ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday February 14th, 2019
Posted by Heather Thamm on 02/14/19 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. There is also a chance of triggering a larger slab avalanche 1-3′ thick on slopes 35 degrees and steeper. Give cornices a wide berth, avoid travel under glide cracks. Several glide cracks have released over the last few days.

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. Strong winds yesterday have added stress to the snowpack across the region. In addition to wind slabs keep in mind this zone may have more potential for triggering a larger avalanche. 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.

Special Announcements

Heading to Hatcher Pass? Be sure to check out the Hatcher Pass Avalanche Center  mid-week snow and avalanche summary and observations page for current conditions.

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

A strong outflow wind event triggered a handful of natural wind slab avalanches across our region yesterday. Most of these occurred in Summit Lake, but one was seen near Bertha Creek on Seattle Ridge and one on Max’s Mountain in Girdwood. Northwest winds averaging 20-40mph were seen drifting snow along many ridge tops. This wind direction creates unusual wind loading patterns opposite our normal Easterly storm track direction. Aspects that are shallow and wind scoured will be more loaded than usual. Another challenge is Sunburst weather station is sheltered from this wind direction and not necessarily representative of the full extent of this wind event. For comparison a weather station along the Alaska railroad corridor, South of Grandview, recorded NW winds 30-40mph with gusts in the 60mph’s most of the day. Keep this in mind if you head into the mountains. Hard supportable snow should be suspect and can release once well onto a slope. Wind slabs could be small and shallow or larger if they step down to older weak snow. Northwest winds have backed down overnight, but will remain Moderate, 10-25mph at higher elevations. Any active wind loading or shooting cracks will be obvious clues wind slabs are still tender. 

CORNICES: Cornices are looming large in some of the Alpine terrain. Yesterday’s wind may have added additional stress. Give them an extra wide berth as they often break farther back than expected.

A wind slab released sometime yesterday afternoon on Max’s West face in Girdwood Valley. This avalanche was likely human triggered, based on the ski tracks above and below, but those details have not been confirmed. 

 

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

Yesterday’s wind event added stress to older weaker layers within our existing snowpack. This was the case in Summit Lake where several avalanches released near the ground in shallow snowpack zones on East facing aspects of Summit Peak. In deeper snowpack zones, like Turnagain and Girdwood, a wide spread layer of buried surface hoar sits 1-3’ below the surface. For those just tuning in, the MLK buried surface hoar (buried on Martin Luther King day) was the culprit in numerous human triggered avalanches over the last two and half weeks. The last human triggered avalanche occurred 8 days ago on Eddies, remotely triggered from a ridge onto a steep, unsupported slope. The question remains, can person or snowmachine tip the balance? At this point the jury is out, but due to the nature of buried surface hoar and yesterday’s wind loading event, this layer is still suspect.

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.

 

Strong winds triggered several wind slabs that stepped down to older weaker snow on East facing terrain of Summit Peak in Summit Lake. Summit Lake in general has a shallower and weak snowpack than Turnagain. Areas like Johnson Pass and Lynx Creek may also harbor a similar structure.

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

Glide cracks are on the move and becoming more active this week. Yesterday a glide avalanched in Summit Lake and another one release in Girdwood Valley on Monday, on Goat Mountain. There are a handful of new cracks opening up including one spotted yesterday on Seattle Ridge, Repeat Offender, just South of the up-track. Several glide cracks threaten popular skin track routes on the non-motorized side of the road, like the South facing slopes of Lipps and Magnum. Glide cracks are unpredictable, not associated with human triggers, and can release without warning at any time. We’ve seen glides avalanche when temperatures drop into the single digits, like yesterday, and when temperatures are above freezing. The best way to manage this problem is to identify their location and avoid traveling under their runout. This can be especially frustrating if glide cracks are opening up in popular terrain.

A glide crack opening on Seattle Ridge, on a Southeast aspect of Repeat Offender.

Mountain Weather

Yesterday: Skies were clear and gusty outflow winds dominated our region. Northwest winds in the upper elevations were Moderate to Strong, averaging 15-40mph. Sunburst weather is commonly sheltered from NW winds and recorded moderate winds 10-15mph with gusts to 30mph. As opposed to the MP 43 weather station along the Railroad corridor, South of Grandview, MP 43 recorded steady NW winds 30-40’s mph with gusts in the 60’s. Temperatures along ridgetops dropped into the single digits (F) yesterday morning. Temperatures near sea level fell into the teens (F). No precipitation was recorded.

Today: Skies will be clear and sunny. Moderate Northwest outflow winds will continue, but will be more Moderate 10-20mph near ridge tops. Some higher elevation peaks and gap zones like Portage Pass may see NW winds 15-30 mph. Temperatures along ridge tops will remain in the single digits (F) and temps near sea level will remain in the teens (F). No precipitation is expected.

Tomorrow: A continuation of cold temps and clear skies are on tap tomorrow. Outflow winds will diminish by tomorrow morning and shift to more of Easterly direction by mid-day. Temperatures will remain cool, low temps in the single digits and teens.   Friday evening into Saturday morning temperatures will start to increase as a low approaches the Gulf of Alaska. Saturday and into Sunday looks like our next chance for snow showers.

 *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′) 16   0   0   57  
Summit Lake (1400′) 18   0   0   24  
Alyeska Mid (1700′) 16    0 0   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 5   WNW   10   30  
Seattle Ridge (2400′) 13    *N/A  *N/A    *N/A