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Avalanche Advisory

Turnagain Area Avalanche Advisory
Thursday, March 8th 2018 5:52 am by Wendy Wagner
ARCHIVED ADVISORY - All advisories expire after 24 hours from the posting date/time.
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The Bottom Line

The avalanche danger will rise to CONSIDERABLE today and to HIGH tonight as a potent storm moves into Western Prince William Sound. Heavy snowfall combined with strong wind will create dangerous avalanche conditions by this evening. Natural and human triggered avalanches will become likely. Elevated caution is advised today as snow begins to accumulate and travel in avalanche terrain is NOT recommended tonight

*The National Weather Service has issued a SPECIAL WEATHER STATEMENT*


 Show the Complete North American Avalanche Danger Scale
4 High Alpine / Above 2,500' Travel Advice: Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
4 High Treeline / 1,000'-2,500' Travel Advice: Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
3 Considerable Below Treeline / Below 1,000' Travel Advice: Dangerous avalanche conditions. Careful snowpack evaluation, cautious routefinding and conservative decision-making essential.
Avalanche Problem 1

Storm Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

A strong storm beginning this morning will bring up to a foot of new snow today and an additional 1-2 feet tonight. This will rapidly load slopes and if the storm verifies, large natural avalanches will be likely along with likely human triggered slides. These avalanches will be large and could run far into runout zones. Areas closer to the coast, such as Portage and Placer valleys will see the majority of the snow, but heavy snow is also expected in Girdwood Valley, Turnagain Pass, Moose Pass and Seward. The Summit Lake area is likely to see lesser amounts (16-20" storm total compared to 3+' of storm total for the other zones). 

Storm snow avalanches will be the primary concern as the day progresses. Avalanche danger is directly related to how much new snow has fallen combined with increasing winds loading slopes. The new snow will fall on a few inches of light powder that sits over hard wind affected surfaces. This set-up creates a slick surface for the new snow and bonding is expected to be poor. It is a day to be mindful of changing conditions and leaving the mountains before sun down. 

Types of storm snow avalanches that are expected to be occurring naturally tonight:

Wind slabs:  1-3 feet of new snow combined with strong Easterly winds = wind slabs up to 4-6' thick 
Storm slabs:  Soft slabs are expected in areas out of the wind due to rapid loading and poor bonding with the old surface
Loose snow sluffs:  Sluffing in the new snow on steep slopes is expected
Cornice falls:  Fresh cornices are expected to build and break off during the storm, often this will trigger a wind slab or storm slab below
 

Forecaster excercise:  Imagine 2-3+ feet of new snow by tomorrow morning, with wind, falling on the slopes pictured below. We are powder starved and in need of snow, but sticking well out of avalanche terrain (including runout areas) as the storm sets in will be key.

Seattle Ridge, with the up-track on right of photo

 

Southwest face of Sunburst

 

Magnum Ridge 


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Avalanche Problem 2

Persistent Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

New snow and wind loading could overload old weak layers buried in the existing snowpack. In the case this occurs, or a storm related avalanche 'steps down' into a deeper weak layer, avalanches could be very large and send debris well into valley bottoms. Another reason to let the mountains sit during the storm. Areas on the South end of the Pass and the Summit Lake area are most suspect for having deeper weak layers release.

Weak faceted snow on a crust sits about a foot below the surface in the mid-elevations in the Summit Lake region. An example of weak layers in the pack that could become overloaded and release during the storm

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Mountain Weather

Partly sunny skies with valley fog was over the region yesterday. Ridgetop winds were light from the North and temperatures were in the teens along ridgelines and in the mid 20'sF at 1,000'. Overnight, winds shifted to the East and are increasing along with temperatures as a small, but strong, low pressure develops South of Seward. 

Today, Thursday, a quick and potent storm will move in with heavy snowfall and strong wind. Snow has just begun this morning and by 6pm this evening should bring 8-14" (.5-.7" water). From 6pm to 6am tomorrow, another 12-24" (1-1.5" water) is expected. Favored areas will be Placer and Portage Valleys with Summit Lake likely seeing the least in these ranges. Ridgetop winds will be Easterly in the 30-50mph with stronger gusts. Temperatures should remain close to 30F at sea level (#snowtosealevel) and in the low 20'sF along ridgetops.

Quote from the NWS's Special Weather Statement:

Depending on the track the low center takes, the road system from
Seward to Portage could see snow accumulations anywhere from one
to three feet through Friday evening. Heaviest snowfall is
expected to be from Moose Pass southward.

Friday, this system will slowly exit and light snow showers are expected. Ridgetop winds are forecast to remain strong from the East and South however. Periods of light snowfall should continue through the weekend as a Southerly flow keeps pumping moisture into our area.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 22  trace  0.05  67 
Summit Lake (1400') 20  29 
Alyeska Mid (1700') 23  trace  0.05  59 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 15  NE   25
Seattle Ridge(2400') 20  SE   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 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
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