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Avalanche Forecast

Archived

Apr 12th, 2017–Apr 13th, 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 avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

Watch for fresh wind slabs on lee slopes in steep terrain mainly above treeline. Sun breaks can quickly create loose wet avalanche conditions on steep slopes facing the sun. Give cornices a wide safety margin.

Detailed Forecast

An approaching front will move across the Mt Hood area Wednesday night and early Thursday with strengthening southerly winds.

Light to briefly moderate snow and strong winds will likely build fresh wind slabs on lee slopes below ridges, near and especially above treeline by Thursday. 

Overnight fresh snow may fall on the previous day's melt-freeze crust, which may act as a good sliding surface, especially during sun breaks and daytime warming.

Watch for new wind slabs, mainly on NW-SE slopes above treeline. Keep an eye out for firmer wind transported snow that is a sign of wind slab.

Strong winds and new snow will continue to build and grow recent large cornices. There have been numerous recent cornice failures with some being very large in the WA Cascades.

Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. Five people were tragically killed by a cornice release in BC on Saturday. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack

Heavy rain in mid March has left behind a well consolidated old snowpack with one or more strong melt freeze crusts in the upper snowpack.

A series of strong spring storms was seen last week that affected mainly the Washington Cascades. At Mt Hood snow levels were generally between 6500-7500 feet on Wednesday, lowering to about 550-6500 feet on Thursday. For the 3 days ending Friday morning NWAC stations at Mt Hood had about .8 inches of WE which will have fallen as snow above the snow levels.

A low pressure system moved north along the coast on Friday. About 4 inches of snow were seen at NWAC stations at Mt Hood. Very strong winds built large wind slabs and fresh cornices during sustained SE-SW winds averaging over 35 mph with gusts over 80 mph! These winds were so strong that wind slabs were less widespread and formed lower on leeward slopes than typical.

A calmer, cooler pattern was seen Saturday and Sunday with about 4-5 inches of snow each day at NWAC stations at Mt Hood. A period of fair weather and sunshine or filtered sun Sunday allowed for more consolidation and some surface snow melt on all but steep shaded slopes in higher terrain.  

A front and upper trough crossed the Northwest on Sunday night, depositing another another 4-6 inches of storm snow at Mt Hood stations by Monday morning. Moderate winds built fresh wind slabs above treeline on steep lee terrain features. 

Sunny weather Tuesday helped storm snow begin to settle, however, winds remained strong enough above treeline Tuesday to continue to transport surface snow, maintaining areas of wind slabs. 

Another frontal band followed a similar path from south to north overnight and early Wednesday, building fresh wind slabs and depositing another 3-5 inches of snow in most areas. 

Recent Observations

On Saturday and Sunday, the Mt Hood Meadows patrol reported that fresh wind slabs were stubborn and isolated. No ski cut avalanches were released. There were about 6-12 inches of new storm snow at the 6600 foot elevation. The strong winds built some hard slab conditions on lee slopes above treeline, ranging from 1-3 feet. These hard slabs were not likely to be released by human trigger.

The Mt Hood Meadows patrol on Monday reported new small sensitive wind slab on the NE slopes above 6000 feet with large wind slab expected at higher elevations. Sensitive loose wet avalanches were also seen below tree line.

Updates from the Meadows patrol Tuesday and again Wednesday indicated daily fresh wind slabs above treeline on isolated features, mainly N-NE facing terrain. These were hard slabs ranging from 4-12 inches and stubborn Tuesday with some 1-2 ft wind slabs by Wednesday. These slabs were mostly released with explosives with good propagation and some were sensitive to ski trigger. Cornices were again noted to be very large!

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.