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Avalanche Forecast

Archived

Mar 14th, 2018–Mar 15th, 2018

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

Olympics.

Avalanche conditions are improving as the snowpack consolidates. Watch for fresh but shallow new Wind Slabs near ridges at higher elevations. Older weak snow may still exist in the snowpack so avoid large open slopes that may still harbor this difficult to predict and manage avalanche problem. Give fresh Cornices a wide margin of safety. Watch for shallow wet snow conditions on steep southerly facing slopes during extended sun breaks.

Detailed Forecast

Cool weather Thursday with a mix of sun and light snow showers with light winds will allow for a gradual decrease in avalanche danger. Fresh shallow wind slabs near and above treeline will continue to strengthen and be more stubborn to trigger by Thursday. Still use caution and avoid steep lee slopes and cross loaded features that have received recent wind re-distributed snow.

The late winter sun is becoming strongly effective at producing wet snow avalanches. Watch for extended sun breaks on steep southerly facing slopes quickly wetting and weakening surface snow. Watch for rollerballs or pinwheels and snow falling from trees as signs of wet snow conditions.

Recent warm weather and snowpack settlement has helped round and strengthen buried persistent weak layers. We have removed the Persistent Slab problem from the forecast problem list, however this does not mean it is impossible to trigger a persistent slab in isolated areas, especially with a larger trigger such as a cornice fall. 

Avoid large open slopes, especially slopes below large overhanging cornices. 

Snowpack Discussion

Sunshine and temperatures in the 40's to lower 50's the first few days of the week changed to a cool showery regime Tuesday. Only a few inches of new snow fell in showers by Wednesday. The overall snowpack has been gaining strength with strengthening melt-freeze crusts near the surface and only shallow recent snow above.

A period of moderate winds during the showers Tuesday may have formed some shallow wind slabs on lee slopes in higher terrain. Any shallow wind slabs should become more stubborn to trigger. 

Several melt-freeze cycles Saturday through Monday of this week continues to consolidate and strengthen the snowpack. 

Older weak snow layers were reported in the Olympics last week. On slopes that receive direct sun, several facet/crust layers may be found. A more widespread layer of weak sugary facets was found just above a very firm melt-freeze (2/8) crust formed in early February, but continues to gain strength. The depth to this weak layer depends on aspect and elevation.

We have not received reports of recent avalanches in the Olympics other than small wet loose on steep sunny slopes.

There are no significant layers of concern below the 2/8 crust layer.

Observations

On Sunday as of 1 PM, NPS rangers reported generally small loose wet avalanches with one larger release caused by a partial cornice collapse. 

On Saturday, Forecaster Robert Hahn and NPS Rangers found the 2/8 weak snow/crust interface in snowpits on a West aspect at 5300' and down 2.5 feet from the surface. No new avalanche activity was observed other than small wet loose avalanches on steep sunny slopes. North facing terrain still held drier settled powder. 

On Wednesday March 7th, a NPS ranger traveled in the Victor Pass area and found many crusts with facets forming on south aspects. Near ridge-tops, the 2/13 weak layer was 8-12" down and 3 mm facets were observed. Mid-slope this layer was down 3-4 feet.

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.