Avalanche Forecast
Issued: Jan 3rd, 2015 9:52AM
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
The avalanche danger will increase later Sunday afternoon and evening with human triggered slides becoming likely in the alpine. New loading will test persistent weak layers along the east slopes, especially on wind loaded aspects. Choose your terrain conservatively as dangerous avalanche conditions should develop.
Summary
Detailed Forecast
Light snowfall Saturday night and Sunday morning will increase in intensity Sunday afternoon along with increasing winds. Shallow wind slab should begin to develop on lee NW through SE slopes near and above treeline and may bond poorly to the existing snow surface.
Persistent slab avalanches are the most difficult to predict and at this time we recommend avoiding avalanche terrain until these layers are reassessed after the storm.
All of this will lead to increasing avalanche danger Sunday. Expect human triggered avalanches to be likely as the afternoon wears on. Be prepared to curtail your trip early if conditions become sensitive earlier than forecast.
More significant warming and precipitation is expected Sunday night and a natural avalanche cycle is expected.
Snowpack Discussion
After last weekend's storm, north through east winds, strongest in the alpine, developed locally sensitive wind slab on unusual aspects throughout the eastern Cascades (see first photo). Sunshine activated small loose wet slides on steeper south facing slopes midweek. Northerly aspects that were not wind affected had soft, right side up snowfall generally well bonded to the underlying crust. Very light snowfall accumulated along near the Cascade crest Friday due to a weak weather system. One of our biggest concerns heading into the upcoming storm will be the new storm snow's ability to bond to a variety of snow surfaces; near surface facets, wind board, melt-freeze crusts, etc.
Recent 35 cm natural wind slab crown that released on a W-NW slope near a ridge at 5800 feet on Jove Peak. Photo taken by Tom Curtis Friday. The weak layer of surface hoar likely formed after the storm last weekend and wind shift early this week.
On Wednesday at Dirty Face Peak above Lake Wenatchee, NWAC observer Tom Curtis found well developed depth hoar over a crust buried 95 cm below the surface around 6000 ft. This was producing very easy, sudden planar test results. Human triggering may be difficult due to a strong 40-50 cm slab above the PWL. A larger load, like a snowmobile, would be more likely to affect this layer.
Depth hoar found above rain crust at 95 cm below the surface giving moderate Q1 SP results at Dirty Face on Wed by Tom Curtis.
A pair of skiers reported a remotely triggered persistent slab avalanche 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 NWAC Recent Observations or on Turns All Year.
Remotely triggered persistent slab avalanche 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 is more dangerous in the central east Cascade zone due to the greater ease of human triggering. 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 week or so. After this weekend's storm, the persistent slab avalanche problem will be removed from the northeast Cascades zone if found nonreactive.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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Jan 4th, 2015 9:52AM
