ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Sunday February 24th, 2019
Posted by Wendy Wagner on 02/24/19 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 on slopes over 35 degrees above 2,000′. Triggering an older wind slab resting on weaker faceted snow is possible. Slabs could be anywhere from 4″ to 2′ thick depending on prior windloading. Steep rocky terrain will be the most likely place to find one of these avalanches.  Additionally, sunshine with warm temperatures may cause wet sluffs on steep southerly terrain. As always, give cornices a wide berth and limit exposure under glide cracks.

GIRDWOOD / PORTAGE / PLACER:   Old wind slabs could be deeper, up to 2′ thick.  

SUMMIT LAKE / JOHNSON PASS:    Triggering a larger, more dangerous slab remains a concern due to  various old weak layers in the mid and base of the snowpack.  Conservative terrain choices and a cautious mindset is advised  if choosing to venture into the further reaches of this area.

SEWARD / LOST LAKE:   Old wind slabs up to 2′ in depth could be found and triggered in this area on steep windloaded slopes.

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Treeline
/ 1,000'-2,500'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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.
     
Avalanche Problem 1
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

After quite the pummeling by the outflow winds last week, the snowpack has had a couple days to rest under clear skies and light winds. As we head into today and into the work week, the quiet weather is expected to remain, allowing the snowpack to continue to adjust and stabilize. We have not heard of, or seen, any avalanche activity since Thursday when the NW wind event wreaked havoc. This event triggered large natural avalanches in the Summit Lake area on the Kenai as well as some other areas along Turnagain Arm and in the Portage Valley. The snow surface above treeline is a mixed bag of sastrugi, scoured areas, wind drifts, wind crust and wind slabs. That said, many areas are reported to still harbor soft fun snow and escaped the brunt of the damage. 

The avalanche issues today revolve around older weak layers within the snowpack (persistent weak layers). The most recent weak layer was buried a week ago and sits around a foot deep under the wind affected snow and wind slabs. This layer has been showing signs that it could still be reactive but is on a downward trend. The second layer down is the MLK Jr buried surface hoar. This layer is showing signs of only being reactive in the Summit Lake zone. Also in the Summit Lake region is weak snow near the ground. 

If you are headed out today watch for:
    –  Wind slabs on steep slopes. Rocky terrain with unsupported slopes (i.e. steep slopes with a cliff underneath) 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 high elevation temperatures can help loosen these monsters and with good days and ridgeline travel, don’t forget to give cornices a wide berth.
    –  Keeping with safe travel protocol such as exposing one person at a time.

 

           

Make sure to check out Heather’s write up from the natural avalanche cycle on Fresno in the Summit area on 2/21. This event highlights the importance of matching avalanche terrain to the potential size of an avalanche. In this case, a portion of the fast moving debris overshot the main avalanche path and pushed up and over a subridge of hemlocks. 

 

For the snow geeks out there, a persistent weak layer round-up:

  • Valentine’s Layer:  Small faceted grains, 10″-16″ deep, last avalanches seen on this layer 2/21. Layer responsible for the 2/19 snowboarder remotely triggered slab on Seattle Ridge.
  • MLK Jr Layer:  Buried surface hoar, 2-3′ deep, last avalanches seen in this layer 2/21 in the Summit Lake region.
  • Basal facets (large faceted snow near the ground): This layer has only been found in the Summit Lake region and produced at least one very large avalanche on Thursday, 2/21.

 

 

Turnagain Pass’s beloved Tincan ridge yesterday, Saturday.

 

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

New glide cracks are opening up around our region and some are filled in and apparently not opening up. The last known glide release was on the south side of Goat mountain in Girdwood Valley on Tuesday 2/19. 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. 


Small older wind slab on the lower SW shoulder of Lipps. Also, an old filled in glide crack is not easily seen, but the ‘crumpled’ up snow in the middle/lower portion of main slope is discernible. 

 

Mountain Weather

Yesterday:   Sunny skies prevailed region-wide. Ridgetop winds were light from a generally westerly direction. Temperatures hit 30F at many lower elevations with ridgelines in the 20’sF. A strong inversion has set up again overnight with cold air pooling in valley bottoms and at sea level. This morning temperatures at the mid and upper elevations are in the mid 20’sF while valley bottoms and sea level temperatures are in the single digits.  

Today:   Other than a possibility for some valley fog, today will be a carbon copy of yesterday. Sunny skies, light winds from the west and temperatures in the 25-30F range above the cold air trapped in valley bottoms. The cold air should mix out again in the afternoon warming parking lots and lower elevations to near 30F.  

Tomorrow:   The ridge of high pressure over the region looks to remain in place through the work week and possibly into the weekend. Sunny skies, some chance for valley fog, cold temperatures in low-lying areas and warm temperatures at the higher elevations are to be expected. Don’t forget your sunglasses!

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   0   0   61  
Summit Lake (1400′) 15   0    0 30  
Alyeska Mid (1700′) 24   0   0   57  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   W   4   9  
Seattle Ridge (2400′) 27   Variable   2   5