Avalanche Forecast

Archived

Apr 21st, 2017–Apr 22nd, 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

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Natural loose wet avalanches are less likely Saturday, but if skier triggered, could entrain older snow layers and become large in specific areas. Keep this in mind around terrain traps. Shallow new wind slab may build above treeline. Cornices are still large so give them a wide safety margin.

Detailed Forecast

Friday night should be mild with increasing mid and high clouds, likely preventing a solid refreeze tonight in many areas and elevations. A relatively weak frontal system will lift over the area from south to north beginning late Saturday morning through Saturday afternoon and will be followed by post-frontal showers. Rain should generally be seen below 5500 feet. There should be just enough new snow in the alpine to build areas of new but generally shallow wind slab on lee aspects above treeline. Look for an abrupt wind shift mid-day. Older wind slab formed earlier in the week above treeline should be unlikely to trigger on Saturday.

Potential afternoon sunbreaks are most likely in the southern part of the Cascades and further east of the Cascade crest, and would help activate loose wet avalanches on solar aspects with fresh snow mainly near and above treeline. Loose wet avalanches at lower elevations should 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.

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. Between Monday and Thursday night, weather stations along the east slopes of the Cascades picked up roughly half an inch of water equivalent (WE). Much or all of this has fallen as snow above 5500-6000 feet. 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.

Friday was a warm day with most NWAC stations in the Olympics and Cascades reaching into the 40s and 50s.

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.