Avalanche Forecast

Archived

Apr 20th, 2017–Apr 21st, 2017

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.

Don't let the sunshine blind you to spring avalanche hazards on Friday! The loose wet potential will ramp up quickly as temperatures warm along with increasing sunshine. A more winter-like snowpack should be found above treeline where lingering wind slabs may still be sensitive. Cornices are still large so give them a wide safety margin.

Detailed Forecast

After a cool start, strong late April sunshine and rising freezing levels will quickly increase the loose wet hazard on all solar aspects. If the alpine winds out of the E-SE and high clouds increase in the afternoon as forecast, the greatest loose wet hazard above treeline will likely peak late morning through early afternoon. Fresh snow near and above tree-line will quickly activate with a natural shed cycle likely on steeper solar slopes. Loose wet avalanches at lower elevation may be less likely to initiate naturally, but if skier triggered, could entrain older snow layers and become large in specific areas. Keep this in mind around terrain traps.

Winds have transported recent snowfall over the past week onto lee slopes, with the most snowfall and transport occurring above treeline. Be suspicious of steeper wind loaded slopes below ridges. Expect the wind slab hazard to increase quickly with elevation.

Cornices are still large so give them a wide safety margin. 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.

Avoid unsupported slopes with overhanging blocks of snow and smooth rock underneath. Glide avalanches can release at any time, not just during the heat of the day, and are by definition difficult to predict and manage.

Snowpack Discussion

The active weather pattern continues into late April for the Pacific Northwest despite what the calendar says. Over the last few days, Mt. Hood stations have picked up 1 - 1.5 inches of water equivalent (WE). Much or all of this has fallen as snow above 5500-6000 feet. Moderate W-SW transport winds have occurred near and above treeline over this stretch as well. Natural loose wet avalanches have likely occurred throughout the week as the snow-line has oscillated or on solar aspects at lower elevations during sunbreaks.

Recent observations

None

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.