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Avalanche Forecast

Archived

Apr 29th, 2017–Apr 30th, 2017

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

Regions

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Avoid steep, exposed wind loaded terrain above treeline where fresh wind slabs may be reactive to human trigger Sunday. Watch for wet and weak surface snow from daytime warming and any afternoon sunshine. Continue to give cornices a wide berth if traveling along ridge-lines and avoid slopes below cornices. 

Detailed Forecast

A front is expected to move across the Cascades Saturday night. This should cause a period of moderate rain and snow with snow levels about 5500 feet and a period of strong SSW winds. Showers should taper Sunday and end with gradual clearing late Sunday. Wind should shift to westerly and diminish through the day Sunday. 

Some shallow fresh wind slabs are likely on lee slopes, mainly NW-SE facing above treeline by early Sunday.  Watch for and avoid areas of firmer wind transported snow on NW-SE aspects and other cross loaded features on other aspects in areas of varied terrain.

Near and below treeline, cooling should allow for previously wet surface snow to begin re-freezing, diminishing the threat of loose wet slides. However, watch for wet surface snow deeper than a few inches, pinwheels and initial small loose wet avalanches that indicate the potential for loose wet avalanches.

Cornices are still large, so give them a wide safety margin. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give potential cornices a wide berth if traveling along ridge-lines and avoid slopes below cornices. See a blog post regarding cornices here.

Avoid unsupported slopes with overhanging blocks of snow and smooth rock underneath. Glide avalanches are difficult to predict and can release at any time, not just during the heat of the day.

 

Snowpack Discussion

A cool, snowy pattern has been seen in late April for the mainly for the Cascade west slopes and especially in the Mt Hood area.

Last week from Monday 4/17 to Thursday 4/20 NWAC weather stations along the Cascade east slopes picked up about 0.5 inches of water equivalent (WE). Much of this modest amount of WE fell as snow above 5500-6000 feet.

The most recent storm cycle began a week ago Saturday. In the 6 days ending Friday morning NWAC stations along the Cascade east slopes had modest amounts of WE and light snowfall.

A fair day Saturday with increasing high clouds allowed temperatures at many NWAC stations along the east slopes of the Cascades to reach the 40's and 50's.

Recent observations

The Washington Pass DOT crew on Thursday reported no significant, recent avalanche activity.

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.