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Avalanche Advisory

Turnagain Area Avalanche Advisory
Saturday, March 3rd 2018 5:38 am by Heather Thamm
ARCHIVED ADVISORY - All advisories expire after 24 hours from the posting date/time.
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The Bottom Line

The avalanche danger is MODERATE above 1000’ where triggering a wind slab will be possible.  Additionally, old weak layers deeper in the pack may be triggered, creating a larger avalanche.  Evaluate snow and terrain carefully. 

**If headed South of Turnagain Pass be aware of recent avalanche activity in Summit Lake and Lost Lake areas. Click HERE for the Summit Summary and click HERE for several observations from Lost Lake. 


 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.
Special Announcement

Be aware of elevated avalanche danger in many areas of Southcentral Alaska including Lost Lake on the Kenai, the Western Chugach, Hatcher Pass, and parts of the Alaska Range. Recent natural and human triggered avalanches have occurred this week due to new snow and strong winds region wide, and more snow is expected in the Anchorage and Mat-Su areas this weekend.


Avalanche Problem 1

Wind Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

Triggering a lingering wind slab is possible today on a variety of aspects due to unusual wind loading patterns this week. Strong Westerly and Northerly winds that ended Thursday morning funneled through our zone loading North, East and South aspects. Ridgetops have been scoured down to old anti-tracks or sastrugi and leeward slopes now have the classic pillow-shaped wind slab look. An inch or two of new snow that may fall today may make it difficult to identify terrain features that have been wind loaded. Smooth supportable snow that is hollow sounding is the most suspect, especially if the slope is unsupported. Shooting cracks may not be present until committed to a slope and the whole slab releases. Several weak layers are sitting within the top 2 feet of the snowpack and triggering a wind slab could propagate a larger avalanche. (More on this below.) Although the sun is not expected today, keep in mind that its that time of year when radiation can make the snow more reactive on Southerly aspects. 

Sunburst is a good example of where W and NW and SW winds were loading a variety of aspects and crossloading many terrain features. 

 

Note the older cornices in this photo and the direction of wind loading. On Wednesday SE winds were loading NE aspects near Silvertip, Lynx and Johnson Pass while at the same time SE and SW aspects were being loaded in the Turnagain Pass.  


Avalanche Problem 2

Persistent Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

Triggering an avalanche 2+ feet thick is possible due to several weak layers buried within our snowpack. More potential exists on the Southern end of Turnagain Pass and Summit Lake zone where a shallow and weaker snowpack remains. This was evident on Wednesday during the wind event where numerous large avalanches released naturally near Silvertip Creek and in Summit Lake. In Turnagain Pass we have been tracking several weak layers buried 1-2 feet deep (facets and buried surface hoar) and both have been reactive in stability tests. This structure can be found on all aspects and will be easier to trigger in thinner areas of the snowpack. These slabs could be triggered in softer more protected snow or in places where the snow is harder and more supportable. Whumpfing has been reported by several parties this week in the mid elevation band, but so far we are not aware of any human triggered activity in Turnagain Pass.  Evaluate the terrain for consequences before committing to a slope. There is still much uncertainty around this avalanche problem. 

Deep Persistent Slabs: Keep in mind that there are deeper persistent layers that could 'wake up' if you find the wrong spot above 3,000' in the Alpine. At these high elevations, old weak layers of facets and buried surface hoar sit in the bottom half of the snowpack. This structure is also more pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass, the Summit Lake area and Crow Pass. 

In the treeline zone weak snow (facets) have been reactive in stability tests over the last few weeks. In the upper elevations a layer of buried surface hoar is mixed with facets and is also suspect. 

 


Mountain Weather

Yesterday was overcast and temperatures average in the 20F’s in the alpine with cooler temps at valley bottoms increasing into the upper 20F’s this morning as low pressure moves into the region. A trace of snow was picked up on a few weather stations. Winds were light from the East (5-15mph) most of the day. Overnight East winds on Seattle Ridge bumped into the 15-20mph range.

Today expect light snow showers in Turnagain Pass and Girdwood and an inch or two of new snow. Skies are expected to be overcast and could start to clear early this evening. Temps at sea level may reach the low 30F’s and temps in alpine will average in the mid 20F’s. Winds should be light 5-15mph from the Southeast, but may pick up into the moderate range this evening. 

Tomorrow looks like a period of clearing skies before another low pressure system move into our region Monday evening. Similar to today’s weather this system is also expected to favor Anchorage and Mat-Su. Depending on how far North the low tracks, we may see an uptick in gap winds tomorrow near Coastal areas, but these winds should remain in the Moderate range. Temperatures are expected to be in the 20s F.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 23  69 
Summit Lake (1400') 21  29 
Alyeska Mid (1700') 23  trace  .05  60 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 19  ESE  17 
Seattle Ridge(2400') 22  10   25

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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