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Avalanche Forecast

Archived

Dec 18th, 2016–Dec 19th, 2016

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.

Regions

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Travel in avalanche terrain is not recommended along the west slopes on Monday. 

Detailed Forecast

Strong west flow will carry a strong cold front across the Northwest Sunday night and Monday morning. This should cause strong alpine winds with moderate to heavy snow along the west slopes and a warming trend. The warming trend should contribute to the formation of both new wind slab and new storm slab layers.

New wind slab is very likely along the west slopes on Monday. Wind transport will deepen these layers mainly on lee north to southeast slopes but wind slab is possible on other aspects.

New storm slab is also very likely along the west slopes on Monday on any slopes that rapidly accumulate new snow of more than a several inches.

Travel in avalanche terrain is not recommended along the west slopes on Monday. 

The December 8th layer continues to round and gain strength and appears to become nonreactive along the west slopes. The upcoming storm cycle should be will be a good test to see if this layer can be put to rest for the winter.

Another strong front should be seen Monday night and Tuesday. This should maintain or increase the avalanche danger in the Olympics and Cascades.

Snowpack Discussion

Weather and Snowpack

The last storm cycle to affect the Olympics and most of the Washington Cascades was about Thursday, December 8th to Monday morning, December 12th. About 2-4 feet of snowfall was seen at NWAC stations along long the west slopes.

There has been a lot of snowpack settlement since the last storm cycle ended, allowing this underlying snow to mostly stabilize.

Clear or fair and cold weather has been seen about Wednesday, December 14th to Sunday, December 18th . This caused widespread surface hoar and near surface faceted snow to develop in the Cascades. Thin sun crusts have also formed on steeper solar slopes. These layers are expected to act as widespread weak layers or sliding surfaces for the upcoming moderate to heavy snowfall.

Recent Observations

NWAC pro-observer Lee Lazzara was in the Yellow Aster Butte area near Twin Lakes near Mt Baker Friday. Strong NE ridge level winds were transporting major surface snow and building sensitive fresh wind slabs on atypical SE-NW facing terrain. These new wind slabs caused a plan change to safer lower elevation terrain. Even so, where small areas of wind slab were encountered, they were sensitive to trigger. Lee also noted new surface hoar and near surface faceted snow on a sun crust on solar slopes. He found that the December 8th persistent weak layer was no longer significant and not showing signs of propagation.

On a different note NWAC pro-observer Jeff Ward was at Stevens Pass on Friday and found light winds and generally stable snow. Isolated wind slab was stabilizing. However there was also widespread large surface hoar.

NWAC pro-observer Ian Nicholson was at Paradise on Saturday and found a mix of surface hoar and faceted surface snow that was reactive in shovel tilt tests. Past wind loading was noted on some west aspects. The December 8th persistent weak layer crystals seen at 52 cm were becoming rounded and were not reactive.

The Snoqualmie DOT crew has noted lots of surface hoar the past few mornings including Sunday morning.

NWAC pro-observer Simon Trautman was on Mt Herman on Sunday and found a widespread weak surface snow sandwich that included faceted snow and a crust. These layers were giving highly reactive cracking and natural and triggered slab avalanches on steep slopes.

Weak layers including faceted snow and a crust seen at Mt Herman on Sunday. Photo by Simon Trautman.

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.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.