Avalanche Forecast

Issued: Dec 18th, 2016 10:29AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Travel in avalanche terrain is not recommended along the west slopes on Monday. 

Summary

Detailed Forecast

Strong west flow will carry a strong cold front across the Northwest Sunday night and Monday morning. This should cause strong alpine winds with moderate to heavy snow along the west slopes and a warming trend. The warming trend should contribute to the formation of both new wind slab and new storm slab layers.

New wind slab is very likely along the west slopes on Monday. Wind transport will deepen these layers mainly on lee north to southeast slopes but wind slab is possible on other aspects.

New storm slab is also very likely along the west slopes on Monday on any slopes that rapidly accumulate new snow of more than a several inches.

Travel in avalanche terrain is not recommended along the west slopes on Monday. 

The December 8th layer continues to round and gain strength and appears to become nonreactive along the west slopes. The upcoming storm cycle should be will be a good test to see if this layer can be put to rest for the winter.

Another strong front should be seen Monday night and Tuesday. This should maintain or increase the avalanche danger in the Olympics and Cascades.

Snowpack Discussion

Weather and Snowpack

The last storm cycle to affect the Olympics and most of the Washington Cascades was about Thursday, December 8th to Monday morning, December 12th. About 2-4 feet of snowfall was seen at NWAC stations along long the west slopes.

There has been a lot of snowpack settlement since the last storm cycle ended, allowing this underlying snow to mostly stabilize.

Clear or fair and cold weather has been seen about Wednesday, December 14th to Sunday, December 18th . This caused widespread surface hoar and near surface faceted snow to develop in the Cascades. Thin sun crusts have also formed on steeper solar slopes. These layers are expected to act as widespread weak layers or sliding surfaces for the upcoming moderate to heavy snowfall.

Recent Observations

NWAC pro-observer Lee Lazzara was in the Yellow Aster Butte area near Twin Lakes near Mt Baker Friday. Strong NE ridge level winds were transporting major surface snow and building sensitive fresh wind slabs on atypical SE-NW facing terrain. These new wind slabs caused a plan change to safer lower elevation terrain. Even so, where small areas of wind slab were encountered, they were sensitive to trigger. Lee also noted new surface hoar and near surface faceted snow on a sun crust on solar slopes. He found that the December 8th persistent weak layer was no longer significant and not showing signs of propagation.

On a different note NWAC pro-observer Jeff Ward was at Stevens Pass on Friday and found light winds and generally stable snow. Isolated wind slab was stabilizing. However there was also widespread large surface hoar.

NWAC pro-observer Ian Nicholson was at Paradise on Saturday and found a mix of surface hoar and faceted surface snow that was reactive in shovel tilt tests. Past wind loading was noted on some west aspects. The December 8th persistent weak layer crystals seen at 52 cm were becoming rounded and were not reactive.

The Snoqualmie DOT crew has noted lots of surface hoar the past few mornings including Sunday morning.

NWAC pro-observer Simon Trautman was on Mt Herman on Sunday and found a widespread weak surface snow sandwich that included faceted snow and a crust. These layers were giving highly reactive cracking and natural and triggered slab avalanches on steep slopes.

Weak layers including faceted snow and a crust seen at Mt Herman on Sunday. Photo by Simon Trautman.

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.

Elevations: Alpine, Treeline.

Likelihood

Very Likely

Expected Size

1 - 1

Storm Slabs

An icon showing Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very 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

Unlikely

Expected Size

1 - 1

Valid until: Dec 19th, 2016 10:29AM