Avalanche Forecast
Issued: Dec 24th, 2016 10:13AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Wind slab may be still be sensitive on lee slopes mainly near and above treeline. While the 12/17 PWL is gaining strength and appears increasingly unlikely to be human triggered, remember that persistent weak layers are generally involved in larger avalanches. Of particular concern is the low-likelihood/high consequence threat of deep persistent slabs in the Mission Ridge area consisting of hard slabs failing on weak layers near the ground - a deadly combination. We recommend a very cautious approach with conservative terrain selection in the Mission Ridge area.
Summary
Detailed Forecast
Christmas should be cold and mostly sunny with generally light winds across the Cascade range.
Shallow wind slab formed Thursday and Friday has become less likely to trigger.
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 but we are gaining confidence that it is less of an issue and have moved the likelihood down a notch to Unlikely.
However, since this layer is still showing the ability to propagate in snowpack tests throughout much of the range, we still advise observing the snowpack structure in your local area and skiing or riding on lower angled slopes until there is more certainty that this layer is no longer a problem. While triggering this layer seems unlikely, remember that persistent weak layers are generally involved in larger avalanches.
Of particular concern is the low-likelihood/high consequence threat of deep persistent slabs in the Mission Ridge area consisting of hard slabs failing on weak layers near the ground - a deadly combination. The avalanche danger for the east slopes will be rated as Moderate for Christmas Day but we recommend a very cautious approach with conservative terrain selection in the Mission Ridge area.
Snowpack Discussion
Weather and Snowpack
Strong westerly flow directed two Pacific frontal systems across the Northwest Sunday night and again Monday night (12/18-12/19) 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- Tuesday 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.
After a fair weather period midweek another front crossed the Northwest on Thursday followed by an upper trough on Friday with low snow levels. About 1-5 inches of snow accumulated along the east slopes.
Scattered snow showers were seen mainly near the crest with ample sunbreaks and generally light winds on Saturday.
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.
A report via the North Cascade Mountain Guides from the north side of Delancy Ridge on Thursday indicated previous strong wind transport but no current signs of instability and a generally right side up snowpack.
The NCMG on Friday and Saturday at Washington Pass had some planar hand shears in wind affected snow but the only instability directly noted was small loose dry avalanches in steep rocky terrain. The 12/17 interface was found to be unreactive in several snowpack tests.
NWAC observer Tom Curtis was on DirtyFace Peak near Lake Wenatchee and found the 12/17 PWL 15-25 cm down but not propagating in snowpack tests on N-E-SE aspects between 4000-5500 feet. Tom also found shallow and stubborn wind slab in the near treeline band.
A different story continues to evolve in the Mission Ridge area. On Wednesday avalanche mitigation produced 1.5 -3 ft hard slab avalanches in 3 separate paths! These avalanches were releasing on basal facets about 15 cm from the ground. On Thursday, snowpits on W-N-E slopes at 6500 ft continued to show hard slab layers giving hard compression tests with moderate quality shears on facets about 15 cm from the ground with about 120 cm (4 ft) of total snow. On Saturday, a backcountry ski tourer in the Lake Clara area near Mission Ridge reported a huge whumpfing noise, likely indicating a collapse of the basal facets. While no avalanche occurred, the terrain where the collapse occurred connected to a large avalanche path that was NE facing near treeline. While deep persistent slabs in this area are unlikely to trigger, they warrant a high level of caution before venturing near or into avalanche terrain.
Problems
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
Expected Size
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
Expected Size
Valid until: Dec 25th, 2016 10:13AM
