Avalanche Forecast

Archived

Feb 8th, 2017–Feb 9th, 2017

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

Regions

.

An approaching storm Wednesday will begin to load and stress the deep recent storm snow by Wednesday afternoon. Watch for changing conditions and be prepared to alter tour plans and leave plenty of safety margin. Travel with deep snow safety precautions in place.

Detailed Forecast

An approaching storm will bring increasing clouds and strengthening winds through the morning Wednesday. Snowfall will arrive midday Wednesday and increase through the afternoon with gradual warming.

The warming and beginning of the new snow loading Wednesday afternoon, will begin to stress the significant recent storm snow. The avalanche danger will gradually increase through the afternoon Wednesday, however the significant loading and heavier precipitation rates are likely to hold off until Wednesday night and into Thursday.

If the approaching storm arrives sooner than forecast or with, initially heavy precipitation, the avalanche danger could increase more rapidly than expected. Watch for changing conditions and be prepared to alter plans accordingly.

Note that deep snow conditions now exist in most terrain. Travel with a partner and use good communication and keep your partner in view at all times.

Snowpack Discussion

Weather and Snowpack

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

Moderate W-SW winds occurred in the Washington Pass zone Saturday night. Light to moderate snow showers have been occurring through Monday afternoon in all areas.

Recent Observations

North

Persistent slab has been re-listed in the northeast zone highlighting a 5 cm layer of facets roughly 10 cm below the 1/17 crust 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, Jeff and NCH 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 the 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 2/7. However, there was a report of a 3 ft natural, possibly releasing Monday or Monday night on a faceted layer below the recent storm snow, unsure of aspect or elevation.

Central

On Sunday, 2/5, ski patrol at Mission Ridge reported several collapses (whumpfs) on deeper weak layers. These occurred on an infrequently skied NE aspect about 6000 ft.

South -

Nice to see a new report from this zone appear on the NWAC observations page Tuesday!

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.