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Avalanche Forecast

Archived

Feb 9th, 2017–Feb 10th, 2017

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
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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A mixed bag of avalanche problems should be expected on Thursday. Back country travel in avalanche terrain is not recommended through Thursday morning.

Detailed Forecast

A strong wet warm front will lift north over the Olympics and Cascades on Wednesday night to Thursday morning. Plan on rapid warming Wednesday night and heavy rain or snow with higher snow levels through Thursday morning. This should cause an avalanche cycle in the Olympics and Cascades which should test the snowpack. Small avalanches may step to deeper layers during the cycle.

A mixed bag of avalanche problems should expected on Thursday with wind slab and storm slab very likely at times in the above treeline band and loose wet avalanches very likely in the below treeline band with some over lap in between in the near treeline band. Back country travel in avalanche terrain is not recommended through Thursday morning.

Although the northeast zone won't be listed separately most precipitation there will probably fall as snow and wind slab and storm slab should be the predominate avalanche problems there.

Due to the numerous other avalanche problems listed for the Cascade east slopes persistent slab won't be included in the list. Potential persistent slab layers should get good test during this avalanche cycle and this is another good reason to avoid avalanche terrain Thursday in case there are any dangerous results. A decision will need to be made on whether to relist persistent slab for the Cascade east slopes later partly based on what happens Thursday.

The cold front should cross the Cascades Thursday late morning or midday. This should cause a change to showers and lowering snow levels. The avalanche danger should begin to decrease following the cold front Thursday afternoon.

  

Snowpack Discussion

Weather and Snowpack

A strong storm cycle was seen Friday through Monday with 2-3 feet of snow reported along theCascade east slopes on average with lesser amounts at lower elevations and in the Mission Ridge/Blewett area. A slow warming trend affected all areas Saturday afternoon.

A nice day was seen Tuesday with light wind and sun or filtered sun and high clouds allowed for snowpack settlement and stabilization of the deep recent storm snow.

Increasing winds, snowfall and warming were generally seen on Wednesday as the next storm approached.

Recent Observations

North

A 5 cm layer of facets roughly 10 cm below the 1/17 crust may still be found in the Washington Pass zone and in areas further east. This reactive PWL has only been found in isolated locations in cold non-wind affected north facing terrain. No avalanche activity has been observed on this layer. 

On Sunday NWAC pro-observer Jeff Ward and the NCHG observed debris from several small wind slab avalanches and one large wind slab avalanche, size D2.5, that ran from ridgeline over 2000 feet on a ENE aspect likely during peak precipitation and wind loading Saturday night. Storm slabs were not particularly sensitive in areas Jeff traveled, with evidence of a few previous natural storm slabs observed on steep solar aspects that likely ran on a 1/30 sun crust.

Reports of gradually settling storm snow from the Washington Pass area are providing excellent conditions on mid angled terrain as of Tuesday. However The NCHG report a1 m X 100 m natural slab avalanche on a run near Harts Pass known as Dogs Bowl that possibly released Monday or Monday night on a faceted layer below the recent storm snow. Details are lacking regarding aspect or elevation.

Central

On Sunday the pro-patrol at Mission Ridge reported several collapses (whumpfs) on deeper weak layers. These occurred on an infrequently skied NE aspect about 6000 feet.

South

A report via the NWAC Observation page for Umptanum Ridge for Tuesday indicates a shallow 70 cm snow pack with whoomping due to buried hoar frost at 47 cm below the surface.

Problems

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.

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.