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Archived

Avalanche Forecast

Jan 31st, 2016–Feb 1st, 2016
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low

Regions: Olympics.

The greatest avalanche problem should be wind slab on lee slopes near and above treeline mainly NW-N-SE facing slopes. Watch for signs of firmer wind transported snow.

Detailed Forecast

Light winds, mostly cloudy weather, a few light snow showers and cool temperatures should be seen in the Olympics on Monday.

This weather should bring a further gradually decreasing avalanche danger on Monday.

The greatest avalanche problem should be wind slab on lee slopes near and above treeline mainly NW-N-SE facing slopes. Watch for signs of firmer wind transported snow.

Watch for storm slab in areas that had rapidly accumulating snowfall. Storm layers that may exist above the new crust will slowly settle but could remain reactive to human triggers mainly near and above treeline.

Continue to evaluate snow and terrain carefully on Monday.

Avoid ridges where cornices may be present and slopes below cornices. Cornices can always be sensitive to human triggers.

Snowpack Discussion

Heavy rain fell in the Olympics Wednesday-Thursday. Over 2 inches of rain was recorded at the NWAC station at Hurricane Ridge by Thursday midday. This rain event should give the snowpack a new horizon going forward.

The "20th of June" path released as a wet slab during rain and warmth last Thursday. Photo taken January 29th by Matt Schonwald.

A strong occluded front with strong winds crossed the Northwest on Friday. The Hurricane rangers reported about 14 inches of new snow for the 2 days ending Saturday morning.

NWAC pro-observer Matt Schonwald visited Hurricane on Friday and found rapidly building wind slab and extensive loading on lee slopes in all 3 elevation bands. New cornices were sensitive to trigger but still relatively small. The bonding of the new snow to the forming crust was strong but there was a thin layer of low density stellar crystals above the crust, forming the weak layer for newly forming wind slabs to fail.

In the below treeline zone, a strong crust with shallow new snow will cause less danger. 

Avalanche Problems

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: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1

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 lips 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.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1