Avalanche Forecast
Issued: Jan 1st, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Persistent slab layers are the most difficult to predict. Avoid avalanche terrain east of the crest unless you can be sure there are no persistent slab layers present.
Summary
Detailed Forecast
A weak cold front will move northwest to southeast over the Olympics and Cascades Friday afternoon and night. This should cause increasing northwest winds Friday in most areas. Light rain if any should quickly change to light snow due to a rapid cooling trend. The main light snow should be seen in the Olympics and north Cascades Friday afternoon spreading to the south Cascades Friday evening with little making it to Mt Hood Friday night.
The light amounts of snow should not greatly change snow conditions east of the crest.
Watch for lingering wind slab from early in the week where the northeast winds redistributed snow mainly above and near treeline.
The most dangerous problem will be to avoid avalanche terrain that is prone to the persistent slab layer. Persistent slab layers are the most difficult to predict. Avoid avalanche terrain east of the crest unless you are sure there are no persistent slab layers present.
Snowpack Discussion
A storm system hit the Northwest last weekend with strong west to northwest winds. NWAC and Snotel sites west of the crest picked up about 6-14 inches of storm snow with a good cooling trend. Rapid loading caused storm slab during and after this storm which is expected to have since stabilized.
Strong northeast winds hit at the tail end of the storm Monday and Tuesday causing easterly upslope winds east of the crest. This likely built new wind slabs on S-W aspects in the near and above treeline zones.
Observations describing the persistent slab due to a facet/crust at 50-100 cm below the surface vary along the east slopes. There has been less evidence at Washington Pass over the past few days. Remotely triggered avalanches were reported there last week but not the past few days.
But at Dirty Face Peak above Lake Wenatchee on Wednesday, NWAC observer Tom Curtis found well developed depth hoar over a crust buried 95 cm below the surface. This was producing very easy, sudden planar test results. A strong 40-50 cm slab layer lay above with the recent storm snow above that. Triggering this layer at this location may have been difficult.
Depth hoar found above rain crust at 95 cm below the surface giving moderate Q1 SP results at Dirty Face above Lake Wenatchee on Wednesday by Tom Curtis.
A pair of skiers also reported lots of settling and remotely triggered a persistent slab in Clara Lake Basin near Mission Ridge on Wednesday. This was on a south slope at 6500 feet due to the facets on the crust at 1 meter below the surface. See the full report via the Recent Observations tab.
Remotely triggered persistent slab in Clara Lake Basin Wednesday by Jamie Tackman.
At this time we believe the persistent weak layer may be more dangerous with a greater ease of human triggering in the central east Cascade zone.
No recent information is available from the south-east Cascade zone, so travelers are urged to use caution and make conservative decisions in case similar conditions are found there.
Problems
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
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: South East, South, South West, West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Jan 2nd, 2015 10:00AM
