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Avalanche Forecast

Archived

Apr 6th, 2017–Apr 7th, 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.

The avalanche danger will increase with elevation on Friday, creating dangerous avalanche conditions above treeline. A mix of wind and storm slab and loose wet concerns should be seen near treeline. Loose wet avalanches are likely below treeline.

Detailed Forecast

Warm frontal moisture will spread northward and move over Mt. Hood late Thursday night with snow levels 6500-7000 ft and increasing S-SE winds. After a brief lull in the precipitation early Friday morning, showers should begin late morning and increase in the afternoon. SW winds will increase and should become strong and gusty by late morning with snow levels falling to 4000-5000 ft in the afternoon.  

The avalanche danger will increase with elevation on Friday, creating dangerous avalanche conditions above treeline. New wind and storm slab problems will initially be found above treeline, however these problems should creep into the near tree-line elevation band by the end of the day.

Travel above treeline is generally not recommended Friday. Southerly winds should primarily load W-N-E aspects, but very strong winds will cross-load slopes and create unusual loading patterns. Unstable storm slabs should develop in wind sheltered areas during periods of intense snowfall accumulation but only wind slab will be listed in the avalanche problem set.  A mix of wind and storm slab and loose wet concerns should be seen near treeline. 

Loose wet avalanches are most likely below treeline. Watch for wet surface snow deeper than a few inches, pinwheels and initial natural releases that indicate an increasing loose wet avalanche danger. Be wary of steep slopes with terrain traps such as cliffs or if heavily treed, where even a small loose wet avalanche could have big consequences. 

Recent cornices are very large and have likely been weakened during this most recent storm cycle. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack

March was a wet and wild month for weather and avalanches in the Cascades. 

Fair but cool weather has been seen in the Olympics and Cascades Monday and Tuesday. Reports generally indicate strong surface or near surface crust layers and shallow recent snow. 

A stalled frontal boundary on Wednesday through the first half of Thursday brought heavy rain and snow to the Mt. Baker area with much lighter precipitation for the central-west and southwest Cascades and Mt. Hood. Snow levels were generally between 6500-7500 feet on Wednesday on Mt. Hood, lowering to 5500-6500 on Thursday. Minor wind transport of shallow new snow above 6500 feet likely occurred on Thursday during periods of moderate W-SW winds above treeline.

Recent Observations

The Meadows pro-patrol on Saturday and Sunday reported triggered loose wet avalanches becoming possible on solar slopes. Otherwise the recent crust was the predominate snow surface with small shallow areas of wind transported snow soaked by rain and frozen in place near and above treeline.

On Thursday, the Meadows pro-patrol checked in reporting no recent avalanche concerns in their area. 

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.