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, especially where poorly bonded to an underlying crust. 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 and above normal caution is still advised.
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. However, wind slab may be locally more sensitive in areas it has poorly bonded to an underlying crust.
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.
Storm slabs that were reactive in the Mt. Baker area Friday should have settled out by Sunday and storm slabs will not be listed as an avalanche problem.
The surface crust formed last week is strong and hard enough in some areas of the west slopes to present an out of control fall danger. Avoid steep slopes where the slide for life hazard is present.
Snowpack Discussion
Weather and Snowpack
Strong westerly flow directed two Pacific frontal systems across the Northwest last Sunday night and again Monday night (12/18-12/19) with generally 1 to 2 inches of water accumulating along the west slopes through early Tuesday morning. A period of rain during this storm cycle allowed crust layers to form in the Baker area from 4000-4500 feet, the Passes up to around 5000 feet (Stevens) to 6000 feet (Snoqualmie) and 6000-7000 feet in the Paradise area. In most areas the crust is strong enough to support a skier's weight.
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 20 inches of snow accumulated at Mt. Baker over this period with about 2-8 inches accumulating elsewhere along the west slopes including the Passes.
Scattered snow showers, sunbreaks and generally light winds summed up the weather on Saturday.
Recent Observations
Several worthwhile observations are available for the west slopes from Wednesday through Thursday, 12/21-12/22 ,via the NWAC Observations tab. In summary no avalanches or direct signs of deeper instability were reported, however the 12/17 PWL showed some propensity for propagation in snowpack tests.
NWAC pro-observer Lee Lazzara was out in heavy snowfall on south slopes on Mt Herman near Mt Baker on Friday in the 4-5000 foot range and reported widespread reactive 15-30 cm storm slab releasing on a recent storm interface. Lee did not find the December 17th PWL in a pit on a north slope. The 12/17 layer was identified but unreactive in snowpack tests at about 70 cm on a south slope.
The Alpental pro-patrol on Friday and Saturday reported shallow wind slab on wind loaded areas which was poorly bonded to the crust formed last week. Elsewhere new snow was not cohesive and was sluffing on the crust. Sensitive and shallow wind slab was also reported in the Silver Basin area of Crystal Friday.
Pro-observer Ian Nicholson was at Mt. Snoqualmie Saturday morning. Ian identified shallow wind slab as his greatest potential concern on steeper wind loaded slopes where it had bonded poorly to a graupel layer overlying the uppermost crust. The 12/17 PWL was showing signs of propagation in snowpit tests, but was requiring more load and showed signs of rounding versus earlier in the week.
Crusts in the Snoqualmie Pass and Paradise area have been reported as especially stout with ski crampons helpful on steeper slopes.
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
