Avalanche Forecast

Issued: Dec 5th, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Beware of potentially dangerous persistent slab this weekend! Persistent slabs may be human triggered where persistent weak layers were buried intact. Stay on low angled slopes and watch and listen for obvious clues like shooting cracks and whumpfing. Use caution until more information is obtained about PWLs throughout the east slopes. 

Summary

Detailed Forecast

Strong alpine winds and moderate snow will accompany a cold front across the Northwest Saturday afternoon to Sunday morning. This should bring .5-1.5 feet of new snow east of the crest with the most expected in the northeast zone. A slight warming trend should be seen east of the crest.

Watch for new storm and wind slab on Sunday. Human triggered storm slab should be likely on varied aspects. Human triggered wind slab should be likely on lee north to east slopes especially above treeline.

Beware of potentially dangerous persistent slab this weekend! Persistent slabs may be human triggered where persistent weak layers were buried intact. Stay on low angled slopes and watch and listen for obvious clues like shooting cracks and whumpfing and use caution until more information is obtained about PWLs spatial variability and likelihood to trigger throughout the east slopes

A short minor break between weather systems should be seen Sunday afternoon. This may slightly decrease the avalanche danger.

The next strong system should begin to move into the Olympics and Cascades Sunday night. But be prepared to curtail your plans later Sunday if conditions deteriorate sooner than expected.

Early season hazards exist for much of the below treeline band throughout the Northwest, so ski and ride with caution.

Snowpack Discussion

Note: The snowpack discussion is for the entire east slopes of the Cascades, but the avalanche danger forecast varies by zone.  

The north central and northeast Cascade zones have been privy to the deepest snowdepths found throughout the Cascades this season thanks to cold air pooling on the east side in otherwise warm storms for the west slopes. Snowdepths are generally between 1 to 2 meters near the crest. Long stretches of cold weather under high pressure has lead to near surface faceting and surface hoar formation around the end of November.  At the end of November, professionals in the Washington Pass area reported faceting near the surface crust, and overall a general variety of snow surfaces.  

Roughly 15-20 inches of new snow fell at the NWAC Washington Pass, Holden Village Co-op site and Lyman Lake Snotel stations over the last few days. Unlike in the Stevens Pass area, this new load may have adequately tested PWLs on the east-side of the Cascades and led to a natural avalanche cycle. Pro-observer Jeff Ward was surprised by the lack of activity on the potential PWL layer at Washington Pass on Friday and considered the conditions uncertain.

Further away from the crest for areas like Mission Ridge and for the southeast Cascades, less avalanche problems are likely as well as less snow. Mission Ridge pro-patrol reported no control work and no PWLs in their area near and above treeline due to warmer and windy recent weather.  

We still have many gaps in our observation network as it's early season, so make your own observations and use them in co-junction to our regional avalanche forecast. Thanks for reading the first NWAC avalanche forecast for 2015-16 season, stay safe and have fun! 

Problems

Persistent Slabs

An icon showing Persistent Slabs

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: All elevations.

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: All elevations.

Likelihood

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: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Dec 6th, 2015 10:00AM