Avalanche Forecast
Regions: Cascades - South East.
Wind slab should be the primary avalanche problem and mainly found near and above treeline Saturday. Wind slab should be less likely to trigger throughout the Cascades but look for wind loaded slopes further downslope than usual particularly in the Mission Ridge/Blewett area. Also, the persistent slab problem still warrants attention in the Mission Ridge/Blewett area where several PWLs represent a touchy snowpack. Avoid all avalanche terrain of consequence in this area.
Detailed Forecast
After a mostly clear start to Saturday for much of the Cascades, a fast moving frontal system will bring light snow to the northeast Cascades by late morning and spread south to the central and southeast Cascades in the afternoon. New snow amounts through 4 pm will generally be light to insignificant, but westerly winds above treeline are expected spike up by mid-day. There is ample new snow available for transport along the east slopes closer to the crest.Â
Wind slab should be the primary avalanche problem and mainly found near and above treeline Saturday. Wind slab should be less likely to trigger throughout the Cascades but look for wind loaded slopes further downslope than usual particularly in the Mission Ridge/Blewett area.
The persistent slab problem still warrants attention in the Mission Ridge/Blewett area where several PWLs represent a touchy snowpack. 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.Â
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.Â
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.  Â
A warm front kept light snow and moderate winds going through much of day in the Washington zone Thursday before pushing south Thursday night. 8-12 inches of snow fell in the Washington Pass zone, Holden and Lake Wentachee while 1-2 inches accumulated further east toward and Blewett/Mission area and further south.Â
Recent Observations
In the Delancey Ridge area in the Northeast zone 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 same area Friday, small loose dry avalanches were the primary avalanche problem but wind affected snow surfaces were visible above treeline. Besides loose dry avalanches in steep terrain, no evidence of natural avalanche activity was observed. In the Washington Pass zone, the 12/17 PWL has been found to be unreactive and has been removed from the set of avalanche problems. Â
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.Â
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.Â
Two reports from the NWAC observation page tell the continuing story of a much shallower snowpack with multiple persistent weak layers in this portion of the central-east zone.Â
Tom Curtis was out at Mt. Lillian Friday and found reactive wind slab along ridges with shooting cracks and whumpfing on north aspects near 5900 feet. Â The wind slab was likely collapsing down to the 12/17 Persistent Weak Layer (buried surface hoar) about 25-35 cm down. Wind slabs were found on NW-E-SE aspects with some wind loading apparent well below treeline.Â
Avalanche 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: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
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: Likely
Expected Size: 1 - 1