Avalanche Forecast
Issued: Dec 27th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs, Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Very dangerous avalanche conditions persist in much of the terrain. Storm, wind or persistent slabs will be sensitive Wednesday. The safest plan is to avoid avalanche terrain of consequence until storm and persistent slabs stabilize.
Summary
Detailed Forecast
Stormy conditions Tuesday night should gradually ease by Wednesday as a brief break in storms occurs through the day Wednesday. Light winds and a lack of significant additional snowfall should allow for a gradual decrease in danger. However, cold temperatures will limit the stabilization Wednesday.
Storm slabs will continue to be sensitive to trigger and widespread Wednesday. The current storm slabs formed over a variety of weak surface conditions, including near surface faceted snow, graupel or possible surface hoar and may be poorly bonded to a slick crust in the Paradise and Snoqualmie areas. Natural or triggered storm slabs may also break down to deeper persistent layers, making larger and more dangerous avalanches possible Wednesday.
The persistent slab problem should remain sensitive to natural or triggered avalanches with the new snow load.
Fresh wind slabs should persist near and above treeline on a variety of lee slopes near ridges and exposed cross loaded features.
The persistent slab problem warrants your attention in the Cascades, especially in areas void of a stout recent crust layer, most notably outside the Paradise, Snoqualmie and Mt Baker area. Heavy recent loading will make this layer more sensitive to trigger where present. Remember that persistent weak layers are generally involved in larger avalanches and cautious route-finding and conservative decision making will be essential for safe travel Wednesday. Continue to exercise caution Wednesday and avoid avalanche terrain of consequence.
Snowpack Discussion
Weather and Snowpack
Strong storms a week ago Sunday and Monday deposited generally 1 to 2 inches of water equivalent along the west slopes through early Tuesday morning. A period of rain or freezing rain (Snoqualmie) 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. The crust is very thin or non existent in the Crystal backcountry.
A front Thursday and upper trough on Friday with low snow levels deposited about 20 inches of snow 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 with fair and cold weather seen on Christmas Day and early Monday, before a strong front moved into the region Monday afternoon.
The strong front moved across the area early Tuesday, followed by heavy snow showers and strong westerly winds through the day Tuesday at low snow levels. The most recent storm totals from Monday morning through Tuesday afternoon have been about 15-30 inches and still snowing Tuesday evening!
Recent Observations
Observations received over the last several days from across the west slopes of the Cascades regarding the 12/17 persistent weak layer (PWL) showed no avalanches or direct signs of deeper instability. However, the 12/17 PWL still shows some propensity for propagation in snowpack tests.
The Alpental pro-patrol on Friday and Saturday reported shallow wind slabs in exposed areas were poorly bonded to the crust formed last week. Elsewhere, the new snow was not cohesive and was sluffing on the crust. Sensitive and shallow wind slab were 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.
Dallas Glass was in the below tree-line band of the Snoqualmie Pass area on Christmas Day and observed 6" (15 cm) of weak snow poorly bonded to the crust, setting up a weak surface snow and slick bed surface combination heading into the upcoming storm cycle, now arriving late Monday. These weak surface snow conditions, and poorly bonded old snow to a smooth underlying crust also exist in the Paradise area, as reported over the past few days by NPS rangers.
Dallas Glass was back in the Crystal area backcountry Monday 12/26 and reports the 12/17 persistent layer is very much in play in that region, buried about 1 foot below the surface. Large column tests continued to show propensity to propagate in multiple tests.
Patrol at Mt Baker ski area Tuesday, 12/27 reported widespread sensitive storm and wind slab releases both in the area during control work and being triggered by the public in the adjacent backcountry. Sounds like there was at least one close call. Storm slabs of 8-12 inches initially Tuesday, became larger as snow loads increased with triggered 2 ft soft slabs reported.
Reports from Alpental patrol Tuesday also reported sensitive soft slabs with good propagation and widespread distribution in mid and upper elevation start zones. These were storm slabs with numerous slides releasing down to the 12/20 crust where a weak bond exists. Sensitive triggered slides ranging up to a foot were reported in the adjacent areas outside the ski area as well. Some of these slides initiated in storm layers about 6 inches down before stepping down to the crust another 6 inches or so.
Problems
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
Expected Size
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 28th, 2016 10:00AM
