Avalanche Forecast
Issued: Dec 11th, 2016 10:04AM
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
Recent storm and wind slab deposits should be given ample time to settle and stabilize, especially due to the slow stabilization rates given the expected cold temperatures. Human triggered avalanches should remain likely on Monday. Choose low angled terrain and avoid slopes connected to avalanche paths with wind loaded start zones. More deeply buried persistent layers are now a factor!
Summary
Detailed Forecast
Continued light storm snow deposits overnight Sunday should give way to partial clearing Monday as high pressure affects the area. Winds should be light Sunday night and Monday with further cooling.
While the ending showers with light winds and cold temperatures should lead to an overall improving avalanche danger Monday, there has been such significant storm snow, along with periods of moderate winds over the past three days, that another day or more of very cautious travel in backcountry terrain is urged.
Storm and wind slabs should begin to slowly settle and stabilize beginning Monday. However, careful snowpack evaluations, cautious routefinding and conservative decision-making will remain essential, as these dangerous avalanche conditions slowly improve.
Be wary of the potential buried persistent layer of surface hoar or near surface facets, potentially in combination with a thin crust layer. A high degree of uncertainty exists as to the spatial variability of this layer and the sensitivity to human trigger as it is now buried 1-2 feet or possibly more as you travel nearer the crest.
Choose low angled terrain and avoid slopes connected to avalanche paths with wind loaded start zones.
Expect new wind slab near and above treeline in all zones, especially on lee easterly aspects. Avoid all wind loaded terrain Monday. In areas away from the crest, such as Blewett Pass and Mission Ridge, expect wind slab to be the primary avalanche problem on lee aspects near and above treeline.
Loose dry will not be listed as 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 many wind protected areas, there remains 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 east slopes recording about 1 to 3 feet of snow.
Cold and fair weather took hold midweek. During this time, there have been many reports of right side up, favorable density profiles with lower density snow nearer the surface. The Washington Pass area in particular has enjoyed excellent skiing and riding conditions with good stability and light winds. Widespread near-surface faceting and surface hoar formation had been observed particularly in this area of the east slopes. The Mission Ridge area experienced moderate easterly winds through Thursday night before switching to moderate westerly winds early Friday morning. Moderate westerly winds continued at Mission Ridge on Saturday.
Light to locally moderate snowfall accumulated Thursday night through Saturday morning along the east slopes. Mission Ridge and Blewett Pass saw a few inches each day, but Holden Village and the Lyman Lake Snotel showed about 12-20 inches of new respectively. In the Southeast zone, Snotels averaged about 6-12 inches of new snow over the last 2 days ending Saturday morning. A slow warming trend began Friday night and continued on Saturday, gradually dislodging the previous arctic air mass.
Another few inches from snow showers likely accumulated Sunday, potentially more nearer the crest.
Recent Observations
Mission Ridge pro-patrol reported the switch to westerly winds had loaded NE aspects below ridgetop on Friday. Easy to trigger wind slabs of 40 cm deep were observed on NE aspects around 6700 feet. Just a few hundred feet below in less wind affected terrain, the snowpack was generally right side up and lacked a cohesive slab. Avalanche concerns were confined to loaded slopes just below ridges again on Saturday in the Mission area. A shallow snowpack still locally limits the avalanche danger to the higher elevations and more wind loaded aspects.
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, 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
Valid until: Dec 12th, 2016 10:04AM
