Avalanche Forecast
Issued: Jan 2nd, 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 difficult to predict. Avoid avalanche terrain east of the crest unless you are sure there are no persistent slab layers present.
Summary
Detailed Forecast
An upper ridge will reassert itself over the US coastal waters on Saturday. Northwest flow and moisture should decrease a bit over the Olympics and Cascades. Clouds should decrease a bit during the day and lead to some partly sunny weather especially in the south and east of the crest.
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.
Watch for lingering wind slab from early in the week where the northeast winds redistributed snow mainly near and above treeline.
Sun crusts, while not an avalanche danger on Saturday, are likely on steep south slopes where the sun warmed the snow on Wednesday and Thursday.
Right side up stable powder may also still be found on shaded sheltered slopes on Saturday.
Heads up in advance for Sunday when moderate to heavy snow with warming should bring a significantly increasing avalanche danger.
Snowpack Discussion
A storm system hit the Northwest last weekend with strong west to northwest winds. NWAC and Snotel sites east of the crest picked up about 6-14 inches of storm snow with a good cooling trend. Strong northeast winds hit at the tail end of the storm Monday and Tuesday. This transported snow and formed some local wind slab on exposed slopes and near ridges.
Recent 35 cm natural wind slab crown that released on a W-NW slope near a ridge at 5800 feet on Jove Peak on Friday by Tom Curtis. The weak layer of thin surface hoar must have had a chance to form between storm and wind changes last weekend and early this week.
An upper ridge over the coastal waters tilted inland Wednesday and Thursday producing sunny weather and warm temperatures at higher elevations. This caused wet loose avalanches on steep slopes facing the sun in most areas including east of the crest.
A weak front is crossing the Northwest on Friday. This may cause some light snow at higher elevations east of the crest and will bring cooling. But lower sites will warm as the cold surface air mass mixes east of the crest. The minor new snow if any Friday should not cause significant new layers.
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.So triggering this layer at this location may be 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 or Turns All Year.
Remotely triggered persistent slab in Clara Lake Basin Wednesday by Jamie Tackman.
The Mission Ridge avalanche above is bigger than it looks in this photo with debris chunks the size of refrigerators at the bottom. 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 southeast 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, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Jan 3rd, 2015 10:00AM
