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Avalanche Forecast

Archived

Mar 27th, 2017–Mar 28th, 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 and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Mt Hood.

You will need to keep a sharper than usual eye on weather conditions if you take a trip into the backcountry on Tuesday. Be prepared to ratchet back your plans if winds become stronger and snow or rain becomes heavier than you expect and you see signs of instability such as cracking of new snow layers or loose wet avalanches before Tuesday night.

Detailed Forecast

Warm front moisture should begin moving into the Northwest on Tuesday especially to the Olympics and north to central Cascades. But rain or snow and warming should also begin to increase over the south Cascades on Tuesday. The avalanche danger levels will be a moving target as they rise on Tuesday especially in the north to central Cascades.

Recent SW alpine winds have built wind slab on mainly NW-SE slopes. But watch for firmer wind transported snow that indicates wind slab on all aspects in areas of more complex terrain.

Loose wet avalanches should become possible in the near and below treeline at Mt Hood mainly due to light rain, solar effects and warming. Watch for wet surface snow deeper than a few inches, pinwheels or initial but growing natural releases.

Recent cornices are very large. Make sure to avoid areas on ridges where there may be an overhanging cornice as well as travel on steep slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.

You will need to keep a sharper than usual eye on weather conditions if you take a trip into the backcountry on Tuesday. Be prepared to ratchet back your plans if winds become stronger and snow or rain becomes heavier than you expect and you see signs of instability such as cracking of new snow layers or loose wet avalanches before Tuesday night.

The avalanche danger should continue to significantly increase on Tuesday night.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy.  NWAC stations at Mt Hood piled up about 6-7 ft of snow.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

This past week has also been active weather-wise, with another strong front on Sunday, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. In the last 5 days ending Monday morning NWAC stations at Mt Hood have picked up another 2-2.5 ft of snow.

Recent Observations

On Monday the Meadows patrol reported touchy 6-12 in but up to 12-18 in wind slab on NE slopes near and above treeline. Surface loose wet snow was also becoming evident in the below treeline areas.

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.