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

GIRDWOOD / PORTAGE / PLACER: More snow fell north of Turnagain Pass over the weekend. Slabs will be deeper and there is more snow available for wind transport today.  

SUMMIT LAKE / JOHNSON PASS:   This area has a very poor snowpack structure with multiple 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 slab avalanches is possible today. 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.
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 are forecast to impact the advisory area today blowing 10-20 mph with gusts into the 30s and 40s. There is snow available for transport. Watch for blowing snow and expect slabs to become more stiff and connected in the Alpine. This wind pattern is opposite the loading direction from the storm this weekend. Be on the lookout for drifting and cracking and listen for hollow sounding snow indicating harder slab over soft snow. Observers yesterday reported an ‘upside down’ snowpack from the weekend’s snow and wind, hand pits failing on isolation and triggering small steep convexities. There was one remotely triggered small slab in Common Bowl on Tincan. Steep, unsupported slopes (35 degrees and steeper) that are wind loaded will be the most suspect today. In addition pay attention to direct sunshine. Steep southerly slopes protected from the wind may see some roller balls and may be more prone to triggering with warming as the recent storm slab becomes more cohesive. 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.

 

Remote triggered small slab on Tincan Common, 2-18-19. Photo: Amanda Compton

 Hand pit failing on isolation on Sunburst at 2700′, 2-18-19. 

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. From Girdwood to Turnagain Pass roughly 2-3′ below the snow surface sits a layer of buried surface hoar.  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. The best way to manage this problem is to avoid traveling on slopes directly below glide cracks.

Mountain Weather

Yesterday: Broken skies with periods of light snow showers, sunshine and patchy fog. Winds were easterly 5-15 mph gusting into the 20s. Winds became calm overnight and shifted to the west. Temperatures were in the 20Fs at upper elevations and mid 30Fs at sea level.  

Today: Mostly cloudy becoming mostly sunny. Northwest winds increasing this morning to 10-20 mph with gusts into the 30s and 40s, decreasing tonight becoming light and westerly. Temperatures in the 20Fs and low 30Fs. Cooling in the afternoon into the evening. Temperatures will be in the teens overnight with mostly clear skies.  

Tomorrow:  Mostly cloudy skies as the next front moves into the area. Winds shift to the east and increase to 10-20 mph with gusts into the 30s. Temperatures will be in the 20Fs.   Snow showers are forecast to start in the afternoon and increase overnight into Thursday.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  29     0   0     60  
Summit Lake (1400′)  29     1   0.1     29  
Alyeska Mid (1700′) 30   2   0.1    60

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20   variable   5   26 (SE)
Seattle Ridge (2400′) 25   variable    6      31 (NW)