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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 25th, 2019 - 7:00AM
Expires
Tue, February 26th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  on slopes over 35 degrees above 2,000′. Triggering a slab resting on weak faceted snow is still possible. Slabs could be anywhere from 6″ to 2′ thick depending on prior wind-loading. Additionally, pay attention to changing conditions with warm temperatures in the Alpine, especially on steep slopes in direct sunshine. As always, give cornices a wide berth and limit exposure under glide cracks.

SUMMIT LAKE / JOHNSON PASS:    Triggering a larger, more dangerous slab remains a concern due to  variety of old weak layers in the mid and base of the snowpack. Extra caution is advised. Evaluate snow and terrain carefully.  

SEWARD / LOST LAKE:   Slabs up to 2′ in depth could be found and triggered in this area on steep wind-loaded slopes.

Special Announcements

Take a minute out of your day to help the Community Snow Observations crew with gathering snow depth information in Alaska!! Website is  communitysnowobs.org. It’s easy to do, you just need a smart phone, 30 seconds and your probe to measure the snow depth.

Mon, February 25th, 2019
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
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

With a strong inversion in place some Alpine weather stations are already near or above freezing this morning. Alyeska Summit (3664′) is at 35F and the base of Alyeska (100′) is at 13F. Sunburst (3812′) is at 30F and Turnagain Pass DOT (1020′) is at 7F. Summit Lake MP 45 (3800′) is at 36F and Summit Lake DOT (1348′) is at 4F.  This is the first day to see temperatures above freezing at upper elevations. It will be important to pay attention to changing conditions. This can sometimes make triggering an avalanche on a persistent weak layer more likely. Be most suspect on steep slopes getting direct sunshine. Look for roller balls and wet sluffs, signs that the snow is heating up. The slab avalanche issues today revolve around older weak layers within the snowpack (persistent weak layers). The most recent weak layer (the Valentine’s near surface facets) sits around a foot deep under the wind affected snow and old wind slabs. This layer has been showing signs that it could still be reactive if you find the trigger point. There was a small skier triggered avalanche observed yesterday looker’s right of Ragged Bowl (near Girdwood) that is suspected to have run on this layer. The second layer down is the MLK Jr buried surface hoar. Although found throughout the advisory area this layer is showing signs of only being reactive in the Summit Lake zone. Additionally in the Summit Lake region there is weak snow near the ground. 

If you are headed out today watch for:
    –  Wind-loaded, steep, unsupported slopes are the most suspect for popping out a wind slab that may be sitting on weak snow.
    –  Larger and more dangerous avalanches are possible in the Summit Lake and Johnson/Bench peak area where a thinner snowpack exists.
    –  SUN EFFECT and moist/wet sluffs on steep rocky southerly terrain. 
    –  Cornices. Warmer temperatures at higher elevations can help loosen these monsters and with good weather and ridgeline travel, don’t forget to give cornices a wide berth.
 And remember to practice safe travel protocol! 

Observer on Max’s yesterday found no signs of instability but found the buried Valentine’s facets to still be reactive, 2.24.19

 

Large natural avalanches triggered by high winds on 2.21.19 in Summit Lake. If heading that way remember the snowpack is harboring a number of weak layers. 

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Glide cracks are unpredictable, not associated with human triggers, and can release without warning at any time.  The best way to manage this problem is to avoid traveling on slopes directly below glide cracks. A short list of known cracks in popular zones:  Magnum, Lipps, Seattle Ridge, Eddies, Lynx Ck. See a new glide crack or one that appears to be opening up? Please snap a photo and send us a quick ob!

Weather
Mon, February 25th, 2019

Yesterday: Mostly sunny with some high clouds, very light westerly winds and temperatures in the 20Fs in mid to upper elevations. Overnight valley bottoms saw temperatures in the single digits with the inversion in place.  

Today: Mostly sunny with a chance of valley fog in the morning. Light winds. With the temperature inversion early morning temperatures in the Alpine are near or above freezing and valley bottoms are in the teens to single digits. Valley bottoms temperatures are forecast to rise into the 20Fs and then dip back down overnight. Upper elevations should stay in the 30Fs to high 20Fs.  

Tomorrow: The ridge of high pressure looks to dominate the weather pattern and similar weather looks to be on tap for the remainder of the week with the high temperatures slightly increasing each day and lows in the teens at night in the valley bottoms.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23   0   0   61  
Summit Lake (1400′)  12     0   0   29  
Alyeska Mid (1700′) 24    0      0      56    

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  24  SW 3   9  
Seattle Ridge (2400′)  24   NW    1      6    
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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.
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