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Avalanche Forecast

Archived

Jan 5th, 2016–Jan 6th, 2016

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

Regions

Stevens Pass.

A variety of avalanche problems are expected on Wednesday. Dangerous avalanche conditions are possible on Wednesday due to likely new storm slab and potential weak layers from around New Years. This forecast errs on the side of caution until more information becomes available.

Detailed Forecast

A large weak low pressure system will drift toward California on Wednesday. A shot of drier air with some weak light showers should eject from the low and over the Northwest. Offshore surface flow should continue to limit showers and bring cooler air to the Cascade Passes. But slightly warmer temperatures should be seen in most areas.

There is some uncertainty in the avalanche danger forecast for Wednesday and this forecast will err on the side of caution until more information becomes available. The storm snowfall of 6-12 inches along the west slopes may be somewhat upside down with denser snow near the surface and will lay over surface hoar and near surface faceted snow from around New Years in many areas. This should make triggered storm slab likely on Wednesday. Small avalanches in many areas or large avalanches in specific areas should be expected Wednesday until more information becomes available.

Mostly older wind slab will still need to be watched for on a variety of aspects. Wind slab from over the weekend is most likely on west aspects. Look for signs of firmer wind transported snow that can indicate wind slab layers.

Some sun or solar effects and slightly warmer temperatures should also be seen on Wednesday. Loose wet avalanches seem like a possible bet as well on steep solar slopes. Watch for wet snow deeper than a few inches and initial natural small loose wet releases on solar slopes that usually precede loose wet avalanches.

Snowpack Discussion

Fair weather was seen for almost a week around the New Year. This weather caused lots of consolidation and stabilizing of the heavy snow from December. It also caused extensive surface hoar and near surface faceted snow on non-solar sheltered slopes according to reports for west and east of the crest in the Washington Cascades.

Strong east winds were also seen New Years Eve through the weekend building wind slab on west aspects, primarily in the central and south Cascades, including the Cascade Passes. A hiker was killed by a natural or triggered wind slab on Granite Mountain near Snoqualmie Pass Thursday, likely due to a wind slab avalanche on a west facing slope near treeline. Other skier triggered wind slab was reported near Paradise and Snoqualmie on north to west aspects on Thursday and Friday.

A change Sunday night to Tuesday has been some snow and a bit of a warming trend. Storm snowfall for this period should range from about 6-12 inches by Wednesday along the west slopes. Heads up because this new snow may accumulate on surface hoar and near surface faceted snow from around the New Year on many non-solar or sheltered slopes!

Three reports via the NWAC observations page for Monday seem to confirm the potential for instability with natural and ski triggered storm slab releases of 6-12 inches for the Stevens Pass and Snoqualmie Pass areas.

 

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.