Avalanche Forecast

Issued: Apr 10th, 2018 11:11AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Light snow and moderate winds will create heightened avalanche danger on wind loaded slopes near and above treeline Wednesday. Use visual clues such as fresh cornices, wind drifted pillows and cracks in the snow all indicating that you could trigger a Wind Slab avalanche. Small Loose Wet avalanches are possible below treeline in isolated areas. 

Summary

Detailed Forecast

Light snow and moderate winds will create heightened avalanche danger on wind loaded slopes near and above treeline Wednesday. Use visual clues such as fresh cornices, wind drifted pillows and cracks in the snow all indicating that you could trigger a Wind Slab avalanche. 

Below treeline you may still trigger Loose Wet avalanches or they may be triggered by additional rainfall. Watch for signs of wet snow conditions such as sluffing, roller-balling and any natural Loose Wet avalanches as signs of an increasing hazard. Avoid terrain traps if you suspect you could trigger a Loose Wet avalanche. 

On northerly aspects near and above treeline, continue to look for and test persistent weak layers within the upper two feet of the snowpack. A refreezing upper snowpack should make lingering persistent weak layers more difficult to trigger but use caution and evaluate the snowpack especially on large open north facing slopes near and above treeline.

Snowpack Discussion

A front brought light snow above 6000 ft Tuesday along with moderate winds near and above treeline. Warm and dry conditions were seen on Monday. The dry weather followed the strong weekend storm which initially pushed rain to high elevations Saturday even along the east slopes of the Cascades. Cooling and new storm snow followed through Sunday afternoon. Limited observations suggest 6-12 inches accumulated above 5000 ft for areas near the Cascade crest. Wet snow avalanches likely occurred earlier in the storm and continued at lower elevations through the weekend. New Wind and Storm Slabs likely developed at higher elevations later in the weekend with fresh cornices developing along ridgelines.   

Moist or wet snow in the upper snowpack should begin to refreeze near and above treeline Wednesday as colder air arrives. 

Older layers of weak snow may still be found in the snowpack. We have one recent observation from the East Slopes - Central zone that identified buried surface hoar 2 feet below the surface at 6800 feet and still reactive in snowpack tests. If you have any information or observations on layers within the snowpack please consider submitting them to NWAC via our public observations page. The exact weak layer and depth depends on your location. Common weak layers found over the last month: 

  • 3/25 and 3/22: Buried surface hoar layers found on shaded aspects near and above treeline.
  • 2/13 Facets above a firm and thick melt-freeze crust can be found in many locations near and above treeline. This layer is thought to be dormant at this time but may re-awaken with a substantial weather input.

Lower elevations further east of the crest have melted back for the season and no longer present an avalanche risk. 

Observations

North

No recent observations. 

Central

On Monday, a public observation from the Entiat River drainage reported wet snow in the upper 16 inches (40 cm)  of the snowpack on a NE aspect at 6800 ft. Below this, colder drier snow was found with buried surface hoar 2 feet down (60 cm). This layer was reactive in snowpack tests, suggesting propagation was possible. The 2/8 crust was 3 feet (1 m) below the snow surface and breaking down. S through E aspects had a more springlike and consolidated snowpack. 

On Friday 4/6, professional observer Matt Primomo traveled up the Icicle to above Colchuck Lake. Matt found a moist and well settled snowpack below 5800' with no persistent weak layers. The 2/8 crust was 1 m down.  At 6700' on a NNE aspect, Matt found preserved stellars 14 in (35 cm) down and showing a likelihood to propagate in snowpack tests. Dry snow was found at this elevation below the top 8 " (20 cm). Higher in the terrain, recent wind slab avalanches likely released on this layer. Matt also observed natural loose wet avalanches and one wet slab avalanche above treeline on a SE aspect.  

Problems

Wind Slabs

An icon showing Wind Slabs

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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Apr 11th, 2018 11:11AM