Avalanche Forecast

Archived

Apr 13th, 2018–Apr 14th, 2018

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

Snoqualmie Pass.

A major storm has created very dangerous avalanche conditions. Avoid all areas where avalanches may start, run, and stop. We are expecting large natural avalanches to occur Saturday as additional precipitation impacts the area. Some of these avalanches may run downslope and cross common travel routes.

Detailed Forecast

Very dangerous avalanche conditions are expected Saturday. Stay out of all avalanche terrain, including areas where avalanches may start, run, and stop. Avalanches may impact common travel routes that cross avalanche tracks and runouts. Spring weather and snowpacks can be tricky. General rules-of-thumb used during the winter often do not apply. With the amount of recent new snow and more on the way, this is a time to step way back.

Fluctuating temperatures and strong wind during the storm created an “up-side-down” snowpack with heavier strong snow over lighter weaker snow. This is a classic strong over weak slab avalanche scenario. You are likely to find this snowpack structure in all avalanche startzones above the rain line. Storm slabs may extend into the upper below treeline zone depending on air temperatures and precipitation type.

At lower elevations, expect wet surface snow conditions. New rollerballs, fresh fan shaped avalanche debris, and rain on snow are all key signs that loose wet avalanches are likely. Some loose wet avalanches this week have entrained large amounts of old wet snow and traveled father than normal.

In locations receiving less snow, such as in the Crystal backcountry, avalanche danger will be slightly lower. However, do not underestimate these avalanche problems. Although potentially smaller you will be able to trigger wind and storm slabs at higher elevations and loose wet avalanches at lower elevations Saturday.

Other spring time avalanche hazards exists. Glide cracks have formed on some slopes. Glide avalanches are difficult to predict and can release at any time. Limit your exposure to any slope with glide cracks. Creeks are beginning to open up at lower elevations. Choose your travel routes wisely if you are planning to over larger creeks

Snowpack Discussion

Over a foot (30cm) of new snow has fallen in most locations as of Friday afternoon and more is expected overnight. Wind redistributed the new snow at higher elevations created obvious new snow drifts in many areas. At lower elevations, snow transitioned to rain late Friday afternoon. This new snow is falling on a melt-freeze crust formed and buried on 4/11. The strength and thickness of this crust is largely dependent on elevation.

Mild weather earlier this week allowed the upper snowpack to gain strength. Observations show the snowpack is comprised of several melt-freeze crusts, refrozen snow, and strong rounded grains. This has created a strong spring-like snowpack. Avalanches should be limited to the snow above the most recent (4/11) crust layer

Observations

Snoqualmie

On Friday, Alpental ski patrol reported several feet of new snow at the top of Alpental since Sunday afternoon. Winds had transported the recent snow forming uneven snow depths in exposed areas.

NWAC staff in the Alpental Valley Friday experienced the snow transition to rain mid-afternoon and observed several new rollerballs as the new snow became loose and wet.

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.

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.

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.