Avalanche Forecast
Issued: Jan 31st, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Shallow storm slabs are possible Sunday afternoon. Watch for new snow that bonds poorly to existing surface crusts or in areas of preserved surface hoar. There is still some uncertainty regarding persistent slabs east of the crest, however recent reports indicate overlying strong snow layers are limiting the potential for human triggering.
Summary
Detailed Forecast
Generally very light precipitation is expected Sunday morning, with light rain and snow in the afternoon along the Cascade crest. A frontal passage is expected later Sunday afternoon and should lead to a slight warming trend late in the day along the east slopes.
New snow may bond poorly to surface crusts, allowing shallow storm slabs to develop Sunday afternoon especially on lee aspects near and above treeline. New storm slabs may also fail on surface hoar that survives on shaded aspects generally below treeline closer to the Cascade crest.
Warmer weather is allowing persistent weak layers from January 15th to become less or non-reactive. This problem will be listed as unlikely for the northeast and central east zones.A fair amount of uncertainty surrounds a potentially stronger frontal system Monday but generally expect increasing avalanche danger Monday.
Snowpack Discussion
Last weekend (Jan 24-25th) a warm weather system caused high snow levels and light rain east of the crest. NWAC sites east of the crest had up to an 1 inch of rain. This caused loose wet avalanches, some possible wet slab avalanches and consolidation.
Mainly mild or sunny weather has been seen so far this week at upper elevations with low clouds or fog mainly below about 3000 feet.
Recent Observations: Extensive surface hoar growth was seen between about 3000-4500 feet where recent low clouds or fog have persisted east of the crest this week.
On Jan 25th, the Mission Ridge ski patrol reported that depth hoar at 45 cm below the surface was becoming rounded but still failed on isolation. NWAC pro-obs or North Cascade Guides reports this week in the northeast and central east zones have found the January 15th facet/crust layers at 40 cm-1 m below the surface with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. So the January 15th facet/crust layers expected in the northeast and central east zones should be strengthening and stabilizing.
NWAC pro-observer Tom Curtis was at 7500 feet on Mt Cashmere in the Wenatchee Mountains on Wednesday and found a local facet/crust combo buried on a lee slope next to a ridge crest under 35 cm of thick, surface crust and 1F-P snow. This sort of slab could be very hard for a skier to trigger, but could be possible, perhaps with a large sudden load, such as snowmobiles.
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
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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Feb 1st, 2015 10:00AM
