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Avalanche Forecast

Archived

Apr 8th, 2018–Apr 9th, 2018

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

Regions

Mt Hood.

The avalanche danger will rise Monday as sunshine and warm temperatures will quickly make recent snow unstable. You can encounter a variety of avalanche dangers, including Wind Slabs, Loose Wet avalanches, Cornices and Glide avalanches. Use visual clues such as fresh cornices, wind drifted pillows and cracks in the snow all indicating that you could trigger a Wind Slab avalanche. Give fresh Cornices a wide margin of safety and limit exposure on steep sun exposed slopes or where the surface snow is becoming wet.  

Detailed Forecast

Rapid clearing, sunshine and warming is expected Monday. This should create dangerous avalanche conditions following the weekend's strong winter-like storm cycle. 

Fresh wind slabs will be easy to trigger on steep lee slopes below ridges and features where wind drifted snow exists.

Recent storm snow, ranging from 18 to 24 inches, will become wet, making Loose Wet avalanches likely, especially on steep sun exposed slopes during the late morning and afternoon. Watch for signs of wet snow conditions such as sluffing, roller-balling and any natural Loose Wet avalanches.

Watch for and avoid exposure to overhead hazards,such as cornices and large unsupported slopes.

Large fresh cornices will become fragile with the sun and warming. Give cornices a large margin of safety and avoid travel on slopes below, as cornice failures may trigger large avalanches on slopes below.

Minimize or avoid exposure below unsupported slopes as the potential for Glide Avalanches will increase during prolonged sunshine and warm temperatures. 

Snowpack Discussion

A strong storm over the weekend caused rain Saturday to high elevations followed by cooling and deep new storm snow of 1.5-2 ft over the Mt Hood area by Sunday afternoon.  There will be a strong difference on new snow amounts with elevation as temperatures were near freezing in the lower terrain below treeline. The new snow was transported by periods strong or very strong winds that built fresh Wind Slabs on a variety of aspects, especially below ridges.

The recent storm increased fresh cornices along exposed ridges. 

Periods of rain and a rising snowline allowed wet snow conditions to extend to near and above treeline Thursday and Friday.  A series of crusts and moist or wet snow exist in the upper snowpack.  

There are currently no significant layers of concern in the mid or lower snowpack.

Observations

Mt Hood Meadows pro patrol reported 8 inch average soft slabs being easily released with ski cuts during avalanche control Sunday morning. In lower elevation terrain from 4500-5400 ft, the new storm snow of 10-14 inches was very reactive to ski trigger with avalanches running long distances and gauging into old wet snow.  

NWAC professional observer Laura Green was in the Timberline area on Friday. She observed some natural pinwheels in the wet surface snow and small broken cornice chunks, but no recent avalanche activity of note. Snow surfaces were wet on all aspects and all elevations that she traveled on up to 6500'. 

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.