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Avalanche Forecast

Archived

Dec 29th, 2016–Dec 30th, 2016

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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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.

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

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.