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Avalanche Forecast

Archived

Mar 19th, 2016–Mar 20th, 2016

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

Regions

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A mixture of typical spring avalanche problems exists with no one problem dominating. Evaluate the local snow and terrain carefully on Sunday.

Detailed Forecast

Cloudy Sunday with periods of mostly light rain or snow showers are expected. Rain and snow amounts should be relatively light and not significantly affect the current danger.

Clouds may allow the temperatures to remain near or above freezing in areas Saturday night and with the addition of light rain near and below treeline, small loose wet avalanches may be possible on Sunday. Loose wet avalanches may start small but could have the ability to entrain recent moist snow in mainly on steep terrain.

Moderate winds near ridgetop may combine with light precipitation to build new areas of shallow wind slab on lee aspects in the upper elevations above treeline. Any new wind slabs will mostly likely be found in NW-NE facing slopes. 

Storm winds 3/13-15 likely built wind slab on lee aspects in the upper elevations of the above treeline band. This wind slab will mostly likely be found on NW-SE facing slopes.

Recently formed cornices have grown large, avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices in general during the spring.

Due to recent sensitive storm slab releases on suspected buried surface hoar layer, March 12th, we are now listing this layer, though it may be spotty and not widespread, it should nonetheless draw attention from travelers in the NE zone. 

Snowpack Discussion

Weather and Snowpack

An active weather pattern in early March has seen several strong storms since March 10th. Cool and showery weather Monday and Tuesday with prolonged westerly winds deposited additional snow at lower snow levels. Storm totals from Thursday 3/10 through Tuesday morning, 3/15 ranged from 1.5 - 3.5 feet. ?

Recent storm instabilities have been slow to consistently stabilize throughout the east slopes. Specifically, in the Washington Pass zone as storm slabs continue to be sensitive to human triggering in places as recent as Friday 3/18. The recent storms have also built large cornices along ridges in many areas with several large cornice releases noted over the last few days.

A spotty layer of surface hoar was likely formed and buried by 3/12 in some areas, with the terrain most suspected N-E facing slopes in the upper elevations of the below treeline and near treeline band.  

During a period of fair weather in late February, widespread surface hoar grew in the northeast zone, mainly surviving outside of steeper solar aspects and wind affected terrain. This layer was buried about 2/27 in the Washington Pass area and to a lesser extent the central-east zone. Avalanches on this layer are now unlikely and no recent avalanches have been noted to this layer.   

We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity at these interfaces and confirming field observations. 

Recent Observations

NWAC pro-observer Tom Curtis tested conditions in the Blewett Pass area Monday 3/14 following the strong storm Sunday. Sensitive storm and wind slabs of up to 20 inches were found in open terrain, especially wind affected slopes. Easy failure in test pits was noted on a crust-facet layer buried beneath the current storm snow, about 16-20 inches. Failures were found with PST and ECT tests on this interface.

Recent reports from the Washington Pass area Friday indicated some shallow wind slabs were remained sensitive to human trigger as well as some shallow storm slabs in lower elevations, possibly on the 3/12 persistent layer. 

At least 2 recent cornice releases in the Washington Pass area caused large size 3 storm slab and car sized blocks running to the valley floor.

Tom Curtis was out again on Mt Cashmere on Thursday 3/17 and found about 35 cm of storm snow well bonded to an underlying crust from last week. An extended column test did not indicate propagation and only small loose wet avalanches were seen on steep solar slopes. Temperatures did not greatly affect the snowpack.

The Mission Ridge pro-patrol reported limited wind transport on Friday, 3/18 from east winds and surface crusts below about 6000 feet from sun and warmth.

 

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.