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Avalanche Advisory

Turnagain Area Avalanche Advisory
Thursday, January 18th 2018 4:40 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 CONSIDERABLE on all slopes above 1,000'. Triggering a 2-3+' large slab avalanche, composed of the new snow, is likely above 1,500'. Naturally occurring avalanches and cornice fall are also possible. Additionally, avalanches could break in deeper layers of the snowpack, causing a much larger slide and a very dangerous avalanche. 

Below 1,000' the avalanche danger is LOW due to a crust forming overnight, and very little snow at this elevation band. 

*Today is a day to let the mountains adjust to 4-6+’ of snow from two storm cycles this week. Cautious route-finding and conservative decision-making will be essential. 


 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.
1 Low Below Treeline / Below 1,000' Travel Advice: Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1

Storm Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

Clear skies following a big storm is a red flag warning for human triggered avalanches. Over the last three days rain, snow and strong winds impacted our region with an additional 2-3’ of snow in the upper elevations on top of 2-3’ that fell over the weekend. Rain/snow line reached around 3000’ on Tuesday and slowly dropped to ~1300’ yesterday morning as temps cooled near the end of the storm. There was a widespread natural wet avalanche cycle observed on Tuesday in steep channelled terrain from Girdwood to Summit Lake.  

STORM TOTALS above 2,500' (1/15-1/17)

Turnagain Pass     20-30"  (2.3” of snow water equivalent)
Girdwood Valley    20-30”  (2.1” of snow water equivalent)
Summit Lake         6-10”  (0.6” of snow water equivalent)

Don’t Forget!!! This was the second storm in one week. Basically 4-6+’ of snow is sitting on old weak surfaces (surface hoar and facets) from last week. In between these two storms, several human triggered avalanches occurred on Seattle Ridge  and Eddies, without incident, and stability tests were showing propagation potential in older layers of the snowpack.  More time and lots of patience is required for all of this new snow to adjust. If you are headed out - keep these points in mind

  1. Slabs triggered will be deep (2-3+ feet) and could step down to a deeper layer. These are dangerous and unmanageable avalanches
  2. Due to the depth of the storm snow, no signs of instability may be present before someone triggers a slab
  3. Remote triggering an avalanche from below, or near a slope is possible
  4. Cooling temperatures will be forming a crust at lower elevation and stabilizing the snowpack below 1500’- don’t let this fool you into thinking the upper elevations are stabilizing quickly

Dry snow is covering and insulating a wet layer of snow within the new storm snow. Until this wet layer freezes, triggering an avalnche will remain possible in mid elevations. 

 

Most of the gullies along Seattle Ridge are full of new debris from a wet snow avalanche cycle that occured on Tuesday. 

 

 


Avalanche Problem 2

Deep Persistent Slabs

Almost Certain
Very Likely
Likely
Possible
Unlikely

Chance

Historic
Very Large
Large
Small

Size

Weak layers within the snowpack have the potential to re-activate with the added load of this week's new snow (5” of water weight). Additionally, avalanches in the storm snow, discussed above, could step down to these deeper layers and produce a very large avalanche depending on the size of the slope. In short, two layers of buried surface hoar and near surface facets sit roughly 4-8’ deep at this point and is a concern at elevations above 2,000'. Basal facets, near the ground, remain a concern at elevations above 3,000'. At this point very little is known about reactivity of these weak layers, and its going to take time and patience for the snowpack to adjust. 

*Giving the snowpack time to heal from these storms is key. Sticking to low angle terrain with nothing steeper above is recommended. Remember, it's the first 2 days after a storm where most avalanche fatalities occur. Although there is nice powder at the upper elevations that can lure us, now is not the time to be sampling it.

A natural avalanche near Bertha Cr on the Southern end of Seattle Ridge (SE aspect) that broke into older layers of the snowpack. 


Additional Concern

Cornice

Cornices have been growing and were unstable in between the two storm cycles this week. Triggering a cornice is likely if you get too close.  A cornice fall could trigger a very large avalanche on the slope below. Give these features lots of space and remember they can break further back from a ridge than expected. 


Mountain Weather

Yesterday morning a potent storm ended with a cooling trend. Freezing temps (32F) creeped down in elevation, and rain/snow line moved from around 3000’ the night before to ~1300’ by late morning. The most intense precipitation ended by 6am yesterday, but rain and snow showers were observed throughout the region most of the day. No measurable amount was recorded at Center Ridge weather station, but .25” was recorded at the Turngain Pass DOT Lot. Girdwood midway station picked up ~.3” of water late morning, which fell as a few inches of heavy wet snow at 1700’. Easterly ridge top winds were moderate, averaging in the 20-40mph becoming light by early evening. Overnight Satellite images indicate clearing skies and temps dropped to mid 20F’s 1000’ at Turnagain Pass.  

Today expect clear skies, temperatures in between 20-30F and light Northerly winds. No precipitation is expected. 

Cold and clear weather is expected over the next two days as high pressure from Siberia settles in over interior Alaska. 

**Seattle Ridge weather station stopped recording wind data yesterday evening due to rime forming on the anemometer 



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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 29  0 56 
Summit Lake (1400') 29  15 
Alyeska Mid (1700') 30  3 0.3  42 

 

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

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 23  ENE  13  44 
Seattle Ridge(2400') 27  **SE  **7  **31 

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.


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