Avalanche Forecast
Issued: Dec 16th, 2015 10:53AM
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
Dangerous avalanche conditions are expected by Thursday afternoon. Reign in your objectives later in the day as snowfall and winds increase by avoiding steep and wind loaded slopes, watching for sensitive storm layers. Recent and newly wind loaded slopes in the near and above treeline zone should be found on a variety of aspects Thursday.
Summary
Detailed Forecast
A weak cold front will cross the Northwest Tuesday night and Wednesday morning. This may bring some very light amounts of new snow but should not greatly change conditions. Some fair weather should be seen by Wednesday afternoon. Light or moderate northwest winds and cool temperatures should also continue on Wednesday.
Previous wind slab layers from the weekend will possibly linger on previous lee mainly north to east slopes near ridges and summits on Wednesday. Storm slab from the weekend should become unlikely by Wednesday but would be found in what were calmer areas if it still exists.
Use more caution and watch for new small wind slab or small storm slab if the weak front Tuesday night and Wednesday morning brings more snow than expected.
Continue to watch for signs of the PWL mainly in the central east zone. Snow pits could be helpful for this. You don't want to take big chance when this type of layer may be present.
Terrain anchors are still causing significant anchoring at the lowest elevations.
Snowpack Discussion
An atmospheric river around Dec 9th pushed heavy rain up to at least 6000 feet in the north Cascades and above 7000 feet in the south. Below these elevations water percolated down below the early December crust (date at which when the crust was buried) and is expected to have eliminated or subdued persistent weak layers formed mainly in late November in the Stevens and Snoqualmie area. An active and cool weather pattern over the last week has produced about 2 to 3 plus feet of new snowfall that now sits over the 12/9 crust. The below treeline zone snowdepth is filling in nicely but still contains many barely hidden hazards like rocks, streams or snags. The skiing in non-wind affected areas with enough snow cover has been good!
Last weekend, most avalanches released within storm layers, but a few released down to the recent rain crust. Storm instabilities have had time to heal and no new avalanches were reported Tuesday or Wednesday. The NWAC staff along with other Stevens Pass professionals had field training Monday and Tuesday in the Stevens Pass area and found a reactive graupel or stellar layer at 35-40 cm down but tests did not indicate propagation was likely. Moderate west-northwest winds in the above treeline zone along with light amounts of new snow received Tuesday night through Wednesday morning have likely redistributed recent and new snow onto lee easterly slopes in the near and above treeline zones.
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, North West.
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 17th, 2015 10:53AM
