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Avalanche Forecast

Archived

Dec 23rd, 2015–Dec 24th, 2015

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

Regions

Stevens Pass.

Storm slab, wind slab, cornices and deep unconsolidated snow are all key features to plan to avoid on Thursday.

Detailed Forecast

A cool upper trough will continue to sink south over the Pacific Northwest on Thursday. West winds should begin to decrease Thursday initially in the Olympics and north Cascades. Snow showers Thursday along the west slopes should bring another few inches to the north and another 5-10 inches to the central and south.

Dangerous avalanche conditions should persist Thursday especially in the near and above treeline zones where new or recent storm and wind slab should be likely. Storm and wind slab avalanches should remain within recent storm layers but could move fast. Watch for evidence of wind deposited snow or pillows. Test for inverted strong over weak storm snow and give cornices a wide margin.

Loose dry avalanches in non-wind affected terrain will be difficult to manage on steep slopes. Even a small loose dry avalanche could have unintended consequences such as knocking you off your feet and into a terrain trap.

The recent unconsolidated storm snow is now very deep in many places!. There was a snow immersion fatality in a tree well in bounds at Snoqualmie on Saturday so always travel with a partner.

Snowpack Discussion

The Christmas snow globe keeps giving here in the Pacific Northwest! It's gotten very deep out there. By Thursday morning sites along the west slopes will have received about 5-9 FEET of snow in less than a week.

Recent pro back country or pro patrol observations indicate extensive avalanches mainly confined to recent storm snow.

A natural avalanche cycle was reported early Tuesday morning at Alpental, likely a response to heavy showers in the waning convergence zone over Snoqualmie Pass Tuesday morning. These storm slab avalanches were extensive ranging mostly from 8-12" but did not propagate greatly.

In the Crystal backcountry on Tuesday NWAC pro observer Dallas Glass found some snow pack whumping and numerous triggered storm slab and some loose dry releases. The storm slab was releasing at about 25 and 45 cm where there may have been storm cycle density changes. The greatest wind loading and cornice formation was noted on generally east facing terrain near ridges. 

Crystal backcountry near Bullion Basin, a shallow 10" skier triggered soft slab, SW aspect, 5800 feet. Released near noon Tuesday, December 22. Photo: NWAC pro observer Dallas Glass   .

The Alpental pro patrol Wednesday reported widespread large 12-14 inch storm slabs from explosives and ski cuts. Up in the north at Mt Baker where there was less new snow the Mt Baker pro patrol reported triggered small loose dry avalanches.

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 Dry

Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.