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Avalanche Advisory

Turnagain Area Avalanche Advisory
Forecaster:   Heather Thamm  
Sunday, April 12th 2015
Created: Apr 12th 5:41 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.
0 NO RATING Below Treeline / Below 1,000'
 Show the Complete North American Avalanche Danger Scale
The John Byrne Family
Special Announcement

Beginning tomorrow, April 13th, we will be issuing advisories 5 days a week.  Advisories will be posted at 7 am on all days of the week except Mondays and Wednesdays.  The final advisory of the season will be posted on Thursday April 30th.


The Bottom Line

Today the avalanche danger is MODERATE where triggering a large avalanche is possible in the Alpine and at Treeline. Below 2500’ a very “upside down” snowpack persists where a slab 1-5’ thick could release. On Northern shaded aspects above 2500' a slab 3-5’ thick is sitting on layer of weak faceted snow and if triggered could have very high consequences.  It will be important to avoid all steep open slopes at Treeline and on Northern aspects in the Alpine. Newly forming isolated windslabs will also be something to watch for if snow showers occur as forecasted today.

* The most desirable snow will be found on shaded aspects and this could make decision-making challenging. Be sure to practice safe travel rituals; identify islands of safety, only expose one person at time, and always have an escape zone.  


Primary Concern

Below 2500’ a layer of wet poorly bonded snow is sitting beneath a slab 1-5’ thick, depending on the location. On the Northern side of the Turnagain pass this slab is closer to 5’ thick. Yesterday on Eddies’ South Face several large (D3) avalanches released naturally on this wet layer. Simultaneously several smaller avalanches released sympathetically on adjacent slopes. It was reported that this occurred in one single event during the warmest part of the day, just after 3pm. Several parties all in different locations described a very loud “Canon” sounding collapse and felt the snowpack drop ½ an inch.  

On the far Southern end of Turnagain Pass where this slab is only 1-2’ thick below 2500’, recent activity and collapsing were not observed yesterday. What this means is that there is more stress on this weak layer where the slab is thicker and thus more likelihood of it reaching its critical tipping point. This problem exists on all aspects below 2500’ and could become active even in places where the slab is thinner. At this elevation band it will be important to avoid being on or near large slopes steeper than 35°. Some examples of these slopes are the East Face of Seattle Ridge and the Library near Center Ridge.

*Even thought temperatures are dropping below freezing at night the wet snow below is unable to re-freeze due to the insulating properties of the slab. This problem is likely to persist for a while and will be most active during the warmest parts of the day. 

Close up photo of one of the many avalanches that occured yesterday on Eddies. This one was on a SW apsect. Photo by Laine Parish.

 

 View of Eddies West Face. Several larger (D3) avalanches occured at a similar elevation band on the South Face, but could not be seen in entirety from the roadside. 

 

Oblique view of the South aspect of Eddies. Several large (D3) avalanches are out of view from roadside perspective. 

 

 


Secondary Concern

Weak layers of faceted snow buried as deep as 5’ sit on some slopes on North facing terrain in the Alpine.  This set up will not be found everywhere.  Because of this variability, it is especially important to treat this terrain with suspicion.  Assessment of this issue is very difficult.  The best strategy for “managing” this problem is to avoid this terrain for the time being and give those underlying weak layers ample time to adjust to the large loads (3’+ snow/3”+ H20 over the last week) that have been placed on them.  This is a low likelihood/high consequence scenario that requires patience and time.


Additional Concern

Moderate ridgetop winds from the Northeast and several inches of new snow could create small isolated wind slabs in the Apline. These wind slabs will likely be small (2-6”) but could be tender if we see 3+ inches of new snow today. Pay attention to the amount of new snow accumulation. Southern aspects have a sun crust that could make a good sliding surface if these wind slabs form today.  


Mountain Weather

Yesterday skies were mostly clear with light winds out of the Northeast along ridgetops. Temperatures reached the low 40s F near 1000’. No precipitation occurred during the day.

Overnight light flurries produced an inch of new snow in Turnagain Pass.  No new snow was recorded in Girdwood. Temperatures dipped into the high 20s F at lower elevations and the low 20s F near ridgetops. Winds were 10-20mph from the NE along ridgetops.

Today will be mostly cloudy with scattered snow and rain showers throughout the day. Up to 3 inches of accumulation is expected. Ridgetop winds will be light to moderate (10-20 mph) from the Northeast. 


*Wind direction data not available at Seattle Ridge Wx Station. Wind amounts only available after 2pm on April 11th.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 33  0.1  70 
Summit Lake (1400') 33  11 
Alyeska Mid (1700') 33  40 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 23  NW  33 
Seattle Ridge(2400') 26  *n/a  *7  *23 

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
If you have comments or questions regarding CNFAIC operations or winter recreation management, please email staff@chugachavalanche.org
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