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Avalanche Forecast

Archived

Dec 18th, 2016–Dec 19th, 2016

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
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 above treeline is not recommended along the east slopes on Monday. Potential dangerous avalanche conditions are expected in the other elevation bands where cautious route finding and conservative decision making will be essential 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 snow along the east 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 east 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 east slopes on Monday on any slopes that rapidly accumulate new snow of more than a several inches.

Travel in avalanche terrain is not recommended in the above treeline band along the east slopes on Monday. 

The December 8th layer is still a question mark along the east slopes. The upcoming storm cycle should be a good test to see if it is still reactive. Head for lower angle terrain if you see collapsing or whoomping or evidence of a buried surface hoar or buried faceted snow and crust layer.

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 1-3 feet of snowfall was seen at NWAC stations along the east 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 Tom Curtis found weak faceted snow above the ground in a still fairly shallow overall snowpack in the Blewett Pass area Thursday. No observed avalanches were noted, but snowpack tests indicate the potential. See a recent NWAC instagram post about the results of a PST test.

On Friday the NCMG at Harts Pass had strong N-NW winds causing wind transport, shooting cracks and wind slab layer collapsing.

Also on Friday the NCMG had strong E-NE winds causing major snow transport seen in the Washington Pass area and off all major peaks south to Bonanza Peak. But near Rainy Pass a generally right side up snowpack was found with a good bond was seen at the December 8th interface.

On Monday the Mission Ridge pro-patrol reported widespread, poorly bonded surface faceted snow about 1 cm deep. The underlying snowpack was strong despite the shallow snow pack and cold temperatures.

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.