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Avalanche Advisory

Turnagain Area Avalanche Advisory
Monday, February 26th 2018 6:04 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 is CONSIDERABLE above treeline due to 6-12" of new snow expected with wind. Natural storm snow avalanches will be possible and human triggered avalanches likely. The danger will be MODERATE in sheltered areas in the treeline band and below where loose snow sluffs composed of the new snow are expected. Debris could run far as these will be dry and fast running slides if the storm verifies. Additionally, old weak layers deeper in the pack may be triggered, creating a larger avalanche.


 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.
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.
Special Announcement

Dangerous avalanche conditions are expected in areas seeing new snow such as Hatcher Pass and the Western Chugach. See the Hatcher Pass Avalanche Center Facebook page - avalanche danger has increased to CONSIDERABLE. Image below is from the National Weather Service.


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Avalanche Problem 1

Storm Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

Storm snow avalanches will be the main concern as a quick moving system is impacting the region. Snowfall totals so far are 4+" in Girdwood Valley and 1-2" at Turnagain Pass. Another 6-8" is expected through the day (to sea level), yet Turnagain Pass may only see a few more inches. The snow is very low-density and should sluff easily on steep slopes, both naturally as well as human triggered. The moderate to strong winds with the storm have a Southerly component, which is unusual and could create wind slabs in unusual places. Wind slabs could be up to a foot thick and will depend on how much snow has fallen. If found, they are likely to be soft and easy to trigger. In areas seeing more than 6" of new snow, and definitely areas with 10" or more, watch for soft storm slabs. 

It is a day to pay attention to how much new snow has fallen and what the winds are doing. If you are in an area with only a few inches of new snow, the main concern will be finding an old wind slab or triggering an avalanche in deeper weak layers of the pack (discussed below). Quick hand pits will be a good tool for assessing new snow amounts. They are also good for determining if there is stronger snow over weaker snow - signs that a wind or storm slab is present.

 

Image below is from Fresno Ridge yesterday (Summit Lake area). This small pocket triggered on a test slope in the trees shows Friday's new snow overloading a faceted weak layer sitting on a crust. Be aware of old weak layers that may become overloaded with today's new snow. (Photo: Chis McNeil)


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Avalanche Problem 2

Persistent Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

The snowpack is very weak in general and we need to keep in mind that larger slides breaking in persistent weak layers could occur; even though most of today's activity will be relegated to the new snow. The new load from today is adding to the weight from Friday and earlier in the week. This is incremental loading and can slowly overload weak layers. Furthermore, new snow avalanches have the potential to step down to these layers. In the upper elevations a layer of buried surface hoar from Jan. 21st continues to show signs of reactivity and in the mid-elevations a layer of facets over a melt-freeze crust is suspect. 

Deep Persistent Slabs:  At the high elevations above 3,000', deeper persistent layers could 'wake up' if the wrong spot is found. Old weak layers of facets and buried surface hoar sit in the bottom half of the snowpack. This structure is most pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass, the Summit Lake area and Crow Pass. 

Photo below is from the mid-elevation slopes of Cornbiscuit (Korn/Biskis) yesterday where a layer of very weak facets sit on a crust below the February snow. 


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Mountain Weather

Sunny skies were over the region yesterday. Ridgetop winds were 15-25mph from the Northwest before backing off around noon. Temperatures were in the teens at most locations. Overnight a low pressure has moved in from the West bringing light snowfall. Between 2-4" of snow has fallen as of 6am (.1 and .2" of water equivalent). Winds shifted to Easterly with the snowfall and are blowing Southerly 10-20mph. 

Today, snowfall will continue as the low pressure moves Eastward. We are expecting an additional 4-8" of low-density snow (.3-.4 water equivalent) through the day before the system moves out this evening. The storm track is favoring the Hatcher Pass and the Front Range (Talkeetna Mtns, Western Chugach), where upwards of a foot of snow is forecast. Ridgetop winds should remain Southerly in the 10-20mph range before turning Westerly and remaining around 15-20mph overnight. Temperatures will be in the upper 20'sF at sea level and lower 20's along ridgetops. 

For Tuesday, clearing skies and windy, cold conditions are expected. Strong ridgetop winds should bring in cold air (single digits) from the Northwest. Stay tuned as to how these winds develop on tomorrow's forecast.



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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 25  1-2"  0.05  68 
Summit Lake (1400') 20  1-2"  0.1  30 
Alyeska Mid (1700') 20  4"  0.2  63 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 11  Variable   12 33 
Seattle Ridge(2400') 17  Southerly    16  58 

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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