Avalanche Forecast

Issued: Jan 21st, 2016 10:33AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

In the northeast and centraleast Cascades, light to moderate snowfall Thursday night through Friday morning and a slow warming trend should gradually stress and load buried weak layers, potentially resulting in large and destructive avalanches. A natural avalanche cycle may occur Thursday night through Friday morning. Travel in backcountry terrain is not recommended Friday due to dangerous avalanche conditions.

Summary

Detailed Forecast

In the northeast and centraleast Cascades, moderate snowfall Thursday night through Friday morning and a slow warming trend should gradually stress and load buried weak layers, potentially resulting in large and destructive avalanches. A natural avalanche cycle may occur Thursday night through Friday morning. The danger from storm and wind slab avalanches will begin high Friday morning, but should decrease as showers taper down Friday afternoon and temperatures cool in the near and above treeline zones.  

Travel in backcountry terrain is not recommended Friday due to these conditions. 

Natural wet loose avalanches are possible predominately below treeline Friday and mainly on steeper slopes where temperatures stay or rise above freezing. Be especially wary near terrain traps, where even a small avalanche could have unintended consequences. Loose wet avalanches may entrain a considerable amount of storm snow, even at low elevations. 

The danger should be lower in the southeast Cascades, where less recent snowfall was received. 

Snowpack Discussion

Weather

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 through Monday from the Methow, the Icicle Creek area, the Blewett Pass area, the Nason Ridge area and the Chiwaukum range east of Stevens Pass. It's safe to say this layer is extensive throughout the east slopes.

A progressive weather pattern this week added about .5-1 feet of snowfall to the east slopes from about Friday to Sunday. Only a few inches of additional snowfall accumulated with a weather system that passed through Oregon Tuesday and Tuesday night. A warm front brought light to moderate amounts of new snow through Thursday morning, with up to 10 to 14 inches of new snow seen at the Hart's Pass snotel and Holden Village respectively, with about 6 inches at Mission Ridge. Temperatures warmed up above freezing along the lower east slopes during the day Thursday. WSW ridgetop winds stayed strong at Mission all well. 

Snow and Avalanche Observations

NWAC pro observer Jeff Ward on Sunday found the reactive persistent weak layer on shaded slopes up to about 5000 feet in the Early Winters drainage. A video by Jeff shows a similar persistent weak layer and a remotely triggered avalanche in the Icicle Creek Drainage near Leavenworth on Monday.

On Sunday the observer at the Scottish Lake High Camp in the Chiwaukums reported a natural 12 inch slab release on a southerly aspect around 5000 feet and easily identified two buried surface hoar layers in snowpits on a north-northwest slope at 5500 feet.

On Monday NWAC pro observer Tom Curtis was on Diamond Head near Blewett Pass in the 4-6000 foot range and found a persistent weak layer at 30-35 cm on north to east aspects with propagation indicated via PST and ECT tests. Tom's video is here.

NCMG on Monday found several persistent weak layers near Goat Peak at 6400' on a north aspect, reactive to column test and sensitive to human triggering.  

Remotely triggered avalanches on persistent weak layers were reported in TAY in the Mission Ridge area Monday. 

NWAC pro observer Tom Curtis visited the Rainy Creek drainage of the central east zone (near Jove Peak) Tuesday and traveled from 2000-5800 feet and did not find any signs of lingering persistent weak layers.

NCMG found touchy conditions below treeline near Delancey Ridge Friday on moderate slopes, with sensitive slabs releasing on the Jan 11th weak layer about 60 cm down. They also heard natural avalanches in their general vicinity. 

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

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, North West.

Elevations: Alpine, Treeline.

Likelihood

Very 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

Likely

Expected Size

1 - 1

Valid until: Jan 22nd, 2016 10:33AM