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Archived

Avalanche Forecast

Dec 26th, 2016–Dec 27th, 2016
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: Mt Hood.

Very dangerous avalanche conditions are expected in much of the terrain Tuesday. Storm and wind slabs will be sensitive Tuesday. The safest plan is to avoid avalanche terrain of consequence until unstable layers stabilize. 

Detailed Forecast

Stormy conditions Monday night and Tuesday will cause an increasing avalanche danger!

New storm and wind slabs will build through Tuesday. Storm or wind slabs will likely be the most sensitive in areas where poorly bonded to an underlying crust.   

Avoid travel in avalanche terrain of consequence Tuesday.

 

 

 

Snowpack Discussion

Weather and Snowpack

Strong storms a week ago Sunday and Monday deposited generally 3 inches of water equivalent recorded at NWAC stations on Mt Hood through early Tuesday morning. 

Unfortunately, much of the heavy precipitation fell in liquid form with rain reaching above 7000 feet by Tuesday morning. 

A sharp cooling trend mid-day Tuesday and showers deposited about 2 inches of snow.  A strengthening rain crust was noted near and below treeline by late in the day Tuesday with the arrival of colder air.  

A front Thursday and upper trough on Friday with low snow levels deposited about 5 inches of snow at NWAC stations on Mt Hood. 

NW winds near and above treeline were moderate at Mt. Hood Friday night through mid-day Saturday.  Christmas Day was partly to mostly sunny with light winds and cold temperatures.

The winds increased to moderate to strong Monday afternoon ahead of the arriving frontal system.  

Recent Observations

Reports from the Mt Hood Meadows pro-patrol Wednesday reported a significantly different snowpack following rain, avalanches and cooling. A stout surface crust was found on all elevations up to at least 7200 feet. On exposed terrain, the crust was very supportable while in treed terrain the crust ranged from breakable to supportable.

The Meadows patrol checked in early Sunday morning to report NW winds had scoured windward aspects near and above treeline exposing the thick crust on many aspects. The crust was slick enough that Santa had trouble landing his sled on any slope steeper than 20 degrees.

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.

Elevations: Treeline, Below Treeline.

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, South, North West.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1