Avalanche Forecast

Issued: Dec 16th, 2016 10:20AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Caution, areas of new wind deposited snow are possible Saturday on unusual aspects, mainly near or above treeline. Watch for wind stiffened surface snow on a variety of aspects and avoid steep terrain with evidence of wind loading. 

Summary

Detailed Forecast

A weak disturbance will move across the region Saturday, causing moderate to strong NW ridge level winds may build new fresh wind slabs on lee slopes, near and especially above treeline. 

Light snow showers Saturday may bury intact a variety of weak surface snow types formed this week under fair cold weather.

Local wind slabs from the recent E-NE winds are likely Saturday. This should be mainly on westerly to southeasterly aspects near ridges.

Watch for firmer wind transported snow on all aspects, mainly near and especially above treeline.

The December 8th layer continues to round and gain strength and should be less sensitive to trigger.  Do head for lower angle slopes and ratchet back your plans if you experience collapsing or whumpfing or have evidence of this layer such as from snowpits.

Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.

Snowpack Discussion

Weather and Snowpack

The last storm cycle to affect the central and south WA Cascades was from late Thursday, December 8th through Monday morning, December 12th. About 2-4 feet of snow were recorded at the NWAC stations in these zones ending Monday morning Dec. 12th. 

Fair and cold weather this week has been interspersed with periods of moderate N-NE-E winds both Wednesday and again Friday. These winds were strong enough to build fresh wind slabs on more atypical SW-NW facing terrain, mainly near and especially above tree line.

There has been about 8-12 inches of snowpack settlement since the storm cycle ended, allowing underlying or storm snow related weak layers to stabilize. 

Several clear nights with light winds this week have allowed for weak surface snow development. On wind and sun sheltered terrain, extensive near surface faceted snow or surface hoar have formed this week and this may be an important weak layer if buried intact by the expected return of light snowfall Saturday.

A thin sun crust is likely to have formed on steeper southerly facing slopes, which also may be a future interface for avalanches when loaded with sufficient wind transported or future storm snow.

Recent Observations

A regional avalanche cycle was seen December 8th-11th especially along the west slopes with some avalanches releasing on the December 8th persistent weak layer in the Washington Cascades. 

Since Monday, in areas not experiencing strong east winds Wednesday, there has been significantly less avalanche activity throughout the region as storm layers continue to stabilize.   

A skier triggered slab avalanche reported Tuesday, released on the December 8th persistent weak layer in the Highland Bowl back country next to the Steven Pass ski area. This slope may have previously released during the storm cycle and reloaded with new wind deposited snow. The Stevens pro-patrol reports it was on a steep SSW slope at 5400 feet with a 40 cm x 100 foot crown releasing on a layer buried surface hoar and facets on a crust.  See recent post on NWAC Instagram.

In the Crystal Mountain area Wednesday, evidence of significant transport and active wind slab formation was ongoing due to strong east winds. Similar conditions were noted by NWAC pro observers training on Mt Rainier Wednesday. 

A member of the Stevens Pass pro-patrol reported wind transport on Rock Mountain and especially in the Chiwaukums on Wednesday.

NWAC pro observer in the Snoqualmie Pass area Thursday, saw evidence of recent wind slab development on atypical westerly facing terrain near and above treeline and avoided travel on this terrain to test for sensitivity.  

There are a variety of current snow surface types awaiting our next storm snow, including surface hoar, near surface faceted snow, sun crusts, wind sculpted snow, and low density snow in protected areas.   

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

Possible

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

Unlikely

Expected Size

1 - 1

Valid until: Dec 17th, 2016 10:20AM