Avalanche Forecast

Archived

Apr 8th, 2017–Apr 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 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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The avalanche danger will increase with elevation on Sunday due to the most recent strong storm. Expect dangerous avalanche conditions above treeline. Large recent wind slabs will require careful snowpack evaluation and cautious routefinding.

Detailed Forecast

A break in storms is expected Sunday, allowing for sunshine and gradual warming. Light winds and sunshine should allow for recent storm snow to quickly become wet and weak, especially on steep slopes facing the sun during the late morning and afternoon. Loose wet avalanches should become increasingly likely in this terrain.

Watch for wet surface snow deeper than a few inches, pinwheels and initial natural releases that indicate an increasing loose wet avalanche danger. Be wary of steep slopes with terrain traps such as cliffs or if heavily treed, where even a small loose wet avalanche could have big consequences. Initial small loose wet avalanches may entrain significant wet snow and become dangerous quickly.

Recent potentially deep wind slabs should persist on various aspects above treeline due to the strong and shifting recent winds. Expect wind slab formation in potentially unusual locations, possibly well below ridges and to lower elevations near treeline.

Recent cornices are very large and have likely been weakened during this most recent storm cycle. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Remember that solar effects can rapidly increase the touchiness of various types of avalanches at this time of year, loose-wet, cornices, glide and wind slab to name a few.

Snowpack Discussion

Weather and Snowpack

March was a wet and wild month for weather and avalanches in the Cascades. The last widespread avalanche cycle during this stretch occurred March 28th-29th.

Fair but cool weather has been seen in the Cascades Monday and Tuesday. Reports generally indicate strong surface or near surface crust layers and shallow recent snow.

A frontal boundary on Wednesday to Thursday morning brought heavy rain and snow to Mt Baker with light rain or snow for the rest of the Cascades. Snow levels were generally between 4500-6000 feet. In the 3 days ending Friday morning NWAC stations along the Cascade east slopes had about an inch of WE which will have fallen as snow above the snow levels.

A deep low pressure system moved north just off the Washington coast on Friday and Friday night. This storm deposited about 2-6 inches along most east slope areas by early Saturday with very strong SE-SW winds Friday afternoon and evening. Winds averaged 40-60 with gusts of 80-95 at Mission Ridge late Friday with strong winds at Dirty Face above Lake Wenatchee as well. These winds were so strong that wind slabs were less widespread and formed lower on leeward slopes than typical.

Cooling, diminishing showers and winds Saturday allowed for the storm snow to begin settling and stabilizing.

Recent Observations

North

No recent observations.

Central

NWAC pro-observer Tom Curtis was at Blewett Pass on Friday and above about 5000 feet reported a thin crust over moist to wet rounded poly crystals with increasing density with depth and no current avalanche problems. Most SW-W slopes had little snow.

South

No recent observations.

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.

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.

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.