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Avalanche Advisory

Turnagain Area Avalanche Advisory
Saturday, March 10th 2018 5:33 am by Wendy Wagner
ARCHIVED ADVISORY - All advisories expire after 24 hours from the posting date/time.
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The Bottom Line

A CONSIDERABLE avalanche danger exists at all elevations and on all aspects in the mountains from Girdwood to Seward. Large slab avalanches composed of 2-4' of yesterday's storm snow are likely to be triggered on slopes steeper than 30 degrees. These may be triggered from a distance or from the bottom of a slope. Larger slabs are possible that break in overloaded weak layers in the snowpack. Cautious route finding and conservative decision-making will be essential for safe backcountry travel this weekend.

Summit Lake area - see today's snowpack and avalanche summary.

*A Special Avalanche Bulletin has been issued through the National Weather Service. This will remain in effect through 6pm Sunday. 


 Show the Complete North American Avalanche Danger Scale
3 Considerable Alpine / Above 2,500' Travel Advice: Dangerous avalanche conditions. Careful snowpack evaluation, cautious routefinding and conservative decision-making essential.
3 Considerable Treeline / 1,000'-2,500' Travel Advice: Dangerous avalanche conditions. Careful snowpack evaluation, cautious routefinding and conservative decision-making essential.
3 Considerable Below Treeline / Below 1,000' Travel Advice: Dangerous avalanche conditions. Careful snowpack evaluation, cautious routefinding and conservative decision-making essential.
Special Announcement

**Dangerous avalanche conditions are being seen region-wide across the Kenai Peninsula and North into the Western Chugach and Anchorage Front Range. Natural avalanche activity was reported in the South Fork of Eagle River yesterday. Expect avalanche danger to remain elevated through the weekend. 

For the Hatcher Pass area, please see today's HPAC avalanche forecast.


Avalanche Problem 1

Storm Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
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Size

A combination of several feet of new snow, powder starved people and clearing skies this weekend all equal a chance for an avalanche incident. Between 2-4' of new snow fell over the region Thursday night through yesterday morning. Many natural avalanches were seen, yet at the same time, many popular areas were obscured in clouds and little information exists. Therefore, we know the new snow that rapidly loaded slopes avalanched immediately in some areas, but we are suspect that more slopes remain intact and could be lying in wait for a human to tip the balance. How quickly the storm snow will bond to the old surface is uncertain and today (and tomorrow) are days to let the slopes adjust. 

Mid-elevation storm totals from Thursday through 6am Saturday:

  • Turnagain Pass: 26-36 inches 
  • Girdwood: 32-42 inches 
  • Portage: 26-36 inches 
  • Summit Lake: 12-16 inches

If choosing to head to the backcountry, keep in mind that triggering large slabs is likely and remote triggering from a ridge or below a slope is also likely. These are unmanageable and dangerous avalanches. Whumpfing (collapsing) and shooting cracks will be keys the snowpack is very unstable. We need to watch our terrain (keeping to the flats and low angle slopes) and watch the other groups around us. Types of storm snow avalanches that could be triggered today:

-  Wind slabs:  Slabs up to 2-4+ feet thick could be found and triggered on slopes that saw wind loading during the storm. They are most likely to be triggered along ridgelines and in cross-loaded gullies.

-  Storm slabs:  Soft slabs (2-3 feet deep) are expected in areas out of the wind, similar to the photos below. With various weak layers under the new snow, this can cause a storm slab that 'seems' to be bonding with the old snow to remain unstable as the weakness may lurk just below the old/new interface.

-  Cornice falls:  Cornices have grown and cornice falls are likely. They are a concern in themselves, but could also trigger a wind slab or storm slab below.

Clearing skies? Sunshine can quickly de-stabilize slopes with enough solar warming. This could occur today if skies open up, but is more likely to be an issue tomorrow. Be suspect of slopes that are 'baking' in the sun, including being under these as they could release naturally.

Natural storm slab avalanches that released yesterday morning due to the rapid loading of 2-3' of new snow. These are small slopes along the cut bank of Granite Creek on the Southern end of Turnagain Pass. Larger terrain out of view in the clouds likely experienced similar avalanche activity.


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Avalanche Problem 2

Persistent Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

Several widespread persistent weak layers exist within our snowpack region-wide, including buried surface hoar and facets 1-2' below the old surface. It's been over 6 weeks since a significant storm has added weight to these layers and a lot of uncertainty exists around how well they will adjust to the new load. Avalanches failing in these layers are possible, along with a storm related avalanche ’stepping down' into these layers. In this case a much larger avalanche is possible. Areas on the South end of Turnagain Pass and the Summit Lake area are most suspect for having deeper weak layers release.

The slabs that released in the lower elevations yesterday, pictured above, were technically failing in facets below the storm snow (pictured below). The snowpack structure changes with elevation and upper slopes could be overloading other weak layers. Nonetheless, weak layers exist at all elevations.


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Mountain Weather

Snowfall continued through yesterday morning before giving way to broken skies and intermittent snow showers. Between 5-10" of snow has been recorded over the past 24-hours with only an inch or so in the Summit Lake area. Ridgetop winds were Easterly in the 15-25mph range with stronger gusts and temperatures were in the teens along ridgelines and the mid 30'sF at sea level. 

For today, Saturday, we are expecting mostly cloudy skies with light snow showers to add 3-5" of snow (.3" water) and another 2-3 tonight (.2 water). Ridgetop winds should remain from the East in the 10-20mph range. Temperatures will stay in the teens along ridgetops and near 30F at sea level. 

An active weather pattern will continue to bring periods of precipitation (mostly in the form of snow) with periods of clear skies this week. Sunday there looks to be a big enough break in clouds to get longer periods of sunshine. 

*Snowfall numbers are estimated 
** Seattle Ridge anemometer (wind sensor) is rimed and not reporting

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 25  *5-6"   0.5   93 
Summit Lake (1400') 27  *1" 0.1 36 
Alyeska Mid (1700') 26  *10"  85 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 16  NE  22  66 
Seattle Ridge(2400') 23  **n/a  **n/a   **n/a  

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 06, 2018 )

AREA STATUS WEATHER AND RIDING CONDITIONS
Glacier District
Johnson Pass: ClosedClosed as of April 20th
Placer River: ClosedClosed as of April 17th
Skookum Drainage: ClosedClosed as of April 1st.
Turnagain Pass: ClosedClosed as of May 7th. Happy summer, see ya when the snow flies!
Twentymile: ClosedClosed as of April 13th
Seward District
Carter Lake: ClosedClosed as of 4/27
Lost Lake Trail: ClosedClosed as of April 13th
Primrose Trail: ClosedClosed as of April 13th
Resurrection Pass Trail: ClosedClosed as of April 20th
Snug Harbor: ClosedClosed as of 4/27
South Fork Snow River Corridor: ClosedClosed as of April 13th
Summit Lake: ClosedClosed as of April 20th

Subscribe to the Turnagain Area Avalanche Advisory:

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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