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Avalanche Forecast

Archived

Mar 28th, 2017–Mar 29th, 2017

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Mt Hood.

Very dangerous avalanche conditions are expected on Wednesday and travel in avalanche terrain is not recommended in this area.

Detailed Forecast

A warm front is moving over the Northwest mainly over the Washington Cascades on Tuesday. However a second frontal system will move over the Northwest Tuesday night and Wednesday. The second system will cause periods of moderate to heavy rain or snow with further gradually rising snow levels Tuesday night and Wednesday including at Mt Hood.

An avalanche cycle is expected by Tuesday night and should last into Wednesday. Snow that has accumulated since the mid-March avalanche cycle may be involved or entrained in avalanches.

SW alpine winds are likely to begin to build new wind slab on mainly N-SE slopes on Tuesday night and Wednesday. Alpine winds have been predominately from the SW-WSW the pasts few days with some SE winds seen at the top of Crystal. Hence NW to SE slopes will be indicated.

Loose wet avalanches will be likely in the near treeline and below treeline where there is significant rain and warming.

Recent cornices are very large. Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.

Although they won't be listed as avalanche problems, new storm slab is very likely at higher elevations and wet slab avalanches are possible at lower elevations.

Very dangerous avalanche conditions are expected on Wednesday and travel in avalanche terrain is not recommended in this area.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy.  NWAC stations at Mt Hood piled up about 6-7 ft of snow.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

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.

Recent Observations

On Monday the Meadows patrol found touchy 6-12 in but up to 12-18 in 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 control 12-20 in storm slab on an E slope at about 6000 ft. Patrollers also triggered a large cornice on approach. Surface snow was getting wet with some pinwheels in the afternoon.

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.