Avalanche Forecast

Issued: Dec 23rd, 2016 12:23PM

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

Recent or new wind slab is expected to still be the main avalanche problem along the east slopes on Saturday. There is still uncertainty regarding the December 17th PWL and skiing or riding on lower angle slopes is the safest bet until there is more certainty that this layer is no longer a problem.

Summary

Detailed Forecast

An upper trough will exit the Northwest on Saturday and high pressure will begin to build offshore. Light snow showers mainly along the west slopes Saturday morning should give way to partial clearing Saturday afternoon with light west to northwest winds and cool temperatures.

Little change is likely along the east slopes on Saturday.

Wind slab should be possible in areas exposed to recent south to west winds. Watch for firmer wind transported snow mainly on northwest to southeast slopes.

The latest tests of the December 17th PWL in the Cascades don't seem to indicate a regionally reactive layer. There is still some uncertainty regarding this layer. Snow pits valid for slopes you intend to ski or ride may give some indication of the presence and reactivity of this layer. But skiing or riding on lower angle slopes is the safest bet until there is more certainty that this layer is no longer a problem. While triggering this layer seems unlikely  remember that PWL's generally cause larger avalanches.

Snowpack Discussion

Weather and Snowpack

Strong westerly flow directed two Pacific frontal systems across the Northwest Sunday night and again Monday night with generally half to 1 inch of water accumulating along the east slopes through early Tuesday morning. Storm totals generally ranged from 6 - 12 inches along the east slopes during this cycle. A brief warming trend peaked mid-morning for many east slope stations before a sharp cooling trend ensued by mid-day.

Westerly winds were especially strong with the 2nd system late Monday night and into Tuesday with gusty winds mixing down into usually more wind sheltered terrain. West winds at Mission Ridge gusted over 100 mph for several consecutive hours Tuesday afternoon!

A fair day was seen on Wednesday.

Another front crossed the Northwest on Thursday followed by an upper trough on Friday. This has caused light amounts of new snow along the east slopes. South to west winds along the crest Thursday and Friday should become light by Friday night with a cooling trend Friday.

Recent Observations

NWAC pro-observer Jeff Ward was in the Icicle Creek area up to about 6300 feet on Wednesday and saw evidence of a widespread natural wind slab avalanche cycle during the last storm, with one very large crown seen on a north aspect. The December 17th PWL was found at 15-30 cm below the surface on W to N to E slopes. The layer was unreactive both in large column snowpack tests, ski tests and cornice drops.

The NCMG were on Delancy Ridge Wednesday and Thursday. By Thursday they were finding moderate to hard shears at the interface of the recent storm snow and the previous or faceted surface. While wind slab was not showing signs of propagation it remains the main concern in the ATL.

A somewhat different story was reported Wednesday and Thursday by the Mission Ridge pro-patrol. On Wednesday avalanche control produced 1.5-3 foot hard slab avalanches in 3 paths. These avalanches were releasing on basal facets about 15 cm from the ground. On Thursday on W-N-E slopes at 6500 feet in snow pits they continue to find hard slab layers giving hard compression tests with moderate quality shears in facets about 15 cm from the ground with about 120 cm total snow. Some stabilizing may have occurred by Friday.

A report via the NCMG for the north side of Delancy Ridge on Thursday indicated previous strong wind transport but no current instability and generally strong right side up snowpack.

The NCMG on Friday at Washington Pass had some planar hand shears in wind affected snow but the only instability was small loose dry avalanches in steep rocky terrain.

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, North West.

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

Possible

Expected Size

1 - 1

Valid until: Dec 24th, 2016 12:23PM