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Avalanche Forecast

Archived

Apr 15th, 2016–Apr 16th, 2016

Alpine
Natural avalanches possible, human triggered probable.
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

Mt Hood.

The amount of sun, heat and recent storm snow should be the main factors to consider on Saturday. More heat and more recent storm snow will generally mean more avalanche danger.

Detailed Forecast

An upper ridge and warm air mass should continue to build over the US west coast on Saturday with offshore surface flow developing over the Northwest. A warm front may clip the Olympics with some light rain or snow and clip the north Cascades with some clouds with fair weather further south and east of the crest.

The amount of sun, heat and recent storm snow should be the main factors to consider on Saturday. More heat and more recent storm snow will generally mean more avalanche danger.

Loose wet avalanches should be likely at Mt Hood Saturday on steeper slopes involving snow received during the week. Watch for wet snow deeper than your boot tops, especially on steep solar slopes during the late morning to afternoon hours. Avoid areas below steep gullies and run out zones.

Wind slab will listed at Mt Hood due to the recent reports. Wind slab should be most likely on N-E slopes in the near and especially the above treeline. Watch for signs of firmer wind transported snow and shooting cracks.

The potential for cornice releases will increase through the weekend. Cornice releases can be unpredictable during the spring so avoid areas below cornices and remember that cornices can break much further back than expected along ridges.

 

 

 

 

Snowpack Discussion

Weather and Snowpack

We've had two big warm-ups and spring shed cycles since the end of March. Over this time period the snowpack has undergone significant settlement with lower elevation snow beginning to fade away and with an overall transition to a spring snowpack.

A front crossed the Northwest Tuesday afternoon and night. Then a cool upper and surface low pressure system moved east along the Washington-Oregon border on Wednesday and Thursday. NWAC sites at Mt Hood had 11-14 inches of late season snow in the 3 days ending Friday morning with 6-8 inches of that snow in just the 24 hours ending Friday morning.

Storms in March continued to build unusually large cornices along many ridges. The potential for low probability/high consequence encounters, such as cornice failures and glide avalanches from steep unsupported slopes and smooth rock faces, should continue for awhile this spring. 

The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

NWAC observer Laura Green was in the Heather, White and Mitchell Canyon areas in low visibility on Thursday. She found new building wind slab of 20-30 cm by late morning with shooting cracks on N-NE slopes at 6600 feet. Ski cuts were giving easy, small wind slab releases on slopes steeper than 30 degrees.

 The Mt. Hood Meadows pro-patrol gave the NWAC an extensive report for Friday morning. In the above treeline explosive control produced extensive wind slab releases on N-E slopes up to 4 feet deep running on the 4/12 crust. It was felt that these releases could have been triggered by skiers. In the near to above treeline they found ski triggered 4-6 inch wind slab also on N-E slopes but more limited in extent and some small ski triggered and natural loose wet avalanches on E-S slopes.

Problems

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.

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.

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.