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Archived

Avalanche Forecast

Mar 22nd, 2018–Mar 23rd, 2018
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
1: Low
The avalanche danger rating at treeline will be low
Below Treeline
1: Low
The avalanche danger rating below treeline will be low

Regions: Olympics.

The avalanche danger will gradually decrease Friday. Wind Slabs formed Thursday will slowly heal and stabilize. However, it will be possible to trigger a shallow Wind Slab on steep lee slopes, mainly below ridges or convexities on steep slopes. Fresh cornices have formed along ridges and if dropped may trigger a slide on slopes below.

Detailed Forecast

Fresh wind slabs will slowly heal Friday under cool temperatures and diminishing winds. Sensitive wind slabs of up to 1' were noted Thursday on lee slopes below ridges, mainly NW-NE facing slopes. 

It will still be possible to trigger a Wind Slab on steep lee slopes, especially below ridges and rollovers or cross loaded features. Give terrain suspected of wind deposited snow extra caution and choose lower angled slopes away from ridges.

Wind slabs will be most likely on shaded slopes, mainly northerly facing at higher elevations.

Give fresh cornices a wide margin of safety and avoid travel on steep open slopes below cornices.  

Snowpack Discussion

As of Thursday afternoon, about 4-6" of new snow had accumulated in the Hurricane Ridge area. The new snow was easily transported by moderate to strong southerly winds, building sensitive Wind Slabs on lee slopes near and above treeline.

Several previous days of sunny, spring-like weather created a strong melt-freeze crust on sun exposed slopes and a weaker, thinner crust on shaded slopes, now below the 6 plus inches of new snow. 

Several layers exist within the snowpack with the potentially most problematic found on east aspects in mid-elevations where faceted crystals were seen on a crust layer buried about 1 foot below the surface.   

Observations

NWAC observer and NPS rangers on Thursday, 3/22 found building Wind Slabs up to 1 ft on lee slopes mainly above 5000 feet on northerly facing terrain. Fresh cornices were building as well with cornice drops triggering small slabs mostly 6-8". Shallow Wind Slabs were triggered on some steep rollovers . On windward slopes, the new snow was stripped to a firmer hardening crust.

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, West, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1