Avalanche Forecast

Issued: Nov 24th, 2017 10:24AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Glide Cracks and Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The likelihood of wet snow avalanches has diminished. However, Glide or wet loose avalanches remain possible in specific terrain, such as steep unsupported slopes. Watch for overhead hazards and non-avalanche related hazards such as open creeks and shallow covered hazards. Watch for unstable storm snow in areas with the most recent snowfall, such as wind loaded terrain in the Mt Baker area.

Summary

Detailed Forecast

Saturday

A brief break in precipitation is expected in most areas through early afternoon with gradual warming. Light precipitation at further warming expected later Saturday afternoon spreading from south to north. The warming temperatures Saturday should allow for any recent storm snow to continue to settle in place and stabilize. Only light amounts of precipitation are expected during daylight hours and should appreciably affect the avalanche danger.

Old wet snow conditions should persist, however, and maintain the potential for glide avalanches on steep unsupported terrain features that may not have released during the recent heavy rains. 

The old wet snowpack should continue to drain and stabilize and slowly re-freeze Saturday, especially at higher elevations. 

Watch for the numerous re-emerging ground hazards, such as open creeks, shallowly covered rocks and trees, creating poor and challenging travel conditions, especially below treeline.

Remember that closed ski areas without avalanche mitigation are equivalent to backcountry terrain!  

 

Snowpack Discussion

Avalanche and Weather Summary

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 of winter 2017/18 in the PNW has taken a recent hit as warm wet weather arrived Tuesday morning and has just begun to cool as of late Thanksgiving. Most west slope areas received 4-9 inches of rain since Tuesday morning, reaching well above the Cascade crest level. The warming and rain initially triggered numerous wet snow and glide avalanches earlier this week, mostly reported on Tuesday and Wednesday. Numerous size 2 avalanches occurred, especially on steep unsupported slopes and rock faces where many glide avalanches released. 

Rain has now melted significant snow with average snow depths decreasing about 35-60% since Monday! 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. 

The wet or saturated snowpack continues to drain and as of Friday afternoon is beginning to solidify and begin slowly re-freezing.

Cooling since Thursday with additional, mostly light precipitation, has begun to add some new snow above the old wet snowpack. The greatest new snow has been in the Mt Baker area where about 4-6 inches of new snow had accumulated as of Friday afternoon.

Observations

A few NWAC observers began assessing the post deluge Friday, Nov 24, both in the Mt Baker and Alpental Valley backcountry. 

In the Baker region, the key take away 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. The most recent storm snow, up to about 6 inches, was reluctant to release on steep test slopes with storm layers appearing to rapidly settle with near freezing temperatures. No direct observations were made above treeline.

Mt. Baker pro-patrol saw a very active day on Tuesday with numerous large glide avalanches releasing down to steep rock surfaces near and below 5000 feet in elevation.  Besides the glide avalanches, widespread natural loose wet activity ranging from small to large was observed on Shuskan Arm.   

In the Alpental Valley, a similar story is told 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

Glide Cracks

An icon showing 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

Possible

Expected Size

1 - 1

Storm Slabs

An icon showing 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

Possible

Expected Size

1 - 1

Valid until: Nov 25th, 2017 10:24AM