Avalanche Forecast

Issued: Jan 26th, 2018 11:37AM

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 Loose Dry.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Additional snow and wind along the east slopes of the Cascades will continue to develop new and sensitive wind and storm slabs. Caution should be taken with increasing elevation as more snow and wind will lead to larger and more dangerous avalanches. 

Summary

Detailed Forecast

Additional snow and wind on Saturday will continue create dangerous avalanche conditions along the east slopes of the Cascades.

Winds will continue to form sensitive wind slabs on a variety of aspects. Strong winds may deposit snow and form wind slabs lower downslope and at lower elevations than normal. Identify and avoid areas of wind deposited snow such as below fresh cornices and on snow drifts.

New shallow and sensitive storm slabs will develop Friday night into Saturday morning in areas that receive higher precipitation amounts in the northern part of the forecast area. Avalanches may trigger on steeper slopes and convex rollovers. Identify and avoid slope features where you are more likely to trigger an avalanche.

Uncertainty still exists around the distribution of buried surface hoar. Travel with caution and ease into terrain if traveling in areas further east of the Cascade crest.

Cornices have grown large over the past week. Give them a wide berth as they can break much farther back from the ridge than anticipated. Consider overhead cornice hazard while traveling and avoid spending time below large cornices.

Snowpack Discussion

An active storm period has deposited new snow across the east slopes of the Cascades over the last several days. Near Washington Pass, 20 inches of settled snow sits over the 1/16 crust. Further south and east lesser amounts of snow have been recorded. (10 inches in the Lake Wenatchee area and 6 inches at Mission Ridge and Blewett Pass)

Winds throughout the storm cycle have redistributed snow forming wind slabs on a variety of aspects.

Several weak layers can be found within the storm snow. Observations demonstrate most of these layers gaining strength.

Observations from around the east slopes central and north found buried surface hoar above the 1/16 crust. There is still quite a bit of uncertainty around the distribution of this layer. Extra caution should be taken when traveling in areas further east  of the crest or in areas where less snow has been received.

Snowdepth still decreases substantially east of the Cascade crest. In many areas below treeline, there has not been enough snow to present an avalanche danger.

Observations

North

On Thursday and Friday, NCH observed evidence of several recent wind slabs that had released in steep terrain above treeline in the Washington Pass area. Away from wind affected terrain and in several snowpits, a generally stable and right-side-up upper snowpack was noted. 

Wednesday, Jan 24th, NCMG observations on Delancy Ridge found nearly 3 ft of low cohesion storm snow in sheltered terrain below treeline with no avalanches noted. No reactivity was found at the 1/16 interface with the melt-freeze crust.

On Monday, snowpit tests by a snow professional on a SE aspect near treeline in the Twisp River Valley gave sudden collapses within faceted grains below the 1/16 crust. Test results on a NE aspect yielded consistent sudden planar results above the 1/16 crust with partially decomposed surface hoar on the interface.

NWAC received a second-hand report of a large skier-triggered avalanche on west-facing slope (likely near treeline) on Abernathy Peak in the upper Twisp River drainage. The avalanche propagated widely and was suspected to have failed on the 1/16 crust.

Central

A public observation from Wednesday in the Blewett Pass area identified basal facets at the bottom of the snowpack and surface hoar above the most recent crust. Snow cover was still regionally low in this area. Small skier triggered wind slabs were noted and rain briefly reached up to 5000' Wednesday morning.

Buried surface was found in the Icicle Creek drainage over the 1/16 crust over a week ago up to 6800 ft. 

South

No recent observations

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: All aspects.

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

Loose Dry

An icon showing Loose Dry

Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.

Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.

 

Loose Dry avalanche with the characteristic point initiation and fan shape.

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 27th, 2018 11:37AM