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Archived

Avalanche Forecast

Dec 26th, 2014–Dec 27th, 2014
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Cascades - North West.

Watch for developing storm slabs, wind slabs and possible buried surface hoar, especially in areas receiving the heaviest snowfall with the strongest winds, such as at higher elevations on the volcanoes. Choose very conservative terrain.

Detailed Forecast

Increasing storm conditions Saturday. Light rain and snow should increase early Saturday through the day, becoming moderate to heavy with increasing westerly crest level winds. This should build unstable storm and wind slab layers through the day, with wind slabs spreading to NE-SE facing exposed terrain, especially below ridges by afternoon. 

Existing surface hoar identified Friday may have become buried intact by the initial snowfall early Saturday, so watch for this potential weak layer by performing quick investigations of new to old snow surfaces.

Expect dangerous avalanche conditions near and above treeline, especially by late Saturday afternoon. Careful snowpack evaluations, cautious routefinding and conservative decision-making will be essential Saturday. 

 

 

Snowpack Discussion

Very light snowfall has occurred over the past two days, ranging from zero to about 3 inches. This has allowed the 6-12 inches that fell Tuesday through Wednesday to further settle. Lesser amounts were received in the Snoqualmie Pass area since Tuesday. Winds turned northwesterly as temperatures cooled down during the snowfall Wednesday, causing some loading onto exposed  SE aspects.  

On Wednesday, NWAC Pro-observer Jeff Hambelton in Mt. Baker area found a right-side upper snowpack. Moderate wind transport had created a stiffer layer of snow on some lee slopes near and above treeline and observed small loose dry sluffs - 

  Jeff Hambelton, Mt Baker area Wednesday. 

Moderate NW winds and light snow showers on Thursday continued to load lee slopes. 

NWAC observer Dallas Glass was touring above Paradise on Mt Rainier Friday, finding the 10 inches of recent storm snow well bonded to a firm rain crust. Of greater interest was the widespread 5 mm surface hoar which had formed Thursday night and was persisting through the day.

Sparkly surface hoar surviving on SW aspect at 6000 feet, Paradise Mt Rainier, Friday, Dec 26th. Dallas Glass

The lower snowpack is generally very strong strong and comprised of crusts and polycrystals.  

 

Avalanche Problems

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

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, South.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1