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Avalanche Forecast

Archived

Mar 1st, 2018–Mar 2nd, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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While the Avalanche Danger is slowly decreasing, dangerous conditions remain at upper elevations. You can trigger Wind Slabs near and above treeline and Persistent Slab avalanches on sunny slopes in the upper snowpack. Avoid wind loaded areas and large open sun-exposed slopes 35 degrees and steeper. 

Detailed Forecast

You can easily trigger Wind Slab avalanches near and above treeline, today. You can avoid these avalanches by staying off of recent snow drifts, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. These areas may exist far below ridge-lines and on mid-slope cross-loaded features. In some location soft non-wind-effected snow may cover new wind slabs making them harder to identify.

You may still be able to trigger recent Storm Slabs about 1 foot deep.There's a good chance that these avalanches will be hard to trigger by Saturday. Small test slopes and quick tests can help you identify whether you could trigger these avalanches. Avoid slopes over 35 degrees with poorly bonded, yet cohesive surface snow. 

You can trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes greater than 35 degrees. Avoid steep, open, sunny slopes as well as large avalanche paths to reduce your risk of these difficult to manage avalanches.If you dig about 2-3 feet below the surface you will find a series of thin sun crusts surrounded by very small facets. These layers have been the source of several avalanches in the Cascades. Snow profiles and snowpack test can confirm the presence of this layer; however they cannot prove its absences.

Observers continue to report potential for a Deep Persistent Slab. Avalanches triggered in the surface snow could step down to release a large and dangerous avalanche in deeper layers in the snowpack. These avalanches are becoming increasingly difficult to trigger. The best way to avoid this low likelihood-high consequence problem is by avoiding triggering smaller avalanches in the surface snow. You may see some small loose avalanches release on the surface of steep slopes today. 

Snowpack Discussion

Over a foot of new snow fell since Wednesday. New snow combined with moderate to strong winds to transport snow in exposed terrain formed fresh wind slabs.

On E-S-W aspects, a thin breakable sun crust was formed early last week and buried on 2/23. Very small weak facets have been reported surrounding the crust. This layer has not yet had significant time to heal. It is found 2-3 feet below the surface on steeper slopes that have received direct sun during the past week.

Some observations suggest the presence of other persistent grains at this same interface on shaded slopes. Buried surface hoar and large preserved stellars have been reported in recent avalanches and snowpack tests at this interface.

Avalanche and snowpack observations continue to indicate that avalanches are possible on a layer of weak sugary facets buried on 2/13. This weak layer is generally 3 to 5 feet below the snow surface just above a very firm melt-freeze crust (2/8).

There are no significant layers of concern below the 2/8 crust.

Observations

On Wednesday NWAC professional observer Lee Lazzara traveled in the Mt Baker backcountry. Lee reported wind slabs forming on a variety of aspects near treeline. Snowpack observations showed a highly variable snowpack, but the facet/crust combination was found on steep sunny aspects 2 feet below the snow surface.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.