Avalanche Forecast

Issued: Feb 2nd, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Heightened avalanche conditions are expected on lee aspects, mainly near and above treeline, with shallow but perhaps sensitive wind slab. 

Summary

Detailed Forecast

Little or no precipitation is expected early Tuesday, with light rain and snow increasing in the afternoon. 

Shallow storm slabs formed Sunday and Monday may be poorly bonded to old surface crusts, especially near and above treeline on lee aspects.   

Deeper persistent weak layers are still considered unlikely for human triggering along the east slopes.  

Snowpack Discussion

During the weekend of Jan 24-25th, a warm weather system caused high snow levels and rain east of the crest. NWAC sites east of the crest had up to an 1 inch of rain. This caused loose wet avalanches, some possible wet slab avalanches and consolidation. Mild or sunny weather settled in last week at upper elevations with low clouds or fog along the lower slopes.

Werak frontal systems crossed the PNW Sunday and again Monday, producing generally light amounts of new snow, highest in the northeast Cascades. Moderate SW winds at crest level likely built shallow new wind slab near and above treeline along the east slopes nearest the Cascade crest.  

Recent Observations: Surface hoar growth was seen between about 3000-4500 feet where recent low clouds or fog have persisted east of the crest this week. Some near surface faceting has been reported on non-solar slopes, along with a varying degree of supportable crusts. These weak layers may have been buried by light snowfall Sunday and should be tracked moving forward. 

On Jan 25th, the Mission Ridge ski patrol reported that depth hoar at 45 cm below the surface was becoming rounded but still failed on isolation. NWAC pro-obs or North Cascade Guides reports last week in the northeast and central east zones have found the January 15th facet/crust layers at 40 cm-1 m below the surface with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. So the January 15th facet/crust layers expected in the northeast and central east zones appears to be strengthening and stabilizing.

NWAC pro-observer Tom Curtis was at 7500 ft on Mt Cashmere in the Wenatchee Mountains on Wednesday and found a local facet/crust combo buried on a lee slope next to a ridge crest under 35 cm of thick, surface crust and 1F-P snow. This sort of slab could be very hard for a skier to trigger, but could be possible, perhaps with a large sudden load, such as snowmobiles.

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

Possible

Expected Size

1 - 1

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

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Feb 3rd, 2015 10:00AM