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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, March 7th, 2018 - 7:00AM
Expires
Thu, March 8th, 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 is possible.  Watch for old wind slabs along ridgelines and in cross-loaded gullies. Evaluate snow and terrain carefully.  

Wed, March 7th, 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

As we anticipate a storm tomorrow it is good to remember the current state of the snowpack. Observers continue to find the buried persistent weak layers in the snowpack to be reactive and today the possibility of triggering a large avalanche remains. At the mid-elevations, buried 1-2′ deep are facets sitting on a crust and at the higher elevations, 1-3′ deep is a buried surface hoar/facet combo. 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. On Monday an observer found the buried surface hoar in Lynx Creek, described the snowpack as “spooky” and changed their plan due to concerning snowpit test results. 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. If we do get heavy snow tomorrow expect the avalanche danger to rise quickly.

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. In addition, yesterday afternoon the winds picked up blowing from the East and gusting into the 30s. Look for cracking and very shallow, fresh wind slabs near ridgelines. 

Solar warming/effects:  Sunshine this afternoon may allow for enough warming to initiate roller balls and small wet loose avalanches on steep Southerly aspects. Warming may also cause slabs to be more reactive; something to keep in mind as we choose our late afternoon terrain.  

State of the snowpack before the storm… 

 The Tenderfoot avalanche that occured last week due to very hard wind-affected snow over the buried facet/crust combination.

 

 

Weather
Wed, March 7th, 2018

Yesterday was mostly cloudy with light snow showers in the afternoon into the evening with little overall accumulation. Temperatures were in the teens to mid 20Fs. Winds were picked up in the afternoon blowing Easterly 10-20 mph with gusts into the 30s.  

Today will start cloudy and become partly sunny in the afternoon during a small window before the next system moves in. Temperatures will be in the 20Fs to low 30Fs. Winds will be light and Northerly. Clouds move in again in this evening with snow showers overnight. Winds will shift back to the East and temperatures will be in the upper teens to mid 20Fs.  

There is still some uncertainty in the storm track that is forecast to impact the area tomorrow.   The difference of the low moving into Cook Inlet versus Prince William Sound will affect how much precipitation the advisory area gets. There is a good possibility that we will see some periods of heavy snow and stronger winds. The timing and snow amounts are still to be determined. Stay tuned!  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  25 trace   0    67
Summit Lake (1400′)   18   trace    0       29  
Alyeska Mid (1700′)  22 1.4   0.11    59

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  13 ENE    11 31  
Seattle Ridge (2400′)  19 ESE    15 32  
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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.