Avalanche Forecast

Issued: Feb 22nd, 2017 10:00AM

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

Lingering but isolated wind slabs, will continue to heal, becoming less sensitive to human triggering. Avoid fresh cornices and watch for any wet snow conditions developing during extended sun breaks. 

Summary

Detailed Forecast

Thursday should remain cool with sunbreaks and scattered snow showers, mainly in the afternoon. Very light new snow is expected, if any. 

Lingering wind slabs should continue to heal and become less sensitive to human triggering Thursday. Wind slabs should be isolated and confined to specific terrain features, more likely on N-E-S aspects near and especially above treeline. 

Storm slabs will not be listed as an avalanche problem Thursday as they continue to strengthen and bond. 

Loose wet avalanches will not be indicated as an avalanche problem, but watch for wet surface snow if you find yourself on sun exposed slopes during extended sunbreaks.

Snowpack Discussion

Weather and Snowpack

The latest of several warm, wet SW streams of moisture this season arrived on Valentines Day 2/14 bringing another round of heavy rain, avalanches, crusts and consolidation through Thursday 2/16 along the Cascade west slope zones. Minor new snow amounts were deposited at the tail end of the storm.

A short period of fair weather on Friday, 2/17 caused another surface crust, especially on solar aspects. Light amounts of new snow accumulated over the weekend, except at Mt. Baker, where about a foot of new snow fell by early Monday morning along with moderate SW winds Sunday night.

A storm that tracked across the southern Washington Cascades Monday, deposited up to a foot of snow in the Paradise/Crystal/White Pass areas with lesser amounts further north. Winds that were easterly initially, switched to westerly Monday afternoon and were the strongest in the South Washington Cascades.

A mix of sun and light snow showers Tuesday and Wednesday freshened the surface with a few inches of snow at cool temperatures and generally light winds.

Recent Observations

North

The Baker area had an eventful Monday with pro-patrol reporting a natural avalanche cycle on Shuskan Arm Sunday night. Widespread and sensitive storm slabs 6-10" deep released during control work Monday morning.  A 6" storm slab was skier triggered on a steep north aspect of Table mountain, carrying the skier 100 or more feet. The skier was partially buried and suffered a knee injury. NWAC observer Lee Lazzara was in the Mt. Herman-Artist Point area Monday and found 15 cm (6") of denser new snow poorly bonded to less dense snow from earlier in storm cycle. In wind loaded areas, the slab was up to 60 cm (2') deep.  

Central

On Tuesday, Alpental pro-patrol reported widespread 6" storm slabs, occasionally up to 10" in isolated locations during control work with explosives. The slabs were soft and generally didn't run far. Shallow loose avalanches were becoming touchy by midday with a slight rise in temperatures even with cloudy skies.

South

Conditions were quite touchy in the Crystal backcountry Monday. Several reports from East Peak indicated shallow natural and skier triggered, loose and storm slab avalanches in the near treeline and the upper portion of the below treeline band. One report indicated a slab near the ridgeline of East Peak on a Westerly aspect had released down to the Valentine's Day crust. 

NWAC pro-observer Ian Nicholson was at Paradise on Monday and observed only minor wind transport of new snow. Ian also reported storm slab instabilities were a bit less touchy than in the Crystal area. 

Backcountry reports from professionals in the Crystal area on Tuesday indicated less sensitive avalanche conditions overall, but storm instabilities were still reactive to ski triggering about 20 cm (8") down on steep slopes. 

Small natural slab avalanche, East Peak, Crystal Mountain backcountry. Shane Robinson, 2-20-17

NWAC forecasters, Garth and Dennis returned to the East Peak area Wednesday, 2/22 finding much improved conditions. Storm layers were still producing moderate test results, but not indicating propagation. Cornice drops onto previously loaded east facing slopes below ridges produced small, isolated slabs, arresting in place. Wind slabs were gaining strength and mainly confined to isolated terrain features below ridges, near and above treeline.

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 23rd, 2017 10:00AM