Avalanche Forecast

Issued: Dec 9th, 2016 11:10AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Wind Slabs, Storm Slabs and Loose Dry.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New storm and wind slab may become increasingly sensitive and widespread Saturday due to additional light loading and a continued slow warming trend. Continue to assess snowpack stability throughout the day and dial back your plans if you experience signs of increasing instability. 

Summary

Detailed Forecast

Another round of light to moderate snow showers Friday night should be followed by increasing shower activity Saturday afternoon and evening. Look for a relative break in precipitation from Saturday mid-morning through mid-day.  Areas outside of the Cascade Passes have already moderated temperature-wise while the Cascade Passes should experience a slow warming trend with the weakening of easterly flow late tonight and into Saturday.  

Once again, dangerous avalanche conditions may develop on Saturday as we look to tip the scales with additional light loading. Gradual warming will help new snowfall settle and perhaps develop more of a slab structure. Look for new wind slab near and above treeline in all zones. All aspects will be listed due to the ongoing reversal from easterly to westerly flow depending on location and elevation.

The upper portion of the below treeline and the near-treeline band should be the trickiest to navigate where persistent weak layers may have survived windy periods and become buried intact by recent light snowfall.

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. 

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 was in place midweek allowing near surface faceting and/or surface hoar to become widespread. Beginning Wednesday night and continuing through Thursday, considerable wind transport from easterly winds 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. 

A switch to westerly flow occurred outside the Cascade Passes on Friday with moderate warming while the Passes stayed easterly and cold. Generally light snowfall accumulated Thursday night through Friday with the most at Mt. Baker and Paradise where roughly 9 inches of snow has accumulated through 5 pm. 

Prior to this new snow, the snowpack in non-wind affected terrain was generally right side up with low density snow at the surface and no problematic layers in the lower snowpack. 

Recent Observations

Widespread near-surface faceting and surface hoar formation has been observed throughout the west slopes including the Passes during the middle of the week along with some excellent ski conditions. Strong easterly winds built sensitive but shallow wind slabs roughly from Stevens Pass and south to White Pass on Thursday. 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.

Light snowfall and/or localized shifting winds and warming Friday did not tip the scales and cause an uptick in reported avalanche activity. Most of the ski areas checked in today from Mt. Baker down to White Pass, and they generally noted shallow storm or wind slabs in their areas that were neither especially sensitive or widespread during control. Pro-observer Lee Lazzarra was in the Mt. Baker backcountry Friday and noted easy hand shears between the old snow/new snow interface but overall the new snow lacked a cohesive slab structure. 

Problems

Wind Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Loose Dry

An icon showing 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

Possible

Expected Size

1 - 1

Valid until: Dec 10th, 2016 11:10AM