepths found throughout the Cascades this season thanks to cold air pooling on the east side in otherwise warm storms for the west.Â
Snowdepths along the east slopes are generally in the 1-5 foot range with the greatest snowdepths in the northeast zone and the shallowest near Blewett Pass and in the southeast zone. However, strong storms Sunday and Monday were heaviest along the south, so snowdepths are beginning to even out from north-south.
We had a wet and wild November and this formed a strong crust by mid November in all areas.
A long stretch of cold weather led to widespread near surface faceting and surface hoar formation along the east slopes in late November.
The weather so far in December has become very active with periods of heavy snowfall over the past week with an overall warming trend Monday, culminating in a downpour of rain all day Tuesday at high freezing levels.
This weather and snowpack scenario caused ripe avalanches conditions, though few reports have been received as it hasn't been the most pleasant of conditions over the past few days.
Reports:
The most important report for along the east slopes comes from Mission Ridge Ski Area on Sunday where the pro patrol reported numerous and often sympathetic easily triggered slab avalanches of about 8-16 inches releasing on a rain crust from early December. One very large explosively triggered slab avalanche on the northeast slope of Windy Ridge was 5 feet deep and propagated several hundred feet which released on the mid November crust.
Increasingly large persistent and storm slab conditions were found in the Smith-Brook area. just east of Stevens Pass Sunday by NWAC's Dallas Glass. These conditions may be susceptible to natural avalanches with additional loading or a change to rain, possibly before the next storm arrives Tuesday!
NWACÂ observer Jeff Ward was at Washington Pass on Friday and was surprised by the lack of activity on the potential PWL and considered the conditions uncertain.
With varying amounts of rain received along the east slopes Tuesday, it is still too early to tell the current state of that persistent slab problem and how it has or has not changed. This layer may still be reactive, though likely less so now following the rain event. Still best to proceed with caution until more definitive observations come in.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: 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: Possible
Expected Size: 1 - 1
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: Possible
Expected Size: 1 - 1