Avalanche Forecast
Issued: Dec 8th, 2016 11:55AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions may develop on Friday. Look for new wind slab near and above treeline especially from Stevens Pass south to White Pass. New snow may bond poorly to weak surface snow and become reactive by the end of Friday with increased loading and a gradual warming trend. Dial back your plans if you experience signs of increasing instability.
Summary
Detailed Forecast
Will we reach a tipping point on Friday with widespread sensitive slab development or will recent weak layers be buried and await the next storm to become active? A first round of light to moderate snow Thursday night followed by increasing shower activity and a gradual warming trend Friday afternoon/evening should lead to an increasingly sensitive snowpack.
Dangerous avalanche conditions may develop on Friday. Look for new wind slab near and above treeline especially from Stevens Pass south to White Pass. All aspects will be listed due to variation in local loading patterns. Also, new snow may bond poorly to weak surface snow and become reactive by the end of Friday with increased loading and a gradual warming trend. The near-treeline band should be the trickiest to navigate where shallow new wind slab may have already buried recent weak surface snow. Above treeline and especially in the south-central Cascades including Crystal and Paradise new wind slabs should be the largest. New storm slabs may the most sensitive by the end of the day in the Mt. Baker area on all aspects. The Considerable danger rating will be the same throughout the west slopes, but the avalanche problems will vary slightly in the forecast zones.
In steep sheltered terrain continue to watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences. If the warming trend doesn't materialize Friday, loose dry will be more likely than storm slabs.
With deep unconsolidated snow in many wind protected areas, there is an increased 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!
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
An active and cool weather pattern has resulted in a snowy December thus far with NWAC stations along the west slopes recording about 2-4 feet of snow.
Cold and fair weather has taken hold since Monday. There have been many reports of right side up, favorable density profiles with lower density snow nearer the surface. We've been hearing the term "as good as it gets" quite a lot lately!
Given the favorable snowpack profile, good bonds between older snow layers and a lack of deeper layer concerns, avalanche problems should be confined to the upcoming old snow/new snow interface or within new storm layering.
In areas not blasted by the easterly winds over the last 24 hours, near surface faceting and/or surface hoar is widespread. Considerable wind transport has occurred near and above treeline from Stevens Pass and south to the Crystal and Paradise environs. Less wind effect occurred in the Mt. Baker area.
Recent Observations
Multiple professional reports received Monday and Tuesday from the Stevens and Snoqualmie Pass areas, and above Paradise on Mt Rainier indicated that the storm related sensitivity experienced last Sunday had improved greatly. No slab like character was noted with a favorable density profile providing excellent surface snow conditions. These were areas where sensitive human triggered soft slab conditions were reported Sunday, indicating an improving stability trend. Tree and rock wells were a significant backcountry travel concern in many areas.
Widespread near-surface faceting and surface hoar formation has been observed throughout the west slopes including the Passes over the last few days. Strong easterly winds have likely built sensitive but shallow wind slabs roughly from Stevens Pass and south to White Pass. We had confirmation of shallow and increasingly sensitive wind slab from Pro-observer Tom Curtis in the Jove Peak area near Stevens Pass Thursday on SW-SE aspects near treeline. Tom also noted a melt freeze crust overlying buried surface hoar just below the surface on N through SE aspects near treeline - a potential bed surface/weak layer interface. A similar report came from a solar slope in the Chair Peak area of Snoqualmie Pass Wednesday. The bottom line is that we have new wind slab for much of the west slopes outside of the Mt. Baker area on some non-traditional aspects near and above treeline and widespread weak surface snow in less wind affected areas heading into the upcoming storm cycle.
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: All aspects.
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
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Dec 9th, 2016 11:55AM
