Avalanche Forecast

Issued: Mar 30th, 2016 11:00AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Loose Wet, Wind Slabs and Cornices.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

We are in the midst of the first extended spring warm up. Very warm temperatures and the late March sun will focus the avalanche hazard on wet snow concerns Thursday, especially highlighted by the significant recent storm snow in many areas. High freezing levels, light winds and sunny skies over the next several days will cause natural loose wet avalanches on solar aspects and increase the odds for low likelihood/high consequence problems such as cornice failures.

Summary

Detailed Forecast

On Thursday, freezing levels should remain near or above 10,000 feet with recent strong NE crest level winds becoming very light. The very warm daytime temperatures and the late March sun will focus the avalanche danger on wet snow concerns. High freezing levels, light winds and sunny skies over the next several days will cause natural loose wet avalanches on solar aspects and increase the odds for low likelihood/high consequence problems, especially 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.    

The likelihood of triggering older wind slabs continues to diminish, however, in isolated areas above treeline, a mix of older wind slab on lee easterly aspects and newer wind slab on the south through westerly aspects will maintain wind slab potential on a variety of aspects. Watch for cracking and firmer or chalky wind transported snow. 

Many areas have massive cornices along ridge-lines, watch for those overhead hazards. 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,  followed by a large upper trough and a cool unstable air mass crossed the Northwest Sunday. 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 accumulated 17". 

Crest level winds were moderate out of the NE on Tuesday through early Wednesday before diminishing Wednesday afternoon.  Most NWAC stations ranged from the mid 40's to upper 50's by Wednesday afternoon, with Snoqualmie Pass into the 60's! 

The frequent March storms have built unusually large cornices along many ridges as noted by field observers over the past several days.

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

An icon showing Loose Wet

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. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. 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.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Aspects: East, South East, South, South West, West.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 1

Wind Slabs

An icon showing 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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Cornices

An icon showing 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 lips 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.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Mar 31st, 2016 11:00AM