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Avalanche Forecast

Archived

Apr 22nd, 2017–Apr 23rd, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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New wind slabs may build above treeline, mainly below ridges. Loose wet avalanches are less likely Sunday, except at the lowest elevations below treeline. Cornices are large, so give them a wide safety margin.   

Detailed Forecast

Cooling with diminishing showers and some partial clearing overnight Saturday and early Sunday should allow for surface snow to partially or completely refreeze and strengthen. This should allow for a brief decrease in avalanche danger through early Sunday.

Another frontal system will lift over the area from south to north Sunday, followed by post-frontal showers late Sunday. Rain should generally be seen below 4500 feet. Light to moderate amounts of new snow should accumulate above 4500 feet Sunday. Above treeline, areas of shallow new wind slabs may develop on lee aspects.

Due to cooling by Sunday, loose wet avalanches should be less likely to initiate, except in the lower elevations below treeline where shallow wet snow is expected. 

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 in the Pacific Northwest despite what the calendar says. Between Monday and Thursday night, Mt. Baker picked up about 3 inches of water equivalent (WE) while other areas along the west slopes picked up 1 - 1.5 inches of WE. Much or all of this fell as snow above 5000 feet in the north and 5500-6000 feet in the central and south Washington Cascades. Natural loose wet avalanches likely occurred throughout this period as the snow-line 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. Breezy E-SE winds were seen in the Passes and in the southwest Cascades including Mt. Rainier and Crystal.

A renewed active weather pattern Saturday brought periods of light rain and snow showers through the day, with a slow cooling trend. These were mainly rain showers below about 5500 feet.   

Recent observations

The Chinook Pass DOT on Thursday reported loose wet avalanches becoming increasingly sensitive during sunbreaks below 5000 feet with ski cuts and explosives, with larger slides gouging down to older wet snow. A 12-16 inch wind slab was triggered below the ridge crest with explosives.

NWAC pro-observer Lee Lazzara was in the Mt. Baker backcountry on Wednesday. Lee found recent wind slabs on lee slopes near and just above 5000 ft up to 12 inches thick but unreactive in ski or snowpack tests. Lee observed a large recent glide avalanche on Mt. Herman (see Lee's Instagram post) as well as evidence of a recent loose wet cycle from earlier in the week.

Lee was back out on Friday in the Baker backcountry on Ptarmigan Ridge. Lee observed numerous loose wet slides on all aspects near and below treeline. Natural cornice fall along Ptarmigan Ridge ripped out a D2.5 wind slab on a north aspect (see photo below, thanks Lee!), and other natural cornice fall was noted as well. Note the massive cornices in the photo! 

Photo by Lee Lazzara, 4-21-17

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.