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Avalanche Forecast

Archived

Apr 9th, 2017–Apr 10th, 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

Mt Hood.

Watch for fresh and recent wind slabs near ridges in steep terrain, mainly above treeline. Any sun breaks can quickly make fresh snow unstable on steep solar slopes. Small loose wet slides may entrain significant snow in steeper sun exposed terrain. Give cornices a wide safety margin.

Detailed Forecast

A front is expected to move through the region overnight Sunday, followed by showers Monday. Light to moderate snow at cool temperatures and moderate winds should build some fresh wind slabs above treeline by Monday. 

Cooler temperatures and light showers Monday should allow for storm layers to begin settling.

Watch for recent wind slabs in higher terrain on steep lee slopes, mainly NW-SE facing.

Recent cornices are very large and have likely been weakened during this most recent storm cycle. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Other avalanche problems not listed that may be encountered on Sunday include:

Storm slab in more sheltered areas above treeline that experienced rapidly accumulating snowfall for more than a few hours. 

Remember that solar effects can rapidly increase the touchiness of various types of avalanches at this time of year, loose-wet, cornices, glide and wind slab to name a few.

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 began about midnight Wednesday of this past week. From midnight Wednesday through Sunday morning, the Mt Baker area has received over 5 inches of water equivalent. Most or all of this fell as snow near and above treeline.

Very strong winds Friday and Friday night 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.  

Cooling, diminishing showers and winds Saturday allowed for the storm snow to begin settling and stabilizing.

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.  

Recent Observations

The Meadows pro-patrol on Saturday and Sunday reported triggered loose wet avalanches becoming possible on solar slopes. Otherwise the recent crust was the predominate snow surface with small shallow areas of wind transported snow soaked by rain and frozen in place near and above treeline.

On Thursday, the Meadows pro-patrol checked in reporting no recent avalanche concerns in their area with the bomber rain crust the dominant feature.

On Friday NWAC pro-observer Laura Green was at Mt Hood Meadows and above treeline indicated a continuing 1 inch plus surface crust over wet snow. Wind transport had created 6-8 inch pocket of snow, but she reported no avalanches and no signs of instability.

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 ft elevation band. 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. 

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.