ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Wednesday February 13th, 2019
Posted by Heather Thamm on 02/13/19 at 7:00 am.
Avalanche risk
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

The avalanche danger is MODERATE above 2000′. Triggering a small wind slab will be possible on steep, leeward terrain features due to active wind loading. There is also still a chance of triggering a large slab avalanche 2-3′ thick on slopes 35 degrees and steeper. Give cornices a wide berth, avoid travel under glide cracks.

SUMMIT LAKE / JOHNSON PASS: Areas south of Turnagain Pass harbor a thinner, weaker snowpack with multiple weak layers present including the MLK buried surface hoar. Similar strong winds will be impacting this area today. Choose terrain wisely and look for signs of instability.

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
  • TYPE
    Wind Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Today strong Northwest winds 20-30mph with gusts in the 40Fs will impact our region. This wind direction can create unusual wind loading patterns opposite our normal Easterly storm track direction. Expect drifting snow in the mid and upper-elevations where 4-6” of loose faceted snow is available for transport. Hard supportable snow should be suspect. Shooting cracks will be an obvious sign that steeper slopes could have tender wind slabs. Although wind slabs are likely to be shallow, they could be more dangerous if they were to step-down and trigger a large slab that breaks in the MLK buried surface hoar.

CORNICES: Cornices are looming large in some of the Alpine terrain. Give them an extra wide berth as they often break farther back than expected.

Yesterday afternoon Westerly winds were seen transporting snow along many ridgetops and some mid elevation zones. Silvertip Creek drainage, 1pm Feb.12, 2019

 

Avalanche Problem 2
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.
  • TYPE
    Persistent Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

The last human triggered avalanche on the MLK buried surface hoar occurred a week ago on Eddies. That avalanche was a 2′-3′ deep and was triggered remotely from the ridge on a steep, unsupported slope. Over the last week many people have been pushing into steeper terrain without incident. Strong winds will be transporting snow onto leeward aspects today. Wind loading has the potential to add additional stress to the snowpack. The question is what could tip the balance? Could wind loading do it or would it require a person or snowmachine? Lots of old tracks may still be visible or partly covered, but this doesn’t necessarily mean the slope is safe. Several tracks may be on a slope before someone finds a trigger point and the whole slope avalanches. Sastrugi, wind eroded snow, on ridgelines could also harbor poor structure due to the widespread nature of the MLK buried surface hoar. Thus remote triggering is still a possiblity.

As winds change the overlaying slab, keep the MLK surface hoar in mind and remember:

1- This weak layer is widespread in the region and seems to be particularly suspect between 2000′-2500′ due to a melt-freeze crust associated with it.

2- Use safe travel protocol. Expose only one person at a time (this includes paying attention to other groups in the area), watch partners, stop in safe zones and be rescue ready.

3- Wind loaded steep features, large connected and unsupported slopes are the most suspect. As always, one can simply avoid high consequence terrain and stick to slopes under 35 degrees with nothing steeper above to avoid the issue.

Yesterday, Feb.12, 2019 on Twin Peaks several test pits showed propagation. The weak layer involved  was the MLK buried surface hoar and melt freeze crust. Facets were also associated with this layer. 

Additional Concern
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. The 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.
  • TYPE
    Glide Avalanches

There was a glide release observed on Monday (2-11-19) in Girdwood on the lower southern shoulder off of Goat Mtn. Glide cracks are unpredictable, not associated with human triggers, and can release without warning at any time.  Look out for glide cracks and limit exposure under them. Several popular routes in Turnagain Pass, Lipps and Magnum’s South aspects, have large glide cracks that are hard to see from below and challenging to avoid. 

Glide avalanche released sometime between the evening of Feb.10th and the morning of Feb.11. Photo courtesy of B. Veatch. 

Mountain Weather

Yesterday: Skies were mostly sunny. Temperatures near sea level were in low 30Fs dropping to mid 20Fs overnight. Temperatures at ridge tops were in mid 20Fs dropping into the teens F overnight. Northwest winds were 10-20mph increased to 15-30 with gust in upper 30mph’s to low 40’s. No precipitation was recorded.

Today: Skies will be mostly sunny. Strong Northwest outflow winds at ridge tops and in Gap Valley’s will be 15-30 mph with gusts in 40mph’s. Temperatures at sea level will be in the low 20F’s. Temperatures near ridge tops will drop from mid teens to low teens F.

Tomorrow: Clear skies, cooling temps and offshore winds will continue Northwest winds 15-30mph should start to decrease Thursday afternoon. Temperatures will continue to fall into the single digits. No precipitation is expected.

Strong offshore winds expected region wide for Southcentral, Alaska

*The Seattle Ridge anemometer (wind sensor) was destroyed. We have a replacement on the way and it should be operational by mid February.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   0   0   58  
Summit Lake (1400′) 25   0   0   25  
Alyeska Mid (1700′) 27   0   0   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17   NW   7   35  
Seattle Ridge (2400′) 24   *N/A   *N/A     *N/A