Avalanche Forecast
Issued: Dec 14th, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Pay extra attention if you see winds transporting snow and find more that a few inches of new snowfall on Tuesday.
Summary
Detailed Forecast
A weak cold front will cross the Northwest on Tuesday. This should bring northwest winds and mostly light snow with a brief, slight rise in the low snow levels.
New small shallow areas of wind and storm slab are possible mainly where the main snowfall is expected in the central Cascades. Wind slab might be found on more south aspects due to northwest winds. Pay extra attention if you see winds transporting snow and find more than a few inches of snowfall.
Previous wind slab layers from strong winds Saturday should further slightly stabilize but could still be triggered on previous lee slopes.
The PWL is getting less likely to trigger in the northeast and central east zones but we will wait for more information before dropping this avalanche problem. Watch for clues to turn around like whumpfing as evidence that the PWL is a problem especially in the near and below treeline zones.
Snowpack Discussion
We have varied conditions along the Cascade east slopes. The forecast discussion is the same for the east slopes but the danger rating and problem varies for a potential persistent weak layer (PWL).
An atmospheric river and warm very wet weather was seen last week. This caused more snowfall in the northeast zone, rain in the southeast zone, with a transition in between in the central east zone.
A stormy pattern with cooling was seen late last week and over the weekend with about 1/2-2 feet of snowfall along the east slopes.
A concern is the extent along the east slopes of the PWL from the cold weather at the end of November. On Wednesday NWAC observer Tom Curtis was at Blewett Pass and found sudden collapse test results in faceted snow 15 cm from the ground. On Thursday Tom found similar sudden collapse results due to buried surface hoar at about 50 cm down at 5100 feet on the north side of Mt Cashmere. Tom also experienced whumpfing and heard a natural avalanche.
Further east in the central east zone, the Mission Ridge ski area did not find evidence of a lingering PWL during control on Friday. Tom Curtis and fellow NWAC observer Ian Nicholson visited Jove Peak east of Stevens Pass on Saturday and did not find a PWL up to 5000 feet on south to west aspects.
The PWL is most likely to linger in the northeast and north central zones. The southeast zone should have a much shallower snowpack more affected by recent rain and warm temperatures. However we have no recent observations from the southeast zone.
A report from Delancy Ridge in the northeast zone on Saturday to near treeline indicated storm snow was well bonded to the underlying crust from last week. Small shallow wind slab were forming above 6000 feet but no instability or avalanches were observed. No observations were made above treeline where more widesperad wind slbb conditions were suspected.
A cool day with little if any new snowfall on Monday will have brought some stabilizing.
Terrain anchors have been somewhat buried by recent snowfall but should still add significant anchoring at lower elevations.
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, South.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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: 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 15th, 2015 10:00AM
