Avalanche Forecast

Issued: Feb 24th, 2018 10:55AM

The alpine rating is high, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Wind Slabs and Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New snow and wind are creating dangerous avalanche conditions. Avoid steep slopes were the wind is depositing snow near and above treeline. It may now be easier to trigger a dangerous Persistent Slab avalanche. Stay away from large open slopes steeper than 35 degrees that are suspect for these avalanches.

Summary

Detailed Forecast

New winds slabs will form on lee aspects. Look for signs of wind transporting snow onto nearby slopes such as blowing snow, fresh cornices, and snow drifts. Wind loading may occur near ridgeline and by cross-loading terrain features mid-slope. Avoid steep slopes where the winds deposited thick pillows of snow or obvious wind features. Shallow slabs and loose surface snow may be found in sheltered terrain. Use caution in terrain where even small avalanches may have large consequences.

The snowpack structure continues to support potential for Persistent Slab avalanches. Any freshly fallen or drifted snow is adding to the slabs. Slopes that previously lacked cohesive snow over the facets, may now have developed slabs. Weak sugary facets can be found 1 to 2 feet below the snow surface and above a firm crust. This is the perfect depth for human triggered avalanches. Two skier triggered Persistent Slabs have occurred this past week in the Washington Pass area. It may now be easier for you to trigger these avalanches than prior to this storm. Continue to stay off of large open slopes where you could trigger a Persistent Slab avalanche. Collapsing and cracking are obvious signs that you could trigger a Persistent Slab. Use extra caution if you observe these signs of instability. 

 

Snowpack Discussion

A round of moderate snow and strong wind will make it's way over the crest to the East Slopes. Wind Slabs will add to the existing wind distributed slabs that formed late in the week. Small isolated wind slab avalanches were reported in the Washington Pass area Thursday and Friday. Winds continued to transport snow in some locations as winds increase Friday afternoon.

In sheltered areas loose surface snow exists. About 1 to 2 feet of generally soft snow has been reported over an old (2/5) crust from early February. Weak sugary facets can be found above this crust in some locations. This persistent weak layer has failed in two reported skier triggered avalanches over the last week.

While several layers exist in the snowpack, there are no significant layers of concern below the 2/5 crust.

Observations

North

Avalanche professionals in the Washington Pass area Thursday through Saturday reported small wind slabs isolated to ridgelines, with several small skier triggered wind slabs observed. Collapsing was reported on Saturday.

The most recent persistent slab avalanche occurred on Wednesday near Harts Pass. The avalanche was about two feet deep and 90 feet wide. It occurred on a NE aspect at 6000’.

Central

NWAC field staff traveled in the Icicle Creek area Wednesday on NE facing slopes up to about 6000 feet. The persistent layer we are tracking was found consistently buried about 2 ft from the surface, though the distribution of this layer was intermittent. In many areas the snow above the facet/crust layer was low cohesion powder, lacking the ability to propagate a slab avalanche. Shallow fresh wind slabs were noted 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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Persistent Slabs

An icon showing Persistent Slabs

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 25th, 2018 10:55AM