Avalanche Forecast
Issued: Dec 12th, 2016 11:45AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
The avalanche danger is less than during the recent storm cycle but careful snowpack evaluation and cautious routefinding will still be essential on Tuesday.
Summary
Detailed Forecast
Light snow showers should end Tuesday morning. A clearing trend should develop initially in the north Cascades Tuesday afternoon. But clouds can often linger at lower elevations along the east slopes in this pattern. Winds should be pretty light Tuesday with cold temperatures.
The avalanche danger is less than during the recent storm cycle but careful snowpack evaluation and cautious routefinding will still be essential on Tuesday.
Wind slab should be most likely to linger on north to southeast aspects due to recent southwest flow aloft. But watch for firmer wind transported snow on other aspects.
Storm slabs in many areas that had r sustained heavy snowfall should further settle on Tuesday.
There is still uncertainty regarding the regional extent, spatial variability and the stability of the potential December 8th persistent weak layer which would be buried at about 1-3 feet along the east slopes. Ratchet back your plans if you experience deep cracking, collapsing or whoomping. Some time and snowpack tests from many areas will be needed to determine is this layer is still a regional problem.
Loose dry avalanches will not be listed as an avalanche problem on Tuesday. But in steep terrain lacking a slab structure continue to watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences.
With deep unconsolidated snow in many wind protected areas there remains a risk for tree well and snow immersion suffocation at this time. Ride or ski with a partner and keep them in sight at all times!
Watch for early season travel hazards at lower elevations such as barely covered rocks and open creeks.
The regional avalanche danger is expected to further slightly decrease on Wednesday. But it is expected that southeast to east winds will transport snow and build new wind slab on Wednesday.
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.
These layers began to get buried in most areas on about December 8th when the latest storm cycle began starting with a couple day warming trend as southwest flow eroded an Arctic air mass over the Northwest. This initially buried the cold low density snow, near surface faceted snow and/or surface hoar with denser snow in most areas. The warming trend leveled off the past couple days. NWAC stations along the east slopes had about 1-3 feet of snowfall for the 4 days ending Monday morning.
Recent Observations
A regional avalanche cycle was seen Friday and Saturday with some avalanches releasing on the December 8th layer.
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 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 but with spatial variation and where steep wind loaded on north to east slopes slopes had a sensitive snowpack.
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, East, South East.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
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 13th, 2016 11:45AM
