Turnagain Pass

Today Northwest ridgetop winds are expected to be in the 15-30mph range with gusts in the 40’s this afternoon.  Loose surface snow (4-8” deep) can still be found in the heart of our forecast zone in the mid elevations and protected areas of the Alpine. Should winds reach the higher side of this spectrum, wind slabs 1-2’ thick will be likely to trigger on steep slopes. These slabs could be soft or hard, and may be forming on top of older wind slabs that exist on a variety of aspects. A brief wind event on Monday night (2/27) caused a natural avalanche cycle on the Southern end of our forecast zone (Lynx Creek to Summit Lake) with wind slabs ranging in the 12-20” deep. Its important to note that a NW flow can funnel winds through Turnagain Pass from a variety of directions and it’s not uncommon to see Southerly winds on the non-motorized side of the road. Today’s avalanche danger will depend on how strong the winds reach and how much snow is available for transport in the area you are traveling in. If large plumes of blowing snow are observed, this could add stress to existing weak layers deeper in the pack, and make it easier to trigger a deeper more dangerous slab. More details in Secondary Concerns. Shooting cracks will be an obvious clue that wind slabs are tender. 

Questions to keep in mind on today:  

• Do I see blowing snow along ridgetops, an obvious sign of rapid loading?  
• Does the snow feel hollow or drum-like below me? 
• What are consequences of the terrain should I trigger a wind slab or a deeper layer within the snowpack?

A wind slab that failed upon isolation in a compression test on the North ridge of Silvertip yesterday afternoon. This particular ridge was impacted significantly by the Monday night wind event. 

Monday/Tuesday winds were likely enough to overload the February 9^th buried surface hoar layers on the southern end of the advisory area.  This layer has been found to be quite a bit shallower (12 – 20” deep) on the southern end of Turnagain Pass, making it easier to trigger. In areas with a deeper overall snowpack (Seattle ridge, Tin Can, Sunburst, Girdwood Valley) this persistent weak layer is 2’-3’ below the surface and has been tougher to trigger in stability tests, but does continue to show the potential to fail and propagate. 

Should strong winds occur today, the Feb. 9^th buried surface hoar could be closer to its tipping point. Winds today could also be creating more trigger spots -thinner areas of the snowpack – scoured terrain features and rocky area. This type of avalanche is likely to break above you and propagate further than expected. Be aware that no red flags may be present.

Deep Persistent Slab: We continue to find various layers of weak faceted snow and depth hoar near the bottom of the pack in certain areas. This includes Summit Lake zone, and some areas in Girdwood Valley and towards the Southern end of Turnagain pass, near Johnson Pass and Lynx Creek. Similar to the problem above, these layers will be very tough to trigger, but a possibility remains in places with this poor structure.

A natural avalanche on the SW shoulder of Captain’s Chair that was triggered naturally during Monday/Tuesday’s wind event. Due to how wide this avalanche propagated the suspected weak layer is the Feb.9 buried surface hoar. This avalanche ran 1000 vertical feet – note the debris in the lower left corner. 

Cornices: Winds today may add stress to already large cornices.  These unpredictable hazards can release naturally and break farther back onto a ridge than expected. They also have the potential to trigger an avalanche on the slope below. Give cornices extra space and avoid being under them.