Avalanche Forecast

Issued: Jan 18th, 2016 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Even with an overall stabilizing trend on Tuesday, dangerous avalanche conditions exist. The tricky nature of persistent layers necessitates conservative decision-making and cautious route-finding is essential.

Summary

Detailed Forecast

A weak front should pass overnight Monday, followed by light showers tapering off Tuesday morning. Another frontal system is expected late Tuesday afternoon and night.  

New, shallow wind slab should be found mainly on lee north to east slopes near and especially above treeline. Generally avoid wind loaded slopes above treeline.  

Older storm slabs should have mostly settled and stabilized with some new storm slabs possible. Choose lower angled slopes if you find sensitive storm layers.  

Persistent weak layers have had a good test over the weekend from new loading. For slopes that have not released on these layers, the likelihood of triggering is slowly decreasing, but the consequences are increasing as the slab depth increases. Storm slabs that step down to persistent weak layer depths could become large enough to bury, injure or kill.

Wind slab should be found mainly on the lee northwest to east slopes near and especially above treeline. Generally avoid wind loaded slopes above treeline.  

There have been numerous reports of large tree bombs with the recent snowfall and fluctuating snow levels. Watch for the hazard above! 

Snowpack Discussion

Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th and have been reported as late as Monday the 18th from the Northeast WA Cascades in the Methow, the Icicle Creek drainage, the Blewett Pass area, the Nason Ridge area and Chiwaukum range east of Stevens Pass?. It's safe to say this layer is extensive throughout the east slopes.

On Monday the 18th, NWAC observer Tom Curtis found this layer buried from 30-35 cm and indicated propagation with PST and ECT tests. A 50 foot crack also released upon approach to a north facing starting zone near Diamond Head, also indicating this layer remains reactive. These weak layers are most likely to be found on non-solar (northerly) aspects and in the upper portion of the below treeline and the near-treeline elevation bands along the east slopes.

Jeff Ward found this layer on shaded slopes up to about 5000 feet Sunday in the Early Winters drainage. This layer was reactive and was remotely triggered by a solo traveler below treeline, prompting very cautious terrain selection of lower angled slopes.   

On Sunday, Scottish Lake High Camps reported a natural 12" slab release on a southerly aspect around 5000' and easily identified two buried surface hoar layers in N-NW snowpits at 5500 feet. 

On Saturday Mission Ridge pro-patrol found easy 6-8" ski cuts on lee aspects near and above treeline involving new storm snow.  It is thought that the buried surface hoar previously found in the Mission Ridge backcountry on northerly aspects was likely thoroughly tested earlier in the week and no longer a widespread issue in this specific area.  However, just east of Blewett Pass a surface hoar layer was identified 1/13 in snowpack tests via the NWAC observation page.

Last week, reports via the North Cascades Guides from Delancey Ridge Wednesday saw easily triggered wind slabs of about 20-25 cm on the north and south slopes at about 6000 feet which was releasing on a persistent layer crust sandwich buried on January 11th. Natural avalanches were also heard in the area.

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

Elevations: All elevations.

Likelihood

Possible

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, North West.

Elevations: Alpine, Treeline.

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

Possible

Expected Size

1 - 1

Valid until: Jan 19th, 2016 10:00AM