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Avalanche Forecast

Archived

Dec 27th, 2017–Dec 28th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

Rain is generally expected near and below treeline Thursday. Loose wet avalanches are possible if rain breaks down the surface crust and taps into softer snow below, so avoid steeper slopes with terrain traps. Due to strong W-SW alpine winds, significant wind transport of recent snow may occur above treeline. Watch for building wind slab on NW - SE aspects.

Detailed Forecast

Rain is generally expected below 6500 feet on Thursday.  With strong W-SW alpine winds, significant wind transport may occur above treeline. If you are so bold to venture above treeline Thursday, watch for building wind slab on NW - SE aspects as there has been a fair amount of low density snow available to transport onto lee slopes where not capped by a rain crust.

Loose wet avalanches are possible if rain breaks down the surface crust and taps into softer snow below the crust. Avoid steeper slopes especially near and below treeline with terrain traps where even a small loose wet avalanche could be problematic.    

Watch for changing conditions especially if conditions develop differently than forecast. See the mountain weather tab for weather forecast details. 

Early season hazards still exist at lower elevations and especially around creek beds that are not filled in.

Snowpack Discussion

Mixed precipitation including yet more freezing rain likely occurred on Wednesday as light warm frontal precipitation brushed Mt. Hood at relatively mild snow levels. No new snow was recorded at NWAC stations on Mt. Hood Wednesday. 

A weather disturbance moved across the southern WA Cascades and Mt Hood area Christmas Eve into Christmas Day, depositing 6-10 inches of snow before unfortunately ending with freezing rain towards the tail end of precipitation and forming a surface crust. Prior to this storm, about 15-18 inches of low density snow fell at Mt Hood in the 24 hours ending Saturday morning 12/23. 

Observations

On Wednesday morning 12/27, Mt. Hood Meadows Patrol reported a rain/freezing rain crust at 5200 ft thickening with elevation up to 6600 ft. Some wind transport of recent snow was occurring above treeline. 

On 12/26 Mt. Hood Meadows Patrol reported that wind slabs were more stubborn to trigger during control work. A 1 cm freezing rain crust had formed near the surface.

On Christmas Day, the Mt Hood Meadows Pro Patrol produced sensitive small to large, 6-12 inches storm and wind slab avalanches near treeline, released with both ski cuts and small explosives on N and NE-facing terrain. There was extensive propagation reported with some of the triggered slides. No control was performed above treeline due to active wind transport.

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Wind Slabs

Wind Slab avalanches are the 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.