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Archived

Avalanche Forecast

Dec 30th, 2014–Dec 31st, 2014
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.

As the weekend storm slabs become less of a problem, older wind slab on easterly aspects and more recent wind slab on south and westerly aspects will require travelers to evaluate snow and terrain carefully and identify features of concern. 

Detailed Forecast

Previously moderate east winds should have diminished by Wednesday under mostly clear skies and gradually warming temperatures. When traveling into the near and above treeline zones, be prepared for recent wind slab formation on more southerly and westerly aspects. Older wind slab on more westerly aspects may still be sensitive in certain areas. Treat wind loaded slopes with caution, it is difficult to determine how sensitive they are with snowpack tests. Loose dry avalanches are possible in wind sheltered areas below treeline... watch for sluffing on steeper slopes especially above terrain traps.

 

Snowpack Discussion

A storm system moved through the Northwest on Saturday and Saturday night delivering some much needed snowfall. West winds changed to northwest, with moderate to heavy snow and a cooling trend. Heavy snowfall continued Sunday and Sunday night with NWAC sites on Mt. Hood picking up about 25-30 inches of storm snow through Monday afternoon!

Mt. Hood Meadows pro patrol found cracking and easy releases of soft and generally shallow wind slab due to the rapid wind loading Monday.   

Moderate to strong northwest winds switched to easterly Monday and have persisted through Tuesday. This built new wind slab on westerly aspects. Due to the cold temperatures, soft storm slabs are not expected to be a widespread problem.  The below treeline zone now has enough snow for a loose dry avalanche problem on steeper slopes that are wind sheltered.

Mt Hood patrol observations from Tuesday indicated that new hard wind slabs had formed but in a highly varied pattern. Only a few locally released smaller slabs were noted with explosives. By Tuesday afternoon however, there was evidence of large new natural releases in the Newton-Clark creek drainages.  These start zones are well above the forecast zone near 9000 ft but show evidence of significant wind slab formation from very strong NE winds.    

 

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 East, East, South East, South, South West, West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 2

Loose Dry

Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.

Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.

 

Loose Dry avalanche with the characteristic point initiation and fan shape.

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

Elevations: Below Treeline.

Likelihood: Possible

Expected Size: 1 - 1