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Archived

Avalanche Forecast

Feb 10th, 2014–Feb 11th, 2014
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
4: High
The avalanche danger rating at treeline will be high
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
4: High
The avalanche danger rating at treeline will be high
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Olympics.

High avalanche danger will be seen at times this week as strong fronts finally begin to roll across the Northwest.

Detailed Forecast

West south west winds and light snow showers should temporarily decrease on Tuesday at lower snow levels.

The main concerns should be new storm slab on a variety of slope aspects and wind slab mainly on lee aspects from Monday and Monday night. Initial lower density snow, weak or low density snow from last week and faceted snow near crusts in the upper snowpack may continue to make these new layers reactive on Tuesday. Slabs may also be more reactive where they slide on the January crust or scoured surfaces.

Travel in avalanche terrain is not recommended above tree-line Tuesday. If you travel near tree-line or below tree-line watch for signs of wind transport and loading on lee slopes. Shooting cracks are always a sign of instability. Even a small wind or storm slab in the wrong terrain such as above cliffs or above steep slopes can have unintended consequences.

Limited snow at the lowest elevations will limit the avalanche danger especially on solar aspects. The mid and base pack should still consist of mostly stable crust and melt form layers from periods of warm weather earlier this winter.

The next front should begin to arrive Tuesday afternoon and evening. The avalanche danger should significantly increase Tuesday night.

Snowpack Discussion

Very cold temperatures last week produced strong temperature gradients in the upper snowpack. This produced a variety of weak surface snow conditions including surface hoar, near surface faceting and preserved the generally light amounts of low density snow that fell last week. Faceting near crusts has also been seen in the upper snowpack. These layers should cause the avalanche danger to significantly increase this week as vigorous fronts move across the Northwest and load these layers.

Surface hoar crystals from a north aspect on Klahhane Ridge, 7 February, Katy Reid

Faceted crystals from above a buried crust from a north aspect on Klahhane Ridge, 7 February, Katy Reid

Vigorous frontal systems will be moving across the Northwest pretty much every day this week. The first front is moving across the area Monday causing increasing west southwest winds, moderate to heavy snow, and a warming trend. The warming trend will generally build upside down heavier snow over initial lower density snow and low density snow from last week. A few inches at lower elevations to a foot or more of snow at higher elevations should be seen by Tuesday morning.

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.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1