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Avalanche Forecast

Archived

Mar 29th, 2017–Mar 30th, 2017

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

Fresh storm and wind slab formed Wednesday night may be found near and above treeline on Thursday, with larger and more dangerous avalanches likely above treeline. If previously wet surface snow has not refrozen, be suspicious of loose wet avalanches that may begin small but entrain older snow and become dangerous and difficult to manage. Sunbreaks Thursday afternoon will likely activate loose wet slides on solar slopes.

Detailed Forecast

Fresh storm and wind slab formed Wednesday night should generally be shallow and found near and above treeline, with larger and more dangerous avalanches likely above treeline. Due to the cooling trend, only wind slab will be listed in the avalanche problem set and emphasized above treeline. W-SW winds should have primarily built fresh wind slab on NW-SE aspects.

Despite the cooler snow levels forecast for Thursday, if previously wet surface snow has not refrozen, be suspicious of loose wet avalanches that may begin small but entrain older snow and become dangerous and difficult to manage. Sunbreaks Thursday afternoon will likely activate loose wet slides on solar slopes. Stay off steeper slopes with terrain traps. Wet slab avalanches won't be listed but are possible in isolated locations at lower elevations that received ample rainfall on Wednesday. Moderate avalanche danger allows for large avalanches in isolated areas.

Recent cornices are very large. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes directly below large cornices. See a blog post regarding cornices here. 

Snowpack Discussion

Weather and Snowpack

Let's just say it's been a wet and wild few weeks regarding weather and avalanches in the Cascades. 

This past week has also been active weather-wise, with another strong front on Sunday, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. In the last 5 days ending Monday morning NWAC stations at Mt Hood have picked up another 2-2.5 ft of snow.

A strong low pressure system brought rising snow levels and locally heavy precipitation Tuesday night through Wednesday along the west slopes of the Cascades including Mt. Hood. After receiving a few inches of snow Tuesday night, Mt. Hood saw periods of moderate to heavy rain above 7000 feet on Wednesday.

Recent Observations

On Monday the Meadows patrol found touchy 6-12" but up to 12-18" wind slab on NE slopes near and above treeline. Surface loose wet snow was also becoming evident in the below treeline areas.

The Meadows patrol on Tuesday reported one explosive released 12-20 inch storm slab on an east slope at about 6000 ft. Patrollers also triggered a large cornice on approach. Surface snow was getting wet with some pinwheels in the afternoon.

The storm limited observations on Wednesday, but rain was quickly penetrating the upper snowpack in the Mt. Hood Meadows base area by late morning. 

Problems

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.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.