Turnagain Pass

Yesterday saw continued snowfall adding a couple more inches of new snow. Rain/snow line fluctuated from 200′-500′.  Overnight winds shifted to the north and gusted into the 60s on Seattle Ridge. The storm total of 6-12″ of snow fell on weak surface snow and/or added load to slopes harboring buried weak layers. We have been talking about the buried January 21st surface hoar needing more of a slab to be reactive and produce large avalanches. The storm snow combined with wind has likely created that slab. We do not yet have enough information about how the snowpack will behave after this loading event. Small avalanches at the old snow/new snow interface could ‘step down’ and release a much larger and unmanageable slide. Don’t let the sunshine blur your judgment. Today is a day to be extra cautious and evaluate the snowpack carefully. Look for recent avalanches, shooting cracks and listen for ‘whumpfs’. 

Crown profile from Twin Peaks shows the January 21st buried surface hoar layer that may now be reactive with added load. 

This avalanche in the North Cornbiscuit Chutes occurred Sunday on recently buried surface hoar and the cracking in the bed surface is believed to be on the January 21st buried surface hoar layer. This illustrates the potential for failure on more than one weak layer.  Photo: Mike Records

Moderate to strong winds coupled with 6-12″ of new snow have likely formed wind slabs 1-2′ thick. Because these slabs are likely sitting on weak old snow, they are expected to be quite sensitive and easy to trigger. The winds shifted from south to north last night so multiple aspects may be loaded. It will be important to pay attention to where the snow feels stiff, looks pillowed or sounds hollow and watch for shooting cracks. 

Cornices: Cornices are unpredictable and can break further back along a ridge than expected. Give these features plenty of space.

Loose snow sluffs: Sluffs on steep slopes are likely with the recent new snow. Warm temperatures and sun hitting steep southerly slopes may also trigger roller balls that may progress to natural loose snow avalanches as they entrain surface snow. 

The recent snowfall is a relatively small load on top of our generally weak snowpack structure in the Alpine (above 3,000′). However, even a small load combined with strong winds and warming temperatures could help tip the balance. Someone might be able to trigger a large deep slab that breaks in the bottom half of the snowpack. It’s good to remember that multiple layers of old buried surface hoar, facets and crusts exist deep in the pack and near the ground. Small incremental loading can sometimes be just enough to ‘wake up’ dormant deep layers. The overall poor structure is worth keeping in mind, as outliers can happen like last week’s Twin Peaks slide.