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Archived

Avalanche Forecast

Dec 18th, 2019–Dec 19th, 2019
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
4: High
The avalanche danger rating at treeline will be high
Below Treeline
4: High
The avalanche danger rating below treeline will be high

Updated 7:50 AM: Wind slabs formed overnight with 5" of new snow combined with sustained moderate winds and strong gusts. Navigate carefully around steep or unsupported slopes where 8” or more of new snow has accumulated. You are most likely to trigger a large and destructive avalanche on steep upper elevation, north-facing, rocky terrain where a layer of weaker snow has been the culprit for a handful of small-large avalanches this past week.

Discussion

Forecast updated at 7:50 AM due to lower than expected precipitation and snow amounts. 5" of new snow accumulated Wednesday night. Winds were strong enough that we expect wind slabs.

A handful of human triggered avalanches (up to D2) and several very small natural avalanches were reported in the Hwy 542 corridor Thursday through Monday. These avalanches occurred in isolated areas: north and east aspects above 5000ft, on rocky, frequently unsupported/convex slopes steeper than 35 degrees. NWAC observers identified weak snow in isolated terrain at the highest, coldest locations on 12/18 and the facets are well developed. A skier triggered an avalanche in Mazama Bowl on Sunday, and was carried downslope, partially buried, but uninjured (photo below). One observer in the Cascade River Road area reported evidence of a large natural avalanche that likely occurred late last week (observation). On Monday and Tuesday, fresh wind drifting at upper elevations was notable in the Mt. Baker backcountry (observation). Travel at lower elevations remains challenging with plenty of shallowly buried obstacles.

Skier-triggered avalanche (D2) on a north aspect in Mazama Bowl at 5400ft. The crown of the avalanche was 2ft deep and 50ft wide, and the debris ran 300ft downslope. The skier was caught, carried, and partially buried by the avalanche, but was uninjured. 12/15/19 Photo: Nick March

In other areas, ie. below 5000 ft or on E-S-W aspects, a generally stable snowpack predates Wednesday night’s storm.

Snowpack Discussion

December 12th, 2019 (The regional synopsis is updated every Thursday @ 6 pm)

After a dry November, this week marks the beginning of more winterlike weather across the region with snow at middle and pass level elevations. While there’s uncertainty in the weather forecast, it does appear that the snowpack will continue to build to some extent over the coming week. 

Throughout much of the region, the slow start to winter has left avalanche conditions distinctly defined by elevation. Slopes above 5,500-6,000ft hold a layered snowpack that provides enough coverage for winter travel and avalanches. Below this elevation, most slopes were bare until the past week. At these lower elevations, it will likely take another round of storms before larger avalanches are possible and travel becomes easier.

 

Image courtesy of the Natural Resources Conservation Services interactive map showing Snotel weather stations measuring well below normal snow water equivalent for this season so far.

Upper Elevations

The lack of significant snowfall has resulted in commonalities throughout the region. Slopes above about 5,500ft currently hold the ”deepest” snow cover and the most layered snowpack. Until the lower elevation terrain builds a more substantial snowpack, the upper elevations will hold the most potential for producing large avalanches. If you dig into the snow in these areas, you’ll find a range of height of snow and a variety of layers. Here are a few layers to note:

  • The interface of older snow and any new incoming snow would be the first interface to check.

  • Snowfall around December 7th and 11th may have buried surface hoar and near-surface facets in some locations. As of Dec 12th, these interfaces can be found 1-2’ below the surface.

  • A layer of facets can be found near the middle of the snowpack, buried just before Thanksgiving. Where found, the facets are often rounded or have even undergone some melt-freeze metamorphism from liquid water.

While these layers give you something to look at in snow profiles, they may not be your main snowpack concern for the day. Continue to check the daily zone forecasts for the most up to date avalanche conditions. We’ll monitor these layers as future weather brings changes to the snow and avalanche conditions

 

A layer of facets in the middle of the snowpack resulted in sudden test results on Dec 11th. Rock Mtn, N, 6270ft. Photo: Josh Hirshberg.

Middle and Lower Elevations

At most locations below 5,500ft, slopes are still building uniform snow cover. In many zones, the hazard of hitting rocks or shallowly buried objects may be more significant than the avalanche danger. Depending on future weather, the snowpack could continue to form or could, unfortunately, melt out to the ground. While there’s little layering of note at these elevations, avalanches aren’t completely out of the question with the right weather input. Further low elevation snowfall or warming could drive avalanche activity. The East North forecast zone, including Washington Pass, has more low elevation snow than other zones and therefore more potential for avalanches at these elevations. As with the upper elevations, we’ll wait and see what the next round of weather brings.

Avalanche Problems

Wind Slabs

Northerly aspects above 5000 ft continue to be our greatest concern for large and reactive storm slabs in shallow, rocky, steep and unsupported slopes. A thin layer of weaker snow observed down 2-3 ft may be more reactive due to additional loading and will be sensitive in wind-loaded areas and SSE winds Wednesday night likely loaded many of these areas of concern. Watch signs of snow transport, such as rounded snow surfaces or wind-stripped ridgelines and then avoid wind loaded areas. "Red flag" signs are imminint indicators that you could trigger an avalanche, such as cracks shooting, whumpfing, collapses, or signs of recent human triggered or natural avalanche activity. When in doubt, defer to lower-angled terrain.

Updated 7:50 AM from storm slab to wind slab due to lower than expected precipitation totals Wednesday night.

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