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Archived

Avalanche Forecast

Dec 21st, 2017–Dec 22nd, 2017
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Olympics.

Watch for recent wind transported snow below exposed ridges or any wind stiffened surface snow, making slab avalanche releases possible. Avoid large open slopes of consequence in higher terrain. Keep eyes on partners at all times when traveling in terrain with deep unconsolidated snow, where a snow immersion hazard may exist.

Detailed Forecast

A weak disturbance is expected to bring clouds and light snowfall across the Olympics overnight Thursday, decreasing Friday. Only light amounts of new snow are expected and should not cause an increase in danger. Moderate NW winds at higher elevations may continue to transport available snow, building areas of wind slab on a variety of a-typical aspects from east to southwest facing. 

While the overall danger should continue to decrease, continue to travel with caution and watch for any slab like structure or cohesion in the upper snowpack, indicating the possibility of slab avalanches propagating.   

Watch for touchy recently formed cornices and keep a safe margin along ridges and slopes below.

Most areas now have deep unconsolidated recent storm snow and this has created a non-avalanche, snow immersion, tree-well hazard in places. Keep partners in sight at all times when traveling in terrain with deep unconsolidated snow.

Snowpack Discussion

Recent deep storm snow has been settling and stabilizing Wednesday and Thursday under fair cool weather. This has allowed for storm related weak layers to settle with no reported storm slab avalanches reported since Tuesday.

Moderate to strong northerly crest level winds have been transporting snow on many exposed ridges both Wednesday and Thursday. This should have built areas of wind slab on a variety of aspects, including non-typical southerly facing slopes near and above treeline.

A strong storm from late Monday through Tuesday afternoon dropped over 3 inches of water equivalent amounting to over 2 feet of new snow in the Hurricane Ridge area by early Wednesday. Strong winds with warmer initial temperatures likely built wind and storm slabs by Tuesday.

The storm snow fell on settled old snow or crust layers formed during the long period of high pressure in early December. 

Observations

No recent observations.

Avalanche Problems

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: Possible

Expected Size: 1 - 1

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.

Elevations: Treeline, Below Treeline.

Likelihood: Possible

Expected Size: 1 - 1