Avalanche Forecast
Regions: Mt Hood.
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.
Detailed Forecast
A fresh 4-8" of snow is forecast Sunday night through Monday morning. Light snow showers are expected for the remainder of Monday.Â
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.Â
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.Â
Remember that closed ski areas without avalanche mitigation are equivalent to backcountry terrain! Â
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Snowpack Discussion
New resources within your Avalanche Forecast this season!Â
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.Â
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 including Mt. Hood. 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.Â
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.Â
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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: Unlikely
Expected Size: 1 - 1