ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Saturday January 12th, 2019
Posted by Aleph Johnston-Bloom on 01/12/19 at 7:00 am.
The Bottom Line
Considerable Avalanche Danger
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Strong winds impacting the region today will cause the avalanche danger to rise to MODERATE at all elevation bands in exposed terrain as triggering a wind slab becomes possible. As these slabs build over the weak surface snow the danger may rise to CONSIDERABLE in the Alpine later in the day. Natural avalanches may become possible and human triggered avalanches may become likely. Pay attention to changing conditions. In addition, glide cracks may release into avalanches. Limiting/avoiding exposure under them is prudent and give cornices a wide berth.  

There is a Winter Weather Advisory issued for 9 pm this evening to 11 am Sunday for Portage Valley and Turnagain Pass.  

SUMMIT LAKE / JOHNSON PASS / LYNX DRAINAGE:  Keep in mind buried weak layers exist in the middle and base of the snowpack. More potential for triggering a large slab avalanche exists in this zone, especially with wind-loading. Choose terrain wisely.

 

3. Considerable
Alpine
/ Above 2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
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

The  11th Annual Alyeska Ski Patrol Auction benefiting the Alyeska Ski Patrol Avalanche Canine Program is tonight from 8-10 pm at the Sitzmark!  Enjoy the “entertainment” and bid on an early morning ski with your favorite patroller.

For the Saturday morning Hatcher Pass avalanche forecast click HERE.  


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

After enjoying sunny skies and a mostly stable snowpack a series of storms will begin to affect the advisory area today. Temperatures are on the rise, winds are increasing and precipitation will start to fall later in the day. Today is the day to expect changing conditions and be on the lookout for signs of instability. Initially increasing winds may trigger small loose snow avalanches but as they continue and intensify, wind slabs will build in exposed terrain. There is soft snow available for transport and wind slabs may be quite tender as they develop over very weak surface snow formed during the cold weather. Well developed surface hoar was observed at lower elevations and along some ridge tops. Near-surface faceted snow was observed at all elevations. Below 1500′ there is a crust below the weak snow. This is not a good set-up for new snow and wind-loading. Look for blowing snow, cracking, collapsing and be especially heads-up if natural wind slabs start releasing. 

Rapidly warming temperatures and the additional blowing snow may also make cornices easy to trigger over the next few days. As always, give these a wide berth. 

 

 Weak surface and just below the surface snow on Center Ridge, 1-10-19. 

 Weak surface snow on the Gold Pan ridge, 1-11-19. Photo: Allen Dahl

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

South of Turnagain – Lynx Creek/Johnson Pass/Summit Lake zone:  As the weather pattern changes and the potential for wind loading increases it becomes even more critical to remember that a poor snowpack structure exists in these areas. The Christmas buried surface hoar has been found as well as concerning facet/crust combinations in the bottom of the snowpack. Avalanches may initiate near the ground and be quite dangerous. If you’re headed this way, evaluate terrain exposure and the snowpack as you travel. Be on the lookout for signs of instability. 

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

Identify glide cracks and avoid spending any time under these features. Glide cracks are opening and have avalanched within the last week. Glides are completely unpredictable and not human triggered. The rapid temperature rise and additional snow load over the next few days may or may not cause an increase in glide activity. 

Mountain Weather

Yesterday:  Skies were clear and temperatures were cold with daytime highs in the single digits. Valley bottoms remained below zero. Winds were light and easterly. Overnight clouds bulit and temperatures began to increase.

Today:  Mostly cloudy skies with an increasing chance of precipitation throughout the day. Temperatures will continue to rise into the 20Fs. Easterly winds will build and blow 15-25 mph with gusts into the 40s. These are forecast to increase overnight. Snowfall will increase overnight and temperatures will rise with the potential for rain to fall at sea level.  

Tomorrow:  Cloudy skies and snow continues tomorrow with the brunt of the storm in the early morning. Winds will remain strong and temperatures will continue to rise with the warmest temperatures on tap for Monday. The series of fronts and lows are forecast to impact the region into the week.  

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 6   0    0  53
Summit Lake (1400′) -1     0    0   20    
Alyeska Mid (1700′) 8   0    0  41  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  6 SE    6 23  
Seattle Ridge (2400′)  8 *N/A   *N/A   *N/A