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Avalanche Advisory

Turnagain Area Avalanche Advisory
Forecaster:   Graham Predeger  
Wednesday, March 1st 2017
Created: Mar 1st 5:14 am
2 Moderate Alpine / Above 2,500' Travel Advice: Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
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 is MODERATE above 1,000’ where it will again be possible to trigger shallow, fresh wind slabs on all aspects today.  We are also tracking several weak layers deeper in the snowpack; keeping persistent slab avalanches as the secondary concern.  Buried surface hoar (2-3’ deep) and weak facets near the ground are stubborn to trigger but continue to show the potential to fail and propagate into large and destructive avalanches.

Though it'll be quite cool today, direct sun may have an impact on surface instabilities on solar aspects.  Cornices and the ever-present glide crack on Seattle ridge round out our bottom line today.   Below 1,000’ the avalanche danger is LOW where triggering an avalanche will be unlikely.

We had a report of several natural wind slab avalanches in the Summit Lake to Moose Pass zones yesterday.  Look for an observation later tonight and check out the Saturday Summit Summary HERE.


Primary Concern

Yesterday definitely saw somewhat of a wind slab avalanche cycle, particularly in the southern end of the advisory area where there is generally a shallower and poorer snowpack structure.  Observations from Lynx Creek found several shallow but very well connected fresh wind slabs.  Though nothing we saw yesterday was super deep (12-18”), many of these slabs were in consequential terrain if a skier or snowmachiner were caught.  It’s important to keep terrain choices in mind with a problem such as this.  Though a 12” wind slab may not scream ‘deadly avalanche’ it is enough to knock you off your feet or push you into a terrain trap, compounding consequences.

Slabs were observed on all aspects yesterday so it’ll be important to suss out any wind-loaded terrain today by paying attention to the surface – shooting cracks, whumphing, textured surface or hollow sounding and drum-like snow are all clues that a wind slab is present.  These are likely to be moderate to easy to trigger again today.  The predominant Northwest flow we saw Monday night tends to be channeled through Turnagain Pass, such that Southerly winds are seen on the East side (non-motorized) of the highway - loading Northerly slopes.  Not intuitive, but mountain weather is complex!  Bottom line is that these wind slabs may be found on all aspects of the compass today.

Fresh wind slab on the west shoulder of Captian's Chair, above Lynx Creek.  This was one of many slabs observed in this area yesterday.

 

Wind textured surface on an East facing slope, above Lynx Creek.

Cornices:  These backcountry bombs are still ripening and can be quite unpredictable this time of year.  Cornices have a tendency to break farther back along ridgelines than expected and have the potential to trigger an avalanche on the slope below. Give them extra space and avoid spending time under them. 


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) we are still finding this persistent weak layer and although it is becoming tougher to trigger, it does continue to show the potential to fail and propagate

With the addition of wind loading, the Feb. 9th buried surface hoar could be closer to a tipping point potentially creating a larger slab avalanche 2-3’ thick.  Likely trigger spots are in places where the snowpack is thinner - near rock bands or on more scoured features. These slabs can break above you, and release after several tracks are on a slope. 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.


Additional Concern

Glide avalanches:  The glide crack just looker's left of the Seattle ridge up-track was blown over and mostly filled in yesterday. It is still discernable and appears to be slowly opening.  Avoid hanging out under this crack and any others you may see, as they are very destructive and unpredictable.  #lowprobabilityhighconsequence 


Mountain Weather

Northwest winds dropped off precipitously by the daylight hours yesterday morning.  Temperatures were also in a free-fall and bottomed out in the single digits at 1,000’ last night under clear skies.  No new snow to report yesterday.

The big story for this first day of March is the cold temps.  Though sunny skies will persist today, don’t expect spring-like temperatures quite yet.  At 1,000’ we’ll likely see temperatures in the mid-teens and cooling off to single digits today at ridge top locations.  Winds will mainly be from the North in the 5 – 15mph range under sunny blue skies.

This general pattern of cold arctic air and outflow winds impacting southcentral Alaska looks like it is here to stay thru the weekend.  For the moment, the eastern Turnagain arm zone is being spared from the worst of the winds which appear to be impacting both Hatcher Pass and Thompson Pass to a greater degree.

PSA: For the internet weather geeks... National Weather Service (Alaska) web addresses are changing today, March 1.  CLICK HERE for the latest info and to update bookmarks.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880')  7 65 
Summit Lake (1400')  9 29 
Alyeska Mid (1700')  11  0  0 60 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812')  2 NW  16 
Seattle Ridge(2400')  N  19

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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