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Avalanche Forecast

Archived

Mar 28th, 2016–Mar 29th, 2016

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Stevens Pass.

Moderate and persistent crest level winds out of the northeast Monday night and Tuesday will redistribute recent snowfall to unusual aspects mainly above treeline. At lower elevations and in wind sheltered terrain, the main avalanche problem will shift from wind slab to loose wet due to sunny skies and warming temperatures. 

Detailed Forecast

Moderate and persistent crest level winds out of the northeast Monday night and Tuesday will redistribute recent snowfall to unusual aspects mainly above treeline. At lower elevations and in wind sheltered terrain, the main avalanche problem will shift from wind slab to loose wet due to sunny skies and warming temperatures.  

A mix of older wind slab on lee easterly aspects and new wind slab on south through westerly aspects will make wind slab avalanches possible on a variety of aspects, and ultimately for tricky and dangerous travel above treeline Tuesday. Watch for cracking and firmer or chalky wind transported snow. Yesterday's windward aspect may be today's loaded lee slope. 

Melt-freeze crusts can quickly break down with intense spring sunshine. In areas that have received more recent snowfall, small loose wet avalanches have the potential to entrain deeper layers. Moderate winds above treeline will limit the loose wet potential except on direct solar slopes. Be aware of terrain traps where even a small loose wet avalanche could have unintended consequences.  

Near treeline should be a mix of loose wet avalanche and wind slab concerns.  

Cornices have grown large recently. Cornices can break much further back on ridges than expected and releases can be unpredictable during the spring.

Although not listed as an avalanche problem, large powerful glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces.

Snowpack Discussion

Weather and Snowpack

Snow accumulations last Wednesday to Friday varied based on elevation with only a few inches below 4000 feet and 6-21 inches above. The most snow was received at the NWAC Baker and Paradise stations. This snow was generally well-bonded to a moist crust buried March 21st/22nd. 

A strong front crossed the Northwest Sunday morning followed by a large upper trough and a cool unstable air mass Sunday afternoon and evening. Along the west slopes, most locations saw a few inches of accumulation with the highest totals of roughly 10" at Snoqualmie Pass and Paradise. Thanks to a convergence zone aimed at Snoqualmie Pass Sunday evening, the top of Alpental ended up with 17". 

Frequent March storms have built large cornices along ridges in many areas.

The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

Dallas Glass was out near Paradise on Friday and found 60 cm of settled, bonded storm snow. He witnessed large rollerballs to 3 feet in diameter - watch out for those! Several small natural and ski triggered loose wet avalanches were seen on east to south facing slopes. There was significant wind redistribution of snow in the above and near treeline.

On Saturday large avalanches were observed on the upper elevations of the south side of Mt. Rainier, likely due to serac fall. Travelers to Camp Muir reported the avalanches on TAY and the NWAC observation page.

Lee Lazarra was in the Table Mountain/Mt. Herman backcountry of Mt. Baker Monday. He found Sunday's storm snow generally right side up and well bonded to a moist rain crust 25-30 cm down. Wind slab was sensitive in isolated areas near treeline, with new wind slab forming on south through west aspects. 

Alpental pro-patrol reported some pockets of soft wind slab up 17" during morning control work Monday morning. Along the lower mountain, the loose wet potential increased as the sun popped out mid-day.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.