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Archived

Avalanche Forecast

Jan 11th, 2014–Jan 12th, 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.

A lot of changing weather is expected starting Friday night. Travel in avalanche terrain is not recommended on Saturday.

Detailed Forecast

The active changing pattern will continue over the Northwest through the weekend.

A strong front will cross the Northwest on Friday night causing increasing moderate to heavy rain or snow and rising snow levels. A natural avalanche cycle seems possible on Friday night.

Stormy weather due to strong orographic west flow and heavy showers with lowering snow levels should be seen Saturday. The main concern during the day Saturday should be rapid snow loading and deep storm slab building especially in the alpine and near treeline and then to below treeline on many or most slopes. Rapid snow loading is generally accepted to mean more than 1 inch of snow per hour. Natural and triggered avalanche should be likely on Saturday.

Strong SW winds will veer to W behind the front later Saturday morning.  Expect rapid wind loading Friday night and throughout the day Saturday.  Strong winds should easily develop wind slab on lee slopes...and due to the strong winds wind loading will occur further downslope than during a more moderate event.   

Remember the danger ratings and icons are for the daylight hours. Less danger might be seen in areas that often receive less snow such as Hurricane and Crystal. The avalanche danger should still be limited at the lowest elevations by shallow snow and terrain and vegetation.

Snowpack Discussion

Sunny weather with warm temperatures were seen Sunday and Monday. This produced some small loose wet slides on solar aspects and crusts on more varied but not all aspects. Less warming was seen in the passes due to cooler easterly flow. 

Weak systems began to move to the Northwest about Tuesday with persisting cool offshore surface flow. This caused light rain or snow and a mix of precipitation in the passes.

A front moved across the Northwest on Wednesday. Snowfall at most NWAC sites near and west of the crest was in the 6-17 inch range with less at Hurricane and Crystal. An overall cooling trend Tuesday and Wednesday helped to bond new snow to older or crusted surfaces from Sunday and Monday and built mostly favorable new snow profiles.

An occluded front crossed the Northwest late on Thursday. NWAC sites near and west of the crest had snowfall mostly in the 3-9 inch range again with less at Hurricane and Crystal. This snow was generally wet and heavy with remarkably high water equivalents.

Avalanche control results from the ski areas were somewhat variable on Friday morning. Little was seen by the Mt Baker crew. The crews at Stevens and Alpental saw local 6-12 inch and one 2 foot heavy slab releases on lee slopes mainly via explosives. But those reporting from each crew said skiers would cause these releases from the right locations. Some of these avalanches entrained snow down to crusts from last weekend.

Westerly flow between the departing occluded front and the approaching system is maintaining mostly light snow showers near and west of the crest at fairly steady temperatures on Friday. This should generally maintain slab layers such as the above mainly on lee slopes.

Reports indicate that hoar frost seen early in this week generally didn't survive the recent changes or is not reactive. Generally stable crusts and melt forms or rounding non-reactive facets should be found in the mid and base pack in this area.

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: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 2

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: Very Likely

Expected Size: 1 - 1