Avalanche Forecast
Issued: Dec 14th, 2016 11:29PM
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
Areas of new local wind slab are possible on Thursday depending on the local strength of the east to southeast winds on Wednesday. The December 8th persistent weak layer may still be a problem in some areas.
Summary
Detailed Forecast
A low pressure and frontal system will pass by to the south across Oregon and north California on Wednesday. This system will cause southeast winds in the Olympics and Cascades, with east winds in the Cascade passes, and will cause some snow mainly in the south Cascades late Wednesday.
Light snow showers mainly along the west slopes should end Thursday morning with light winds and continued cool temperatures.
New local wind slab is possible on Thursday depending on the local strength of the east to southeast winds on Wednesday. This should be mainly on northeast to southwest aspects near ridges and passes in the central to south Cascades. Older wind slab from the latest storm cycle should have mostly stabilized so the northeast to southwest aspects will be indicated. But watch for firmer wind transported snow on all aspects mainly near and above treeline.
The Tuesday report from Stevens Pass indicates the December 8th layer is still possibly reactive in the Washington Cascades. It should be a good plan to stick to lower angle slopes and ratchet back your plans if you experience collapsing or whoomping or have evidence of this layer such as from snowpits. Persistent slab avalanches may be harder trigger but have a tendency to propagate to a larger size.
Storm slabs from the latest storm cycle should have mostly stabilized by this time. New snow amounts late Wednesday in the south Cascades should be limited enough to not build significant new storm slab. But change your plans if necessary if more snow falls late Wednesday than expected and you find you are in an area where there has been more than a few inches of rapidly accumulated snow and if it seems that storm slab might be present.
Loose dry avalanches from snow from the latest storm aren't expected to still be an avalanche problem. 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 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 layers began to get buried 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 with temperatures staying well below freezing December 10th and 11th and snowfall tapered off by December 12th. NWAC stations along the west slopes had about 2-4 feet of snowfall for the 4 days ending Monday morning.
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. There was also a snow immersion fatality at Snoqualmie on December 11th and apparently a snow immersion close call at Stevens on December 10th.
The Mt Baker pro-patrol reported decreasing avalanche control results on Monday with only local new 7 inch storm slab on sheltered slopes.
The Crystal pro-patrol skied extensively in the back country on Monday while searching for a missing skier and noted recent settlement and no avalanche activity. The skier was later found by Highway 410.
The NWAC pro-observers were at Paradise for a class on Tuesday and generally found a right side up snowpack. Soft wind slab on the surface did not show propagation. The December 8th layer was only seen in some pits and was not reactive. Wind effects could be seen on Mazama Ridge indicating some forming wind slab.
Some reports are available for Tuesday via the NWAC Observation tab. A report from Snoqualmie indicated good stability. A report from Stevens indicated the December 8th layer at 60 cm but it was not reactive.
The most important report from Tuesday is that another slab avalanche release on the December 8th layer was skier triggered in 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.
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 15th, 2016 11:29PM
