Avalanche Forecast
Issued: Dec 15th, 2016 10:42AM
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
Caution, areas of new wind deposited snow are possible Friday on unusual aspects, mainly near or above treeline. Watch for wind stiffened surface snow on a variety of aspects and avoid steep terrain with evidence of wind loading.
Summary
Detailed Forecast
A storm system that moved inland to the south late Wednesday cause a period of moderate easterly winds Wednesday, likely redistributing surface snow to unusual aspects, mostly westerly facing near and above treeline.
Locally new wind slab from the recent easterly winds is possible Friday. This should be mainly on westerly aspects near ridges.
Mostly fair weather with cold temperatures and light winds are expected Friday. The cold temperatures should slow the stabilization of recent wind deposited layers.
Older wind slab from the latest storm cycle should have mostly stabilized, but watch for firmer wind transported snow on all aspects mainly near and above treeline.
The December 8th layer continues to round and gain strength and should be less sensitive to trigger, but remains a concern. Head for lower angle slopes and ratchet back your plans if you experience collapsing or whumpfing or have evidence from snow test results.
Storm slabs from the latest storm cycle should have stabilized in the Cascades by this time.
Loose dry avalanches remain possible on some steep terrain, protected from recent winds.
With deep unconsolidated snow in sheltered areas there remains a risk for tree well and snow immersion suffocation. When near trees, ride or ski with a partner and keep them in sight at all times!
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.
Snowpack Discussion
Weather and Snowpack
Cold and fair weather early last week allowed for a sun crust to form on many solar aspects and for cold low density snow, near surface faceting and/or surface hoar to become fairly widespread in the Washington Cascades.
These surface snow types were buried late December 8th when the latest storm cycle began. The four day storm cycle, beginning a week ago Thursday, deposited about 1-3 feet of snowfall over most east slope areas ending Monday morning.
Another short period of fair cold weather earlier this week allowed for another round of surface hoar or near surface faceted snow to develop as well as a sun crust on some directly south face terrain.
Locally, strong easterly winds Wednesday, December 14th transported loose surface snow and built new fresh wind slab on less typical westerly facing terrain, mainly near and above treeline and nearer to the crest.
The central and southern portions of the forecast area received up to a few inches of snow overnight Wednesday which may have buried any preserved weak surface snow.
Recent Observations
A regional avalanche cycle was seen December 8th-11th with some avalanches releasing on the December 8th persistent weak layer in the Washington Cascades.
Observations from Washington Pass Saturday indicated about 12 inches of storm snow was still lacking much slab character, but had gained minor slab stiffness in areas. One natural size 1.5 wind slab was noted releasing from below a ridge. No triggered slides were reported but some cracking was noted as the surface snow gained some cohesion. The December 8th layer of buried surface hoar and/or buried near surface facets was about a foot down as of Saturday.
An observation for the Mission Ridge via the NWAC Observations tab for Sunday in the Lake Marion area indicates a right side up snowpack, but also noted spatial variability and 10-35 cm wind slab giving shooting cracks on loaded slopes.
Pro-observer Jeff Ward was at Washington Pass on Sunday and noted slightly increasing slab structure near ridges above about 7000 feet with some cracking and limited propagation when testing steep convex slopes.
The pro-patrol at Mission Ridge on Monday gave a very similar report to the Lake Marion report above, that of a generally shallow, but stable snowpack, also noting spatial variation and a sensitive snowpack on local steep wind loaded N-E slopes.
More critically, on Tuesday there was a skier triggered slab avalanche release on the December 8th Persist weak layer in the Highland Bowl back country next to the Steven Pass ski area. The Stevens pro-patrol reports it was on a steep SSW slope at 5400 feet with a 40 cm x 100 foot crown releasing in buried hoar frost and facets on a crust.
A member of the Stevens Pass pro-patrol reported wind transport on Rock Mountain and especially in the Chiwaukums on Wednesday.
The most recent reports for the Washington Pass is well stated on our recent observations page!
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: North, North East, South West, West, North West.
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 16th, 2016 10:42AM
