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Avalanche Forecast

Archived

Mar 30th, 2016–Mar 31st, 2016

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

Regions

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Warmth and the late March sun will shift the primary avalanche hazard to wet snow concerns Wednesday. High freezing levels, light winds and sunny skies over the next several days will increase the hazard, causing natural loose wet avalanches on solar aspects and increasing the odds for low likelihood/high consequence problems such as cornice failures and glide avalanches.  

Detailed Forecast

On Wednesday, alpine winds will ease during the day and freezing levels should rise to around 10000 feet. Warmth and the late March sun will shift the avalanche hazard to wet snow concerns Wednesday. High freezing levels, light winds and sunny skies over the next several days will increase the hazard, causing natural loose wet avalanches on solar aspects and increasing the odds for low likelihood/high consequence problems such as cornice failures and glide avalanches.  

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. Be aware of terrain traps where even a small loose wet avalanche could have unintended consequences. Plan to avoid steep solar slopes by late morning/mid-day to minimize the problem.    

A mix of older wind slab on lee easterly aspects and newer wind slab on south through westerly aspects will make wind slab avalanches possible on a variety of aspects, primarily above treeline. Watch for cracking and firmer or chalky wind transported snow.   

Many areas have massive cornices along ridge-lines so be cognizant of the hazard above. Unlike the loose wet problem, you may not have much notice that a cornice is about to fail. 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 (see Lee's filmstrip photo for a recent example). 

Snowpack Discussion

Weather and Snowpack

Snow accumulations Wednesday 3/23 to Friday 3/25 varied based on elevation with only a few inches below 4000 feet and 6-21 inches above. 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". 

Crest level winds were moderate out of the NE on Tuesday. Most NWAC stations were pushing into the 40s by mid-Tuesday afternoon. 

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

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.

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.

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 older snow or a sun crust 25-30 cm down. Wind slab was sensitive in isolated areas near treeline, with new wind slab forming on south through west aspects. Lee also observed debris from a glide avalanche on Mt. Herman that released either Friday or Saturday, highlighting this sporadic yet ongoing problem in the Baker area. 

Via the NWAC observation page, a skier triggered and was carried in a small slab avalanche above Source Lake on the Chair Peak side Monday evening.  

Problems

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.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.