Avalanche Forecast

Issued: Mar 11th, 2016 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Despite a favorable cooling trend Saturday, wind loading near and above treeline will maintain a Considerable rating with human triggered avalanches likely on lee slopes. A stronger storm with an uncertain track on Sunday should result in rising avalanche danger for the latter half of the weekend. 

Summary

Detailed Forecast

We just can't shake the Considerable rating near and above treeline in this storm pattern despite a favorable cooling trend Saturday and only light to moderate new loading expected. Lingering wind slab formed earlier in the week shouldn't be much of an issue, but once again persistent SE to SW transport winds will build new wind slab on lee slopes Friday night and Saturday. Watch for firmer wind transported snow and be aware of local loading patterns in your terrain. Shallow storm slabs may build late Friday night through Saturday morning.   

Cornices should be less sensitive on Saturday, but they are large so continue to give them a wide berth when travelling along ridgelines since they can break further back then expected. 

The loose wet potential should be minimized by the cooling trend and cloudy skies. If the sun does break out Saturday, mid-March sunshine will rapidly increase the likelihood of loose wet avalanches on steeper solar aspects.  

Powerful glide avalanches remain possible in isolated terrain features such as on steep slopes with smooth bed surfaces and especially where the slabs are unsupported from below.

A cool yet powerful storm system on Sunday may cause high avalanche danger for the southern Cascades by late Sunday afternoon. However, the storm track is uncertain and the avalanche danger will be reassessed with Saturday's forecast issuance. 

Snowpack Discussion

Weather and Snowpack

The nonstop active weather pattern continues to push a storm system through the PNW almost every day or two with fluctuating but generally moderate snow levels. The last significant storm impacted the area Wednesday through Thursday with a warming trend that peaked Wednesday night with the frontal passage in the early morning hours and also featured very strong winds seen throughout the Cascade range. Winds decreased Thursday afternoon along with decreasing shower activity. Generally 1.5 to 3 inches of water accumulated along the west slopes in the 36 hours ending 4 pm Thursday. Outside the Cascade Passes, rain reached up to 5000' feet in the north and 6500 feet in the south with snow levels only rising to about 4000 feet at Stevens and 4500 feet at Snoqualmie Pass Wednesday night.  About 4-14 inches of snow accumulated at NWAC stations through Thursday morning. Additional accumulations were light, except above 5000 feet in the Mt. Baker area Thursday where another foot of snow likely fell . 

There remains a significant amount of water in the snowpack. The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

Stevens Pass and Alpental Pro Patrol reported an extensive natural cycle Wednesday night. Control results were sensitive and widespread Thursday AM involving new storm snow. As the sun poked out and temperatures rose Thursday, large loose wet avalanches, both natural and skier triggered were reported by the pro patrol in the Stevens area with smaller loose wet noted in the Alpental area. Crystal patrol generally had 1-2 ft slab releases during control work Thursday with large and sensitive cornices along ridgelines. 

In areas that experienced rain Wednesday night, mainly outside the Passes and in the below and near treeline bands, a breakable crust made for tough skiing Thursday. 

Problems

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

Elevations: Alpine, Treeline.

Likelihood

Likely

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: All elevations.

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

Unlikely

Expected Size

1 - 1

Valid until: Mar 12th, 2016 10:00AM