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Avalanche Advisory

Turnagain Area Avalanche Advisory
Forecaster:   Heather Thamm  
Thursday, March 2nd 2017
Created: Mar 2nd 6:13 am
3 Considerable Alpine / Above 2,500' Travel Advice: Dangerous avalanche conditions. Careful snowpack evaluation, cautious routefinding and conservative decision-making essential.
2 Moderate Treeline / 1,000'-2,500' Travel Advice: Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1 Low Below Treeline / Below 1,000' Travel Advice: Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
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The Bottom Line

The avalanche danger could rise to CONSIDERABLE in the Alpine (above 2,500’) where triggering a fresh wind slab will be likely should sustained winds reach 30mph today. There also remains a possibility of triggering a more stubborn, and more dangerous avalanche 2-3' deep on all aspects above 2000’. Today’s winds are also expected to be adding stress to cornices. Cautious route-finding and conservative decision making will be essential. 

The avalanche danger in the tree line zone (1000’-2500’) is MODERATE where triggering a fresh wind slab on steep features will be possible.

Below 1,000’ the avalanche danger is LOW where triggering an avalanche will be unlikely.

In Summit Lake, Girdwood, and on the southern end of Turnagain near Johnson Pass triggering a deeper more dangerous avalanche near the ground is still possible, but will be hard to trigger. Check out the Saturday Summit Summary HERE.


Primary Concern

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. 


Secondary Concern

Monday/Tuesday winds were likely enough to overload the February 9th 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. 9th 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

 


Additional Concern

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. 


Mountain Weather

Yesterday skies were partly cloudy and temperatures ranged between 0F and 15F. Winds were light from the NE and no precipitation was recorded.  Overnight temperatures dropped back to around 0F and winds were light. 

Today expect clear and sunny skies, and similar tempatures 0F-15F.  Northwest ridgetop winds will increase late morning to 15-30mph and gusts may reach the 40’s mph. This wind event is expected to peak this evening, but moderate winds are expected to last through tomorrow evening.   

Cold temperatures associated with a high pressure system over interior Alaska will continue to be our dominant weather pattern. This pattern will dominate Southcentral, Alaska into next week without much on the horizon of major changes in the extended forecast.

PRECIPITATION 24-hour data (6am - 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 64 
Summit Lake (1400')  30
Alyeska Mid (1700') 59 

 

RIDGETOP 24-hour data (6am - 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812')  NE
Seattle Ridge(2400') NE  21 

This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area (this advisory does not apply to highways, railroads, or operating ski areas).

Winter snowmachine use open/closed status and riding conditions updates

Riding status is not associated with avalanche danger. An area will be open to motorized use in accordance to the Forest Management Plan when snow coverage is adequate to protect underlying vegetation. Backcountry hazards including avalanche hazard are always present regardless of the open status of motorized use areas.

(Updated: May 16, 2017 )

AREA STATUS WEATHER AND RIDING CONDITIONS
Glacier District
Johnson Pass: Closed
Placer River: Closed
Skookum Drainage: Closed
Turnagain Pass: ClosedThanks all for a safe and fun season on the Chugach NF! Stay tuned for the 2017/18 season. #playsafe #snowtosealevel
Twentymile: Closed
Seward District
Carter Lake: Closed
Lost Lake Trail: Closed
Primrose Trail: Closed
Resurrection Pass Trail: ClosedResurrection Pass trail will be open to snowmachine use during the 2017/18 winter season.
Snug Harbor: Closed
South Fork Snow River Corridor: Closed
Summit Lake: Closed

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The information in this advisory is from the U.S. Forest Service, which is solely responsible for its content. This advisory describes general avalanche conditions and local variations always occur. This advisory provided by the Chugach National Forest, in partnership with Friends of the Chugach National Forest Avalanche Information Center.


USFS SNOW AND AVALANCHE HOTLINE (907) 754-2369
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