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Avalanche Forecast

Archived

Feb 27th, 2015–Feb 28th, 2015

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

Regions

Mt Hood.

Winds slabs will be the primarily concern Saturday with westerly aspects receiving new wind loading but other aspects may be cross-loaded as well. Careful route finding and attention to local loading patterns will be necessary to have a safe and enjoyable day in avalanche terrain especially above treeline.

Detailed Forecast

East winds will increase and become strong E-NE Friday night and continue significant loading of westerly aspects through most of Saturday. 

Winds slabs will be the primarily concern Saturday, so pay attention to the local loading pattern in your area, but generally expect non-traditional westerly aspects to have significant wind loading. Some slopes may be cross-loaded by the shifting winds over the life of the storm.   

Lesser avalanche problems that are still possible are storm slabs where storm instabilities persist in wind sheltered terrain and loose wet avalanches on steeper solar slopes. The sun is strong enough at the end of February to activate loose wet avalanches even on cooler days. 

Due to the low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation. Many areas below treeline do not have enough snow (new or existing) to pose an avalanche hazard. 

Snowpack Discussion

Since the wet start to February, fair weather through mid-February has led to generally thick surface crusts, as well as further deterioration of the snow cover below treeline. Weak weather disturbances passed through in NW flow aloft last week producing light snowfall but no new avalanche concerns. 

New snowfall at NWAC Mt. Hood stations from Wednesday night through Thursday afternoon ranged from less than 1 inch at Meadows to a soggy 3 inches at Timberline with likely higher accumulations and some loading of easterly aspects above treeline. 

A low pressure system brought variable amounts of new snow to Mt. Hood on Friday, with Timberline base station receiving roughly 6 inches during the day but Mt. Hood Meadows base only receiving a few inches. Still this was enough to build sensitive new wind slab Friday morning at about 6600 ft on a NE aspect at Meadows, with soft wind slabs around 1 ft easily releasing. 

The mid and lower snowpack at Mt Hood snow consist of layers of stable consolidated rounded grains or melt forms and crusts from multiple warm periods this winter.

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.