Avalanche Forecast

Issued: Feb 22nd, 2018 11:32AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New shallow wind slabs will form during the day Friday as snow and wind impact the Cascades. Use visual clues to identify and avoid slopes where the wind is depositing blowing snow. Persistent slab conditions have not changed. Stay away from large steep open slopes where these large dangerous avalanches may occur.

Summary

Detailed Forecast

New shallow wind slabs will develop throughout the day Friday on wind loaded slopes near and above treeline. These avalanches will grow larger and become easier to trigger as the day goes on. Use visual clues such as blowing snow, plumes, snow drifts, and fresh cornices to identify and avoid slopes where wind is depositing snow. With plenty of snow available for transportation, wind slabs may grow large in the afternoon.

Conditions will not allow for much, if any change in the persistent slab problem. You are most likely to trigger this weak faceted layer on large open slopes greater than 35 degrees. Other avalanches may also step-down and trigger this weak layer. Stay away from large open terrain where these large persistent slabs may occur. This avalanche problem is not going away. Continue to give yourself wide safety buffer to handle the uncertainty of this difficult problem.

Excellent travel conditions can still be found in many locations. Lower angled slopes, treed terrain, areas away from larger avalanche paths can provide more conservative terrain options.

Snowpack Discussion

Following last weekend's storm cycle, the upper snowpack consists of 2-4 feet of soft settled snow. In many locations this snow sits on top of weak sugary facets. These facets can be easily located just above the most recent (2/8) crust layer. Several avalanches, collapses, and whumphs have been triggered on this persistent weak layer. Snowpack test and observations from around the region continue to demonstrate that this layer can fail and produce avalanches.

Loose surface snow conditions have been reported in most areas. Small loose dry avalanches have been observed in very steep terrain.

A thin rain crust formed and was buried on 2/17 near and below 4000’ in the Snoqualmie area. Limited information about this snowpack layer has been reported other than it is ski-supportable. 

Below the 2/8 crust there are no significant layers of concern.

Observations

Stevens and Snoqualmie Pass

An avalanche professional at Stevens Pass reported snowpack tests Wednesday indicating the weak facet layer could fail and produce avalanches. These were the most recent of numerous snowpack tests conducted over the last several days. All observations supported the same conclusion.

Several large persistent slab avalanches were triggered Saturday through Monday in both the Snoqualmie and Stevens Pass backcountry (Stevens Pass-Hollywood Bowl-2-19). These avalanches failed on facets just above the 2/5 crust.

South

On Friday NPS rangers in the Paradise area reported 3-4 feet of snow over weak facets. No wind transported snow was observed.

NWAC Forecaster Dallas Glass observed a large audible whumph caused by a collapsing weak layer Tuesday near Paradise. Snowpack tests in the area indicated the persistent weak layer could fail and produce avalanches up to 4 feet deep.

No recent observations have been received from the Crystal backcountry. A higher level of uncertainty exists in this area.

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

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Feb 23rd, 2018 11:32AM