Avalanche Forecast

Issued: Dec 26th, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Lingering wind slabs instabilities will continue to slowly settle on Sunday. Watch for sensitive wind-loaded slopes especially near and above treeline, and be wary of terrain traps where even a small loose dry avalanche could have unintended consquences. 

Summary

Detailed Forecast

A frontal system on Sunday should drop a few inches of new snow with light to moderate southerly transport winds. This system will stack up short compared to some of the burly snow totals received over the last week. 

However, recent cold temperatures should continue to slow the otherwise stabilizing trend in the upper snowpack. 

Recent wind slab will most likely linger on north to east slopes in the near and above tree line. Wind and storm slab avalanches should stay within recent storm layers but could still move fast. Test for inverted strong over weak storm snow and give cornices a wide margin.

Beware of loose dry avalanches in steep non-wind affected terrain that could have unintended consequences such as knocking you off your feet and into a terrain trap.  Loose dry avalanches are listed above storm slab along the east slopes because of the colder temperatures, loose dry avalanches are more likely to be found and triggered than storm slabs. 

Remember to discuss plans and maintain visual contact with your partners. Recent unconsolidated storm snow is very deep in many places with possible large tree wells. There have been snow immersion fatalities in tree wells already this season at Snoqualmie and in Canada.

Snowpack Discussion

We have had about a week of heavy snow and cool temperatures in the Olympics and Cascades. Sites east of the crest have had 2-4 feet of snowfall during this time. There has been slow settlement over the last few days, but the main message is that it is still deep out there; Washington Pass area snowdepths are over 2 meters!

A snow pit report from 5700-6500 foot range in Stemilt Basin near Mission Ridge also on Tuesday indicated a buried surface hoar at 45 cm. This layer gave some cracking and whumping. 

NWAC pro observer Tom Curtis was at Iron Mountain a bit west of Blewett Pass on Wednesday and found some wind slab on steep open slopes even to below treeline and often above terrain traps like creeks or trees. A storm layer was seen at 35 cm with wind and storm slabs reacting to tests to this interface. 

Recent reports from the North Cascade Mountain guides near Washington Pass:

Wednesday indicated a storm layer at 15-25 cm but ski cuts on test slopes did not give any results. On Thursday a report indicated a storm layer at 30 cm with isolated small ski triggered storm slab and some loose dry releases and good stability. Quite a bit of wind effect was seen on some nearby slopes. On Christmas Day, a buried surface hoar layer was found in the Washington Pass area around 40 cm down and gave sudden planar (pop!) results in compression tests but was not found to be involved in any natural or skier triggered avalanches. This layer probably needs more of a cohesive slab above it before it becomes apparent whether this is an PWL issue or not. 

The southeast zone has a relatively shallower snow pack versus areas further north, but after the snowy December has more than enough to warrant an avalanche risk. No snowpack observations have been received from this zone. 

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.

Elevations: Alpine, Treeline.

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.

Aspects: All aspects.

Elevations: All elevations.

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: Dec 27th, 2015 10:00AM