Avalanche Forecast
Issued: Dec 29th, 2016 10:20AM
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
Wind slab should be the primary avalanche problem due to a period of strong westerly winds Thursday afternoon and/or night that may load leeward aspects further downslope than usual. Treat wind loaded slopes in all elevation bands with caution on Friday. The persistent slab problem still warrants attention in the Cascades especially in the Mission Ridge/Blewett area where full depth avalanches are still possible.
Summary
Detailed Forecast
A strong but quick moving front will bring a period of strong winds along with precipitation Thursday afternoon and night. Winds and shower intensity should quickly taper down on Friday with a clearing trend expected along with cool temperatures.
Wind slab should be the primary avalanche problem due to a period of moderate to strong westerly winds Thursday afternoon and/or Thursday night that may load leeward aspects further downslope than usual. Treat wind loaded slopes in all elevation bands with caution on Friday.
Storm slabs are expected to be the most reactive and likely to trigger Thursday night during peak warming and storm intensity. However, sensitive storm slabs may linger and be found in wind sheltered terrain Friday. More snow is expected from Snoqualmie Pass and north to Washington Pass with this storm and the avalanche danger will be rated higher to account for the additional expected snowfall. Lower snowfall totals are expected further east of the crest and in the southeast zone.
The persistent slab problem still warrants attention in the Cascades especially in the Mission Ridge area where recent full depth avalanches have occurred. Recent and new loading may make this layer more sensitive to triggering where it is still present. Remember that persistent weak layers are generally involved in larger avalanches. Avoid steeper slopes in areas where you still find this layer in snow pits or especially if you experience direct observations of this layer such as whumpfing or shooting cracks.
Snowpack Discussion
Weather and Snowpack
Strong storms around the Solstice deposited generally half to 1 inch of water equivalent along the east slopes. Storm totals generally ranged from 6 to 12 inches.
A strong front and strong west flow aloft was seen over the Cascades on Monday and Tuesday. NWAC and Snotel stations along the east slopes had 4 to 18 inches of new snow through 48 hours ending Wednesday morning along with a cooling trend. The higher amounts were seen closer to the crest while the lower amounts represented the Blewett/Mission area.
Recent Observations
The NCMG was on Delancey Ridge on Wednesday and reported good conditions with low quality shears in the recent snow which were cleaner on sun crusts on SSW slopes. In same area Thursday, NCMG reported the new snow totaled 15-20 cm (6-8 inches) through the afternoon but was lacking a slab structure near and below treeline. In the Washington Pass zone, the 12/17 PWL has been found to be unreactive.
A different story continues in the Mission Ridge/Blewett area.
Last week Mission Ridge ski patrol produced 1.5 to 3 ft hard slab avalanches during control work. These avalanches were releasing on basal facets about 15 cm from the ground.
On Saturday 12/24, a backcountry ski tourer in the Lake Clara area near Mission Ridge reported a huge whumpfing noise, likely indicating a collapse of the basal facets. While no avalanche occurred, the terrain where the collapse occurred connected to a large avalanche path that was NE facing near treeline. While deep, persistent slabs in this area are unlikely to trigger it will be best to avoid avalanche terrain in the Mission Ridge area!
Two reports from the NWAC observation page tell the continuing story of a much shallower snowpack and deep persistent weak layers in this portion of the zone.
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, North West.
Elevations: All elevations.
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: 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: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 30th, 2016 10:20AM
