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Avalanche Advisory

Turnagain Area Avalanche Advisory
Forecaster:   Heather Thamm  
Thursday, April 13th 2017
Created: Apr 13th 5:29 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.
2 Moderate Below Treeline / Below 1,000' Travel Advice: Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
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Alpine Air Alaska Inc
Special Announcement
  • Due to low snow cover, Twentymile drainage is now closed to snowmachines. You can check the status of riding areas at the bottom of this page.

The Bottom Line

The overall avalanche danger is MODERATE at all elevations due to a handful of avalanche concerns and springtime condtions. Triggering a large and dangerous deep slab avalanche (3-6' deep) on slopes over 35 degrees remains possible, but will be hard to trigger. More potential for this problem exists on Northerly aspects above 1500' that haven’t avalanched already. Warm spring like temperatures will also increase the potential for wet snow avalanches in the afternoon and evening. These could be wet slabs or wet loose avalanches on Southerly aspects, or on all aspects below 1,500'. Identify glide cracks and cornices and give these unpredictable features lots of space as daily warming will be making these features more unstable. 

***The danger could rise to CONSIDERABLE, especially in shallow snowpack zones where wet snow avalanches may occur naturally.

Hiking in Portage Valley and on summer trails around the Advisory area (including the Turnagain Arm Trail a.k.a the bike path).  Extra caution is advised during the afternoon and evening hours for trails that cross under avalanche paths. Avalanches are still possible at the higher elevations that could send debris over snow-free hiking trails.

Summit Lake:  See the Saturday Summit Summary HERE and recent avalanche observations from the last few days HERE.


Primary Concern

There are several weak layers within our snowpack, including a layer of buried surface hoar anywhere from 2-6' below the surface. This buried surface hoar layer was the culprit weak layer in many human triggered deep slab avalanches over last weekend in Seattle Creek. Shaded aspects (NE - NW - W) in the mid and upper elevations that haven’t avalanched already are the most suspect places for triggering a deep slab avalanche. There is also some potential on steeper solar aspects later in the day as sun heats up the slab and breaks down the surface crust. (More on this in Secondary Concern.) 

This problem is one of low probability but high consequence as these are large and potentially unsurvivable slides. As the snowpack continues to adjust, triggering will become more stubborn and less likely with time, but the hazard remains. Keep these point in mind:  

  • It will take someone hitting a 'thin spot' in the slab, or a large trigger such as a snowmachine and/or groups of people.
  • These slides can be triggered remotely, for example, from a ridge or bench on the top/side or below. 
  • There may be no signs of instability before the slope shatters 
  • Several tracks may be on a slope before it releases
  • Stability tests may not produce any noteable results

If wishing not to roll the dice on this deep slab possibility, one can always stick to slopes under 35 degrees or go to places that have already avalanched.

A snowmachine triggered deep slab that occured on Juniors in Seattle Creek on Saturday. More details about the incident can be found HERE and snowpack details HERE

 

 A recent slab avalanche on a Northeast aspect of the Headwall in Seattle Creek. The trigger is unknown, but apears to be pretty fresh.  


Secondary Concern

Springtime melt-freeze cycle (Southerly aspects): We are moving into what we call a melt-freeze cycle. Low avalanche danger in the morning, Moderate to Considerable danger in the afternoon. When the snowpack is frozen in the morning from nighttime cooling, it's stable. During the course of the day the pack warms and becomes unsupportable, this is when the avalanche danger rises. Temperatures today could reach the 50'sF in the parking lots and near 40F at ridgetops. This is when Southerly aspects and the lower elevation snowpack will become wet and unsupportable to snowmachines, skis or boots. When the pack gets this loose and punchy it's time to head to a different aspect as slopes steep enough to slide will be suspect for wet snow avalanches. Human triggered wet loose slides and wet slabs are a possibility later in the day and are nothing to mess with. Keep a close eye on Southerly slopes, especially near rocks and thinner snow zones, and avoid being in the runtout of these areas.

Wet loose avalanches that occured two nights ago on a South aspect of Penguin ridge.This is a good example of a slope with lots of rocks and thin snow coverage that can heat up and loose strength easily with such warm day time tempertures. 


Additional Concern

Glide Avalanches: Yesterday a glide avalanche occurred on the South face of Eddies just before 8pm and was captured on the DOT web cam. There are a handful of glide cracks in specific places like the South face of Eddies, Lynx Creek and even near the up-track along Seattle Ridge. Warm temperatures and prolonged solar energy, expect more of these cracks to release without warming in the next few day. Identify existing cracks and avoid travel in their runout zone.

CORNICES: Daytime warming and solar heating will be adding stress to Cornices as well. Avoid travel on or underneath these backcountry bombs and remember that they often break further back on a ridge than expected. Triggering a cornice has the potential to initiate a large avalanche on the slope below.

This glide release occured yesterday evening on a South aspect of Eddies just before 8pm. Captured on DOT web cam.

 

 


Mountain Weather

Yesterday skies were clear and sunny. Temperatures in valley bottom climbed into the low 50F’s and ridge tops reached nearly 40F. Winds were calm becoming light 5-10mph from the West in the evening. Overnight clear skies remained and temperatures dipped down into the low 30F’s. 

Expect similar weather today. Day time temps could reach the mid 50F’s this afternoon in the lower elevations and mid 40F’s in the upper. Temperatures overnight are expected to reach the low 30F’s.  Ridge top winds from the N should range from 5-15mph. No precip is in the forecast. 

Clear skies and warm temperatures are expected through Saturday morning. There remains much uncertainty through the weekend, but there is talk of cloud cover and a possibility of snow/rain showers.

*Some temperature data is missing, making this average not as accurate.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 39  70 
Summit Lake (1400') 35   0 24 
Alyeska Mid (1700')  38 64 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 33  12 
Seattle Ridge(2400') *36  var.  11 

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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