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Archived

Avalanche Forecast

Nov 29th, 2016–Nov 30th, 2016
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
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: Mt Hood.

Fresh wind slab mainly found above treeline band will create dangerous avalanche conditions and will be the primary avalanche problem on Wednesday. 

Detailed Forecast

A weak frontal system passing through the Cascades early Wednesday morning should bring light amounts of new snow above 4500 feet. However, intense post-frontal showers are forecast for Mt. Hood during the day Wednesday adding several more inches of snow. Brisk westerly winds Wednesday should also build fresh wind slabs on lee easterly aspects mainly above treeline.

Fresh wind slabs found mainly above treeline band on lee easterly aspects will be the main avalanche problem Wednesday. Watch for signs of firmer wind transported snow that indicates wind slab. Shallow storm slabs should be possible to trigger in areas with less wind effect. 

In much of the below treeline band there is not enough snow yet for avalanches.  Avalanche problems should be limited to the upper part of this elevation band.

 

Snowpack Discussion

Weather stations on Mt. Hood picked up 11 and 15 inches at Timberline and Mt. Hood Meadows respectively through early Monday morning. Another 7 to 9 inches fell during heavy showers through Monday evening. W to NW winds remained moderate on Monday helping to transport new snow to lee easterly aspects. 

The Mt. Hood Meadows pro-patrol found sensitive but soft 6-12" storm slabs on most aspects Monday morning failing within the new storm snow.  Wind slabs averaged 18" on north aspects above treeline with a crust for a bed surface that formed during a warm-up last Friday. 

On Tuesday, Mt. Hood Meadows patrol found shallow storm slabs near and below treeline generally unresponsive to ski cuts and explosives.  However evidence of two larger natural wind slab avalanches were observed in White River Canyon, on E-SE aspects, both with start zones around 7000'. One stepped down to deeper layers. 

In general the below tree-line elevation band has plenty of terrain anchors and offers lower avalanche potential due to the shallow snowpack. 

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, North West.

Elevations: Alpine, Treeline.

Likelihood: Likely

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.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible

Expected Size: 1 - 1