Avalanche Forecast

Issued: Jan 29th, 2018 11:50AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Dangerous avalanche conditions continue near and above treeline. Recent heavy snowfall at higher elevations have created large storm and wind slabs that will not have time to heal under additional wind loading. Watch for lower density newer snow actively forming wind slab on Tuesday. Avoid steep, wind-loaded, or convex slopes and give recent storm snow instabilities time to heal. 

Summary

Detailed Forecast

Relatively brief, but heavy precipitation along with strong winds Monday afternoon and evening is depositing significant snow in near and above treeline terrain. Sharp cooling accompanies a change to much lower snow levels and light snow showers overnight. The cooling trend and light to moderate shower activity (in locally intense bands) will continue through the day on Tuesday, helping to refreeze the upper snowpack and adding additional snow in all elevation bands. 

Large wind slabs were created during a series of moist and windy storm systems from Saturday through Monday night. These wind slabs need time to heal and possibly buried under lower-density snow. Consistent W-SW winds will continue to transport lower density snow and will build fresh wind slabs, particularly on NW-N-NE-E-SE slopes near and above treeline. These fresh wind slabs may become large in size by the end of the day Tuesday. Travel cautiously in wind-affected terrain, avoiding wind-loaded terrain near ridgelines and watch for signs of wind-transported snow, including snow that feels stiff or chalky.

Large storm slabs have likely developed in areas/elevations that stayed mostly snow during this storm cycle and also need to time heal. These areas may include near and above treeline terrain in the central and south, or any elevation band in the north. In areas receiving more more intense snowfall rates, fresh storm slabs will build and may become touchy to trigger. Constantly evaluate the sensitivity of the snow, checking small test slopes.

In areas below the snowline, the refreezing upper snowpack should generally put to rest some of the deeper slab avalanche concerns we've been having in recent days as well as any loose wet avalanches seen Monday evening and night before the cooling takes place.

 

Snowpack Discussion

Heavy rain has fallen to the top of the near treeline band on Monday afternoon. This has saturated the significant new snow across the west slopes which has fallen over the last week and a half. This recent 4 feet of accumulated dry snow has settled by 10-15" as of Monday morning with further settlement anticipated during the day and overnight hours Monday. Prior to the the warm temperatures, observers reported many natural and triggered slab avalanches in the recent snow from a variety of aspects and elevations.

Poor visibility and frequent storms have limited observations near and above treeline.

Observations

Central

On Sunday, an avalanche forecaster on Snoqualmie Pass reported wet loose avalanches in steep, rocky terrain below treeline. Other reports including rain in the morning, runnels forming in the snow below treeline, and water draining through the recent snow.

Also on Sunday an observer on Snoqualmie pass reported tests results at 3,000ft indicating propagation on a melt-freeze crust about 2 feet below the surface. This crust may be harder to find at higher elevations.

South

NPS Rangers reported a very large natural slab avalanche on Sunday from an east aspect at 6,800ft in Van Trump Park on Mt Rainier. This released in recently wind drifted snow.

Problems

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Jan 30th, 2018 11:50AM