ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Tuesday February 12th, 2019
Posted by Aleph Johnston-Bloom on 02/12/19 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′. Look for signs of recent wind loading. Triggering a small wind slab will be possible on steep, leeward terrain features. 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 and watch for sluffing.

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.  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

Last night saw another bump in wind speeds with southwesterly winds averaging 10-20 mph and gusts into the 30s. These are forecast to continue today and ramp up as winds shift to the northwest overnight.  As wind directions change look for different loading patterns in the terrain. Remember Sunday night’s winds were from the northeast.  Look for drifting, pillowed slopes and scouring. Wind slabs maybe found today on steep, unsupported slopes. Watch for cracking in the surface snow and stiffer snow over softer snow. Listen for hollow sounds. 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. Alternatively on steep, protected slopes watch for sluffing. 

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.

Recent wind scouring and cross-loading on Magnum, 2-11-19. Photo: Nikki Champion.

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 6 days ago on Eddies. That avalanche was a 2′-3′ deep avalanche 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. There is still a lingering concern that this is the kind of avalanche problem where several tracks may be on a slope before someone finds a trigger point and the whole slope avalanches. The other factor is that the current snowpack structure (with the MLK buried surface hoar 1-3′ deep) has the potential to be reactivated with additional loading. The question is what could tip the balance? Could the wind loading?

As each wind event changes 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.

The MLK buried surface hoar at 2500′ on Repeat Offender, 2-7-19. 

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 yesterday in Girdwood on the lower southeast shoulder off of Goat Mtn. Glide cracks are unpredictable, not associated with human triggers, and can release without warning at any time. One pattern that has been anecdotally observed is increased glide activity with cooling temperatures and clear skies, which are in the forecast this week. Look out for glide cracks and limit exposure under them.

Glide avalanche on Goat Mtn. 2-11-19. Photo: Rich DiJulia

Mountain Weather

Yesterday: Skies were mostly clear in the morning with some valley fog becoming mostly cloudy in the late afternoon. Temperatures were in the 20Fs. Winds were light and westerly until they increased to 10-20 mph with gusts into the 30s at 1 am last night. Overnight there was snow showers with around an inch of accumulation.  

Today: Skies are forecast to become partly sunny with temperatures in the mid 20Fs. There may be some morning valley fog. Winds will be westerly shifting to northwest 10-15 mph with gusts into the 20s and 30s. Northwest winds will increase overnight gusting into the 30s and 40s and temperatures will dip into the teens.  

Tomorrow: Partly sunny with temperatures in the teens and low 20Fs. Northwest winds are forecast to continue to increase through the day averaging 20-30 mph with gusts into the 50s and peak late Wednesday night. Overnight temperatures drop to the low teens and single digits.   Thursday looks to be cold and clear. The next chance for snow is over the weekend.  

*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   1   0.1 56  
Summit Lake (1400′)  23       0       0    25    
Alyeska Mid (1700′) 27    1      0.04 50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  23 SW   7    37
Seattle Ridge (2400′)  25  *N/A *N/A   *N/A