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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, March 6th, 2018 - 7:00AM
Expires
Wed, March 7th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger remains  MODERATE  above 1000′ on all aspects. Triggering a large slab avalanche  breaking in weak layers 1-3′ deep in the snowpack  remains possible.  Watch for old wind slabs along ridgelines and in cross-loaded gullies. Evaluate snow and terrain carefully.  

Special Announcements

Human triggered avalanches continue to be reported region-wide, one at  Hatcher Pass  and another  in  the  Western Chugach (Hiland Road)  occurred Sunday. Snow last week followed by strong winds increased the avalanche conditions, which may continue to linger in many areas of Southcentral, Alaska.

Tue, March 6th, 2018
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Observers continue to find the buried persistent weak layers in the snowpack to be reactive and the possibility of triggering a large avalanche remains.  One observer yesterday found the January 21st buried surface hoar, described the snowpack they found in Lynx Creek as “spooky” and changed their plan due to concerning snowpit test results. This buried surface hoar layer was also observed to be reactive on Magnum yesterday as well. As snow starts today and hopefully falls throughout the week it will be important to remember the current state of the snowpack. Buried 1-2′ deep are facets sitting on a crust at the mid-elevations and 1-3′ deep is a buried surface hoar/facet combo at the higher elevations. The slab over the weak layers could be very hard if it is in terrain that was affected by the strong winds last week. This was the case in the skier triggered avalanche in Summit Lake a week ago, the mid-elevation faceted layer was under very hard wind-affected snow.  Although the heart of Turnagain Pass has the buried weak layers, they are more pronounced and developed on the Southern end of Turnagain Pass and in Summit Lake where the snowpack is shallower. Areas to the North, such as Crow Pass, are also suspect along with those that have not seen much traffic this season. Using safe travel protocols and assessing the consequences if a slab does release is key in choosing terrain. Weak layers like these can become more reactive after even a small additional load. As the snow falls this week keep that in mind and as always be alert for signs of instability. 

Wind slabs: Hard wind-affected snow on steep, unsupported slopes may still triggered if you find the wrong spot. Be suspect of very stiff snow over soft snow or hollow sounding snow near upper elevation ridgelines and cross-loaded gullies. If winds really pick up this afternoon pay attention to changing conditions. 

Snowpit on Magnum yesterday at 3100′. The January 21st buried surface hoar is easy to spot and still reactive in stability tests. 

 

Cross-loaded gullies on Seattle Ridge

 

Weather
Tue, March 6th, 2018

Yesterday was mostly to partly cloudy with some afternoon sunshine peeking through. Temperatures were in the teens to mid 20Fs. Winds were light and variable. Overnight temperatures were in the teens to low 20Fs and winds remained light.  

Today will be mostly cloudy with afternoon snow showers. Temperatures will be in the teens to high 20Fs and winds will start off light and increase from the Southeast 10-20 mph gusting into the 30s. Tonight there is snow in the forecast with 2-6″ possible. Temperatures will be a bit cooler in the low 20Fs to mid teens. Winds will continue from the Southeast.  

Tomorrow’s forecast has continued snow showers in the morning, clearing in the afternoon and then more snow in the forecast overnight. The next low pressure is lining up for more snow on Thursday but details are uncertain. However, the week ahead looks to be snowy.   From the NWS discussion this morning:  Despite March and April being  the driest months of the year (climatologically), Mother Nature is  giving that statistic a run for its money. Fingers crossed!    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22    0 0    68
Summit Lake (1400′)  12      0  0      29
Alyeska Mid (1700′)  19      0  0        59  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 W    6  14
Seattle Ridge (2400′)  20 varible    3  11
Observations
Recent Observations for Turnagain Pass
Date Region Location
02/25/24 Turnagain Observation: Kickstep NE Bowl
02/24/24 Turnagain Observation: TinCan Backdoor/ Center Ridge
02/22/24 Turnagain Avalanche: Lynx Creek
02/22/24 Turnagain Observation: Turnagain, Seattle, Mt Ascension
02/21/24 Turnagain Observation: Tincan Trees
02/21/24 Turnagain Observation: Sunburst
02/20/24 Turnagain Avalanche: Tincan
02/20/24 Turnagain Observation: Seward Highway across from Johnson Pass TH
02/19/24 Turnagain Avalanche: Base of Seattle Ridge
02/18/24 Turnagain Observation: Lynx creek
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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.