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Avalanche Forecast

Archived

Feb 8th, 2017–Feb 9th, 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 possible, human triggered probable.
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.

Wet slab avalanches and cornice failures should also be likely in areas with heavy rain Wednesday night to Thursday morning.

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 about 3-5 feet of snow recorded along the Cascade west slopes.

A slow warming trend affected all areas Saturday 2/4 with rain reaching 4000-4500 feet in the south Cascades. Cool easterly flow kept temperatures locally cool at Pass level at Stevens and Snoqualmie Pass through the day Saturday, with a warm-up Saturday midday following a switch to westerly flow. This created a weak crust layer that has now been found both at Stevens and Snoqualmie down about 3 feet. 

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. Most NWAC snowdepth gages settled about 6 inches or more during the day Tuesday.

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

Recent Observations

North

On Tuesday, the Mt Baker pro-patrol reported ski triggered soft slab avalanches of 8-10 inches on average, though they were not propagating widely as the older deep storm snow was showing signs of settlement and stabilization. There was evidence of a widespread natural cycle, likely Monday night on Table Mountain, with many crowns estimated in the 2-4 foot range.

NWAC pro-observer Lee Lazzara was in the Glacier Creek area Tuesday, reporting numerous natural and ski triggered storm snow soft slabs, on average 40 cm (16") on steep test slopes on both southerly and northerly facing slopes. Another ski triggered slide was evident from a nearby party that may have been a bit deeper, about 2 feet. 

Central

Reports from both Snoqualmie and Stevens passes Tuesday indicated continued storm snow settlement and stabilization, in the near and below treeline terrain. The thin melt freeze crust formed midday Saturday, 2/4 produced positive snowpit results but no avalanches were produced. The storm snow is gaining strength, showing increasing resistance with depth and providing some excellent conditions.

NWAC pro-observer Jeremy Allyn was in the Henry Creek drainage below Jim Hill on Wednesday and reported that wind transport was underway due to the incoming storm. Quick tests of the near surface snow did not show significant results and ECT and PST tests on the 2/4 interface at 75 cm down did not indicate propagation.

NWAC pro-observers Jeff Ward at Skyline at Stevens and Ian Nicholson near Alpental on Wednesday did find significant signs of instability.

Reports via the NWAC Observations page for Mt Snoqualmie for Tuesday and Wednesday indicate several recent D1-2 slab avalanches on steep terrain features.

South

NWAC pro-observer Dallas Glass was in the Crystal backcountry in Bullion Basin Sunday and found a reactive storm layer 35-40 cm down in snowpit tests on all aspects below and near treeline. Shallower storm slabs could be triggered on test slopes. Storm snow from this cycle had settled to about 60-65 cm. No natural avalanches were observed and recent wind transport was localized to near the ridgeline.  

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.