Turnagain Pass

After a cold and dry March, April is starting us off with a wet sucker punch. Yesterday was the 5th day in a row of elevated avalanche danger due to a series of storm systems moving through. The cycle began with a system bringing snow to sea level (1-3+’ throughout the forecast zone) and this last system (Friday and Saturday) bringing rain to 1,800′. Over the last 24-hours we have picked up ~.4-.7″ of rain below 1,200′ and 4-8″ of new snow above. Today will be a short break between storms before another warm low-pressure moves in tonight – see the satellite image HERE. Although precipitation and temperatures are backing off today, the winds are not, and all of these will ramp back up tomorrow, keeping the snowpack in an unstable state. 

RECENT AVALANCHE ACTIVITY:
Significant natural wet slab avalanche activity was seen yesterday as rain fell and saturated the 1-3′ of new snow from last week. Through thick clouds, two large wet slabs were seen at Turnagain Pass, photos below. How many more exist – and just not visible – is a mystery for now. An avalanche professional in the Placer Valley yesterday noted widespread natural wet slab activity occurring in the 1,500′ elevation band. This was along the steep rollovers that line the Valley, such as near Skookum Valley, Spencer Glacier and into Grandview. Wet slabs were also seen in Portage Valley near the Whittier Tunnel entrance – again all releasing near 1,500′.

Dry snow avalanche activity has been prevalent over the last 5 days in the mid and high elevations – though again much is obscured by poor visibility. Debris piles littering the bottom of large and small avalanche paths assures us that widespread naturals have been occurring. If skies part enough today, hopefully we can get a glimpse of the start zones for some of these storm snow avalanches. 

Wet slab releasing yesterday on Pete’s South ridge, Southwest aspect ~1,800′ (estimated 12-18″ thick, 300′ wide). 

Another wet slab releasing yesterday between 1 and 2pm on Eddies ridge, also a Southwest aspect at 1,500′ (estimated 1-2′ thick and 300′ wide). 

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WET AVALANCHES:
Natural wet slabs and wet loose sluffs below 1,800′ are possible today with human triggered slabs and sluffs likely. Once the skies clear and/or temperatures cool enough for the saturated snow in this lower elevation band to begin to freeze up – this issue will get locked into place – but until then, wet avalanches are not manageable and nothing to mess with.

Above 1,500′ where the snow is drier we can expect storm snow instabilities to persist today. These are:

WIND SLABS and CORNICES:
Non-stop Easterly winds will continue to load slopes and build cornices with the new warm, sticky snow. Ridgetop Easterly winds are expected to average 20-35 mph, perfect for keeping the pack unstable. Natural cornice falls are likely and could easily trigger a slab below them.

LOOSE SNOW AVALANCHES:
Sluffs in the dry snow are likely, but much less of an issue compared to the much larger slab avalanche and cornice fall problem.

Debris from the lower slopes of Seattle Ridge – an indication of dry snow avalanches occurring above. 

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Several old layers of weak snow (near surface facets and surface hoar) sit buried in the snowpack. Avalanches releasing in the storm snow have the potential to ‘step down’ into them. Furthermore, as the new snow piles up and begins to stick to the old snow surface, avalanches breaking into deeper layers are also possible. Both of these situations can create a much larger avalanche – something to keep in mind as the storm cycle continues. Snowpack tests are still showing propagation potential in these layers. The question is how much incremental loading will tip the balance?