Avalanche Forecast
Issued: Dec 17th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Watch for firmer wind transported snow on a variety of aspects and avoid steep terrain with evidence of wind loading on Sunday.
Summary
Detailed Forecast
Light snow showers should resume on Sunday with light to moderate west winds. A slight warming trend should be seen by Sunday afternoon.
A broad brush will be used and all aspects will be indicated for wind slab on Sunday. The greatest danger should be in the Northwest zone where the most transport and greatest snowpack sensitivity was seen on Friday. But watch for firmer wind transported snow on all aspects in all the west slope zones due to recent varied winds especially above treeline.
Light snow showers on Sunday may further bury intact a variety of weak surface snow types formed this week under fair cold weather. This is likely to become a weak layer starting Monday.
The December 8th layer continues to round and gain strength and should be less sensitive to trigger. Head for lower angle slopes and ratchet back your plans if you experience collapsing or whumpfing or have evidence of this layer such as from snowpits.
The danger of snow immersion may not be past due to all the heavy snowfall in December. Always ski or ride with a partner and keep them in sight at all times especially if you are moving past trees.
Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.
Note that the first in a series of storms is expected to begin to move across the Northwest Sunday night. This will bring a significantly increasing avalanche danger by Monday.
Snowpack Discussion
Weather and Snowpack
The last storm cycle to affect the Olympics and most of the Washington Cascades was from Thursday, December 8th to Monday morning, December 12th. About 2-4 feet of snowfall was seen at NWAC stations along long the west slopes.
Fair and cold weather this week has been interspersed with periods of moderate or strong NE-E winds both Wednesday and Friday. These winds were strong enough to build fresh wind slabs on more atypical SW-NW facing terrain, mainly near and especially above tree line.
There has been about 12 inches of snowpack settlement since the storm cycle ended, allowing underlying or storm snow related weak layers to stabilize.
West winds in the 20-30 mph range are seen some higher elevation NWAC stations on Saturday such as Crystal Mountain and White Pass. These winds are strong enough to transport snow to mainly NE to SE aspects.
Several clear nights with light winds this week have also allowed for weak surface snow development. On wind and sun sheltered terrain, extensive near surface faceted snow or surface hoar has formed this week and this may be an important weak layer if buried intact by the expected return of light snowfall Saturday and Sunday.
A thin sun crust is likely to have formed on steeper southerly facing slopes, which also may be a future interface for avalanches when loaded with sufficient wind transported or future storm snow.
Recent Observations
A regional avalanche cycle was seen December 8th-11th especially along the west slopes with some avalanches releasing on the December 8th persistent weak layer in the Washington Cascades.
NWAC pro-observer Lee Lazzara was in the Yellow Aster Butte area near Twin Lakes 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 brought 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 that will be a weak layer if buried by future snowfall. 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 that will be a weak layer if buried by future snowfall.
Problems
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 18th, 2016 10:00AM
