Avalanche Forecast

Archived

Apr 8th, 2017–Apr 9th, 2017

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

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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Dangerous avalanche conditions are expected near and above treeline in the central west, passes and southwest zones on Sunday. Recent storm snow will become wet and unstable with direct sun and warming, late morning and afternoon. Small loose wet slides may entrain significant snow in steeper sun exposed terrain. Wind slabs will stabilize, but due to potentially large recent wind slabs, extra caution is advised.

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.

Other avalanche problems not listed that may be encountered on Sunday include:

Storm slab in more sheltered areas above treeline that experienced rapidly accumulating snowfall for more than a few hours.

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 was seen in the Cascades Monday and Tuesday. Reports generally indicated 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 Cascade west slopes. Snow levels were generally between 4500-5000 feet in the northwest Cascades and 5000-6000 feet in the central-west and southwest Cascades. For the 3 days ending Friday morning NWAC stations along the Cascade west slopes had about 1-1.5 inches of WE except with about 3.4 inches of WE at Mt Baker. This WE 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 8-12 inches in the Crystal and Paradise areas and significantly less in the passes by early Saturday with very strong SE-SW winds Friday afternoon and evening. Winds averaged 40-60 with gusts of 80-100 at NWAC sites around the Mt Rainier area late Friday! 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

NWAC pro-observer Lee Lazzara was at Little Devil Peak in North Cascades on Monday 4/3. Near and above treeline Lee found about 4 inches of recent storm snow over the most recent crust, and wind slabs averaging about 1 foot on N-E aspects below ridges. Snowpack tests indicated triggering of wind slabs to be stubborn. There was evidence of recent large cornice failures. The recent strong rain crust dominated the snowpack below treeline with shallow wet snow conditions during the afternoon. Shallow cold snow was preserved on steep shaded terrain near and above treeline.

The Mt. Baker pro-patrol reported easily ski triggered loose wet avalanches Thursday morning peeling off the most recent 4-6" inches above 4500 ft and running well. Below 4000 feet, the random snow pillow collapse would trigger natural loose wet avalanches due to above freezing temperatures and continuing rain.

Saturday morning, the Mt Baker patrol reported recent wind slabs were more isolated in the terrain and had formed lower down below ridges due to the strong transporting winds Friday.

Central

The Alpental pro-patrol in a pit on the upper mountain reported alternating layers of stable crusts and wet snow in the upper snow pack on Friday. At the base they found gradually increasing density with depth of large wet grains of wet snow.

A report via the NWAC Observations page for Friday indicated easily triggered, loose wet avalanches on Tonga Ridge, west of Stevens Pass on Friday.

South

No recent observations.

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.

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.