Avalanche Forecast
Issued: Dec 11th, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs, Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Travel conservatively by staying off steeper slopes and avoiding wind loaded terrain near and above treeline, especially later in the day. The above treeline winds are forecast to be quite strong Saturday afternoon, so don't underestimate the rapid loading occurring at higher elevations. Persistent slabs are still possible to trigger in the CE zone, although there is still a fair amount of uncertainty regarding how spatially extensive this problem remains. Look for obvious clues like whumpfing or shooting cracks as evidence that persistent slab potential exists.
Summary
Detailed Forecast
Light snow should begin by mid-day and slowly increase throughout the afternoon and early evening. Combined with increasing southerly transport winds in the afternoon, there will be an increasing avalanche risk late in the day for the near and above treeline zones.
New wind slab quickly building on lee NW through E slopes Saturday afternoon will be the primary storm related avalanche problem near and above treeline. In contrast, storm slabs are less likely by later in the day.
Due to recent observations received Wednesday and Thursday, persistent slabs are still possible to trigger in the CE zone, although there is still a fair amount of uncertainty regarding how spatially extensive this problem remains. Look for obvious clues like whumpfing or shooting cracks as evidence that persistent slab potential exists. Give this deadly avalanche problem the respect it deserves and continue to play it conservatively.
Use extra caution if traveling into the above treeline zone (alpine) as there have not been any recent observations from this elevation band.
Snowpack Discussion
Snowfall over the east slopes the first week of December was about 1-4 feet. Then an atmospheric river arrived early in the week with additional snow in the northeast zone, rain in the southeast zone, with a transition in between in the central east zone. Almost 4 inches of water accumulated in 48 hours for Washington Pass and Holden ending early Thursday morning.
Avalanche and Snowpack Observations
We have varied conditions along the Cascade east slopes! The forecast discussion is the same for the east slopes, but the danger ratings and problems differ by zone.
A regionally deep snowpack exists in the northeast Cascades. Recent heavy precipitation along with a warming trend has likely thoroughly tested or buried persistent weak layers from earlier in the season. Avalanche problems are more likely to be storm related in the northeast zone. Persistent slabs now seem unlikely to trigger, but we will wait for more information before dropping this avalanche problem all together.
NWAC observer Tom Curtis was at Blewett Pass in the central east zone Wednesday and found sudden collapse test results in faceted snow at the November crust 15 cm from the ground. On Thursday, Tom found similar sudden collapses with buried surface hoar averaging 50 cm down at 5100 ft on the north side of Mt. Cashmere. More importantly Tom experienced whumpfing as this layer collasped and heard a natural avalanche release far from his observation location. We don't know how this layer fares as one moves further up in elevation, thus we'll forecast conservatively until more information is known about lingering PWLs in the central-east Cascades. Further east in this zone, Mission Ridge pro patrol had did not find the recent storm snow particularly reactive during control work Friday morning, nor was there any evidence of lingering weak layers.
The southeast zone should have a much shallower and more stable snowpack affected by recent rain and warm temperatures. We have not received any observations from this zone.
Problems
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
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, 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
Valid until: Dec 12th, 2015 10:00AM
