ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Wednesday February 20th, 2019
Posted by Aleph Johnston-Bloom on 02/20/19 at 7:00 am.
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  due to recent snowfall and wind loading.  Human triggered slab avalanches 1-2′ thick are possible on wind loaded slopes 35 degrees and steeper.  Additionally, give cornices a wide berth and avoid travel under glide cracks. Watch for changing conditions as the storm arrives today and look for signs of instability.  

GIRDWOOD / PORTAGE / PLACER:  More snow fell north of Turnagain Pass over the weekend and wind slabs may be deeper in this part of the advisory area today.  

SUMMIT LAKE / JOHNSON PASS:   This area has a very poor snowpack structure with multiple persistent weak layers. Triggering an avalanche in recently wind loaded terrain has the potential to initiate a more dangerous slab,  breaking deeper in the snowpack.  

SEWARD / LOST LAKE:   Recent snow and wind loading has impacted this region as well and triggering a slab avalanche is possible today. Look for signs of instability. Check out a Mt. Marathon observation from yesterday  HERE.  

 

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.
2. Moderate
Below Treeline
/ Below 1,000'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Special Announcements

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

Northwest winds averaging 10-20 mph with gusts into the 30s were sustained and strong enough to naturally trigger small wind slabs yesterday. These wind slabs were observed throughout the advisory area from Portage to Summit Lake. There were also a handful of human triggered wind slabs reported from Turnagain and Placer. In some spots the winds were just enough to increase the connectivity/stiffness of the storm slab. In Turnagain northwest winds have an interesting way of channeling from the south on the southern end of the pass in some terrain i.e. scouring the southwest side of Sunburst and loading onto the north. Towards the north end of the pass northwest winds top load the front side of Seattle Ridge including the Repeat Offender slide path near the uptrack. Keep the somewhat unusual loading patterns in mind as you choose terrain today.  Triggering a wind slab is possible on steep wind loaded slopes. The winds are forecast to pick up again this afternoon from the opposite direction blowing 10-20 mph with gusts into the 30s from the southeast. With a couple of inches of snow likely this afternoon expect blowing snow.  Be on the lookout for drifting and cracking and listen for hollow sounding snow indicating harder slab over soft snow.  Be suspect of hard snow on steep convex slopes. Wind slabs can often break when you are well onto the slope. A couple of the human triggered avalanches were reported to have been remotely triggered.  Remember slab depth may be thicker on the northern end of Turnagain Pass, in Placer, Portage and Girdwood. More snow fell in this part of the advisory area over the holiday weekend. Practice safe travel and evaluate snow and terrain carefully. Cornices are looming large in some of the Alpine terrain. Give them an extra wide berth as they often break farther back than expected. 

Snowboard triggered avalanche Seattle Ridge, 2-19-19. Photo: David Evans. 

Small storm slab at 2000′ above the Spencer Bench Cabin. Likely triggered by two sledders on the ridge, 2-19-19. Photo: Graham Predeger

Northwest winds flagging as they are channeled from the south on Magnum around 12 pm, 2-19-19.  

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

Periphery zones such as Summit Lake and Johnson Pass harbor a poor overall snowpack structure with a variety of weak layers, including facets and the MLK buried surface hoar. The buried surface hoar has been documented throughout the entire advisory region roughly 1-3′ below the snow surface. Although these persistent weak layers have not been reactive lately, additional load from recent snow and increasing winds may start to tip the balance. It is good to keep in mind that triggering an avalanche today could to step down into old weak layers and initiate a larger more dangerous slide. This is more likely in and around Summit Lake where the structure is the most suspect. 

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 unpredictable, not associated with human triggers, and can release without warning at any time. We received a report that another glide crack released yesterday on the south side of Goat mountain in Crow Creek. The best way to manage this problem is to avoid traveling on slopes directly below glide cracks. 

Mountain Weather

Yesterday: Scattered clouds and sunshine. Northwest winds 10-20 mph with gusts into the 30s. Temperatures in the teens to 20Fs. Winds became calm in the evening and shifted to the east. Skies were mostly clear overnight with clouds building in the early morning.  

Today: Mostly cloudy skies and snow starting in the afternoon, 1-3″ of snow is forecast.   Temperatures remain in the teens to 20Fs. Southeast winds will increase to 10-20 mph with gusts into the 30s and continue through the night. Overnight snow also continues with another 4-8″.  

Tomorrow:   Mostly cloudy with snow showers ending in the morning. Winds will shift from the southeast to the northwest similar to the winds yesterday. Temperatures will be in the 20Fs. A ridge of high pressure will develop over the area on Friday and is forecast to dictate the weather through the weekend. Temperatures are forecast to be unseasonably warm… Stay tuned!  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 19   0    0 61  
Summit Lake (1400′) 13   0    0 29  
Alyeska Mid (1700′) 19   0    0 58  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  12  NW 6   29  
Seattle Ridge (2400′)   18     NW   7   35