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Avalanche Advisory

Turnagain Area Avalanche Advisory
Sunday, February 11th 2018 5:31 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 currently MODERATE and expected to rise to CONSIDERABLE late tonight and tomorrow in conjunction with snowfall and strong wind. For the daylight hours today, wind slabs up to a foot thick along with lingering larger slabs up to 2' thick will be possible to trigger on slopes with wind deposited snow. Watch for changing conditions and in the event snowfall begins earlier than expected, avalanche danger will begin to rise.


 Show the Complete North American Avalanche Danger Scale
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.
1 Low Below Treeline / Below 1,000' Travel Advice: Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1

Persistent Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

One more breezy overcast day is on tap before snowfall begins to fall in earnest tonight. We could see light snowfall add a couple inches through the day, but the bulk of the expected storm snow (6-12") should fall late tonight through Monday morning. Keep in mind, avalanche danger will rise along with snowfall. The new snow will fall on a weak surface and storm snow avalanches should be expected if this storm verifies. 

Prior to tonight's snow, our main concern for triggering an avalanche remains with slopes that have seen, or are seeing, wind loading. Ridgetop winds have been moderate to strong over the past 48 hours from a generally East direction. These have scoured some ridges and loaded leeward slopes with the existing loose snow available for transport. Small pockets of wind slabs could be triggered on these leeward slopes and these should be relatively easy to identify (smooth pillowed surfaces and stiffer snow over softer snow). More concerning however, is the possibility of finding a wind slab that is sitting on buried surface hoar 1-2' deep. There are multiple layers of buried surface hoar  that sit in the top 2' of the snowpack, which may now be overloaded by wind deposited snow. These types of slabs could be larger than expected and break deeper than your 'typical' wind slab. If you head out today, watch for how the winds are transporting the snow and avoid wind loaded slopes. Additionally, slabs may range from soft or hard and have the potential to break above you once committed to a steep slope. 

Wind scouring along the South end of Seattle Ridge yesterday, looking SW from the Eddies ridge (photo: Allen Dahl) 

 

Wind transport just above the trees on Eddies (video: Allen Dahl). Winds are not only loading certain slopes and scouring others, but creating wind crusts as well.


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Additional Concern

Deep Persistent Slabs

Our snowpack structure is generally poor and weak snow can still be found in the bottom portion of the snowpack at the higher elevations, above 3,000'. Shallow snowpack zones, such as on the South end of Turnagain Pass, Summit Lake and portions of the Girdwood Valley are more pronounced. Although triggering an avalanche breaking deeper in the pack is unlikely, a chance does remain, as a party found the right trigger spot last weekend in the Twin Peaks area. It is worth keeping this poor structure in mind and considering a conservative approach to avoid large steep terrain with shallow snow cover or exposed rock. 


Mountain Weather

Overcast skies filled the region yesterday with windy conditions near and above the trees. Ridgetop winds averaged 15-25mph with gusts to 40mph from a generally East direction. Temperatures were on the warm side, low 20'sF in the Alpine and 30F at 1,000'. A few flurries fell overnight with around an inch of snow recorded at mid-elevations in Girdwood Valley and Turnagain Pass.

Today, a quick moving warm(ish) storm is moving into the area. Snowfall is expected to be light through the daylight hours, with around 1-3" accumulation, then intensify this evening through Monday morning, adding another 6-12". Ridgetop winds are forecast to shift Southeasterly and remain in the 10-20mph range until tonight when they bump up into the 25-35mph range with the heavier snowfall. Temperatures are expected to rise to the mid 20'sF along ridgetops and the mid 30'sF at sea level, this will bring a rain/snow mix to sea level and possibly raise the snow line to ~500'-1,000'. 

Monday, light snowfall is expected as the system moves out with a possible 1-3" accumulation. Ridgetop winds look to remain moderate to strong with a shift to Westerly bringing cooler temperatures. Storm total by Monday evening is looking to be around 6-12" of medium density snow at the mid to upper-elevations. 

NWS graphic for the Monday storm:

 

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 27  0.06  61 
Summit Lake (1400') 28  19 
Alyeska Mid (1700') 29  0.1  52 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 21  NE  13  41 
Seattle Ridge(2400') 25  SE  21  41 

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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