Avalanche Forecast
Issued: Nov 26th, 2017 10:08AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Glide Cracks.
NWAC,
Northwest Avalanche Center
As the snowpack refreezes, there is still a low chance for glide avalanches near and below treeline on steep unsupported terrain features. Much of the below treeline elevation band does not have enough snow cover to support an avalanche hazard. Isolated and shallow pockets of storm or wind slab may exist near and above treeline. Watch for non-avalanche related hazards such as open creeks and barely buried rocks and trees.
Summary
Detailed Forecast
A fresh 3-8" of snow with potentially more at Mt. Baker is forecast Sunday night through Monday morning. Light snow showers are expected for the remainder of Monday.
The snowpack will begin to refreeze Sunday night and Monday with cooler weather. There is still a low chance for glide avalanches near and below treeline on steep unsupported terrain features that did not release during the recent heavy rains. Much of the below treeline elevation band does not have enough snow cover to support an avalanche hazard. Otherwise shallow isolated pockets of storm or wind slab may exist near and above treeline. The Moderate hazard will be maintained near and above treeline in part due to the lack of direct observations over the last few days.
Watch for the numerous travel hazards such as open creeks, barely buried rocks and trees, and glide cracks, creating poor and challenging travel conditions, especially below treeline.
Remember that closed ski areas without avalanche mitigation are equivalent to backcountry terrain!
Snowpack Discussion
New resources within your Avalanche Forecast this season!
The Mountain Weather tab will take you to the most recently issued Mountain Weather Forecast. The Observations & Weather Data tab will allow an easy view of the various weather station graphs within your zone of interest and provide easy access to the most recent avalanche and snowpack observations.
The great start to the 2017/18 PNW Winter took a giant step backwards over the last week. During an extended period of warmth and wet weather in the days leading into Thanksgiving, an initial round of wet snow and glide avalanches occurred, especially on steep unsupported slopes and rock faces. Another round of rain Saturday night through mid-day Sunday likely did not cause widespread wet snow avalanches on an already beat up and saturated snowpack. This warm, wet and windy period melted significant snow with total snow depths decreasing by 50% or more from their mid-November peak depths at many NWAC stations. This has allowed much of the previously well snow-covered terrain to open with many creeks and snow bridges re-appearing, especially near and below treeline.
Rain had changed to snow Sunday afternoon with further cooling and generally light snow accumulations expected overnight.
Observations
No weekend observations due the wet weather, numerous ski area closures and lack of snow in WSDOT avalanche paths.
Older observations from Friday November 24th in the Mt Baker and Alpental Valley backcountry:
In the Baker region, the key takeaway is just how much the snow cover has diminished with travel becoming significantly more challenging and hazardous due to open creeks and shallow snow. Most of the observed recent wet snow or glide avalanches were estimated to be two or more days old. No direct observations were made above treeline.
In the Alpental Valley, a similar story with evidence of numerous glide avalanche releases from steep unsupported terrain and rock faces. Travel conditions, especially below treeline, are now difficult with numerous early season challenges and hazards having quickly returned.
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: Alpine, Treeline.
Likelihood
Expected Size
Glide Cracks
A release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often preceded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
Predicting the release of Glide avalanches is very challenging. Because Glide avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide avalanches.
This Glide avalanche broke to the ground on a smooth, grassy slope. From all the mud on the bed surface, water pooling at the base of the snowpack likely caused the failure.
Glide avalanches occur when water lubricates the interface between the snowpack and the ground. These avalanches are difficult to predict and best managed by avoiding terrain below glide cracks.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Nov 27th, 2017 10:08AM
