ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday December 13th, 2018
Posted by Heather Thamm on 12/13/18 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  in the Alpine and at Treeline where triggering a slab 1-3′ deep is possible. Triggering a windslab or getting caught up in a loose snow avalanche will be possible in steep terrain. Keep an eye out for glide cracks and avoid traveling underneath this unpredictable avalanche hazard.

Assess the snowpack as you travel, identify areas of concern and evaluate consequences.

GIRDWOOD:  Windslab 1-2′ thick are possible on leeward features in the alpine due to higher snow totals and elevated ridgetops winds yesterday.

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.

Special Announcements

If you are heading to Hatcher Pass  make sure  to read a recent report from  Hatch Peak on Sunday where a skier was fully buried and recovered without injury.   Be aware alaskasnow.org is undergoing a system-wide website update and  Hatcher Pass Avalanche Center  webpage may look very different when it is finished. For now stay current by following the new Hatcher Pass Avalanche Center mid-week update HERE.  

Looking for avalanche courses or evening presentations? Check out our  calendar page! There are two CNFAIC evening discussions coming up – these are FREE and a great way to get your head back in the avalanche game. TONIGHT 7pm-8:30pm –  ‘Tales from the Pit’ at Blue & Gold Boardshop in Anchorage!

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

Yesterday morning an unexpected sleeper storm was centered over Girdwood and dumped 6-12” of very low-density snow. Snow totals were greater in the alpine. Turnagain Pass received 3” along the road and up to 10” at higher elevations. Winds in the afternoon along ridgetops picked up for a few hours 10-15mph with some gusts in the 20s-40s mph. Triggering an isolated windslab on leeward and cross-loaded features will be possible. Look for smooth or pillow shaped features. Evaluate the snow and terrain as you travel and keep in mind the consequences should even a small rug get pulled out from underneath you.

In areas where winds didn’t change the surface expect new snow to be loose and unconsolidated. Don’t be surprised by ‘sluffing’ and loose snow moving faster than expected.

Low density loose snow was sluffing easily on steeper features yesterday at Tincan. Photo credit: Tully LaBelle-Hamer

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

There remains some uncertainty around weak snow within older layers of the snowpack. Observations this week have found a facet/crust combo 1-2’ below the surface in the mid-elevations (2000’ – 2700’.) Large collapses “whumpfs” were experienced on Magnum’s NW shoulder on Tuesday and stability tests were quite reactive in this zone. It’s unknown how intact this structure exists into the Alpine. On the Northern end of Turnagain Pass stability tests have been showing a strengthening snowpack at higher elevations. However keep in mind that we’ve seen a lot of snow over the last week and we don’t have a lot of snowpack info. It’s still early season. Areas with a thinner snowpack are more suspect for weaker and unstable snow including the Southern-end of Turnagain Pass, Summit Lake and Girdwood Valley. In general North and East aspects have a tendency to be thinner and there is a zone in the mid-elevations where the snowpack quickly transitions to shallower depths. 

Be on the lookout for red flag warnings like whumpfing’, shooting cracks, new avalanche activity or any changes in weather. Keep in mind there is a lot of snow available for transport. Any sign of increased winds could form reactive wind slabs or add stress to a persistent slab.

This is a good example of where the snow quickly transitions shallow and weaker snowpack in the mid elevations. This structure may be more widespread on the Southern end of Turnagain Pass and in Summit Lake. 

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

The first glide cracks of the season were seen on Sunburst SW face under the weather station. A glide crack is the snowpack being pulled by gravity downhill along the ground. They can release at any moment without warning and are usually not associated with human triggers. The best way to manage this hazard is to avoid being on or beneath any slopes with cracks opening up. 

Photo taken on Tues December 11, 2018 of new glide cracks on SW face of Sunburst

Mountain Weather

Yesterday: Snow flurries were the most intense in the morning between 6am to 10am. Turnagain received 3 € of low-density snow along the road and Girdwood 6-10 € at valley bottom. Winds were light with the exception of a few hours along ridgetops where winds increased to 10-15mph with gusts in the 20-40s. Temps at sea level were in the low 20Fs and ridgetops dropped into the single digits.

Today:  Temps will be similar with single digits at ridgetops and low-20Fs near sea level. Snow flurries will diminish becoming mostly sunny in the afternoon. Winds are expected to be light (5-10mph) from the West and shift to the East by this evening. Tonight snow flurries will return with a few inches possible overnight.

Tomorrow: An active winter weather pattern will persist tomorrow and into the weekend. A continuation of more snow showers, cool temps and light to moderate winds is expected.

*Seattle Ridge weather station anemometer is rimed and not recording wind data.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18   4   0.1 33  
Summit Lake (1400′) 15   3   0.3   10  
Alyeska Mid (1700′) 17   7   0.11   20  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8   ENE   5   30  
Seattle Ridge (2400′) 14   *N/A   *N/A   *N/A