Avalanche Forecast

Issued: Jan 18th, 2018 12:31PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Human-triggered avalanches are likely on Friday. New snow and moderate SW winds will combine to create sensitive wind slabs near and above treeline. The danger increase will be storm snow dependent. Both storm and wind slab will increase in size throughout the day. 

Summary

Detailed Forecast

Cool temperatures, moderate snow showers and periods of moderate S-SW winds will continue to deposit new snow and further build existing wind and storm slab layers.

Note that a refreezing 1/17 rain crust will continue to harden and may provide a bed surface for wind and storm slab avalanches on Friday.

Building wind and storm slabs should maintain dangerous avalanche conditions near and above treeline. Storm slabs may form quickly and become sensitive to human triggering, especially in those areas receiving the greatest amounts of new snow (more than about 8" of new snow accumulated) and higher snowfall rates (extended periods of more than about 2" per hour). 

Conservative terrain choices will be essential in higher and wind exposed terrain. Best to travel in wind sheltered, lower elevation terrain well away from steep open slopes until this new storm snow stabilizes. 

 

Snowpack Discussion

New storm snow amounts varied according to elevations as rain gradually changed to snow Wednesday night. As of Thursday evening, storm snow since Wednesday below 4000' is about 4-6 ", near 4000' about 8-10' and above 5000' about 12-16".

Above treeline, greater new snow amounts and periods of very strong winds have caused dangerous avalanche conditions. New wind and storm slabs in higher exposed terrain will require careful terrain choices and conservative decision making.  

Near and below treeline the precipitation began as rain and transitioned to snow. This has caused less dangerous conditions with new snow forming good bonding to the old rain soaked snowpack. However, heavy snowfall and moderate to strong winds have built increasing wind and storm slabs at progressively lower elevations. There are still dangerous avalanche conditions with new and increasingly deep wind and storm slabs of 1 ft or more as of late Thursday.

The old rain wet snow continues to gradually re-freeze and form a new crust layer.

The older snowpack has undergone several rain or warm periods and remains void of any significant layers of concern.

Observations

North

Thursday morning, the Mt Baker Pro Patrol reported fresh wind slabs were sensitive to ski cuts and were releasing in the 4-5 inch range, near treeline. Winds were reportedly very strong along ridges and actively transporting the new snow. 

On Wednesday afternoon, Mt. Baker Pro Patrol observed evidence of several large, natural loose wet avalanches visible on Shuksan arm at approximately 3000 ft. The avalanches are suspected to be from earlier Wednesday. 

Central

NWAC forecaster, Dallas Glass was on Skyline Ridge at Stevens Pass Thursday and reported new storm snow of 8 inches at 4000 feet and 14 inches above 5000 feet. The bonding was good to the old rain wet snow. The crust had yet to re-freeze but was in the process. There was evidence of recent wind transport with a fresh natural avalanche having released on a steep east face about 5300 feet, likely during a brief warm up Wednesday night. There were storm slab layers noted from fluctuating temperatures overnight, but were not sensitive to human trigger in that location. 

South

Pro Patrol at Crystal Mountain Thursday morning reported shallow 2-4" very soft slabs were sensitive to ski triggers on control. These slabs were breaking up and running as very soft slabs and were generally shallow. The underlying rain crust have become very firm by Thursday morning above 6000 ft and gradually softer at lower elevations.

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

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

Valid until: Jan 19th, 2018 12:31PM