ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Saturday December 22nd, 2018
Posted by Wendy Wagner on 12/22/18 at 7:00 am.
Avalanche risk
The Bottom Line
Considerable Avalanche Danger
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

A CONSIDERABLE avalanche danger exists for wind slab avalanches on exposed slopes above treeline. An increase in winds will form touchy wind slabs 1-3′ thick. Triggering one of these will be likely in areas with recent wind loaded snow. A MODERATE danger exists at all elevations for loose snow avalanches on steeper slopes out of the wind, sluffs could be large. Wet sluffs may be seen below 1,000′. Glide cracks are opening and could release at anytime, watch for cracks and avoid being under them.

JOHNSON PASS / LYNX DRAINAGE / SUMMIT LAKE:   South of Turnagain Pass weak layers exists under the new snow. Human triggered slab avalanches over 2′ thick are possible on slopes over 35 degrees. Heads up in these areas!  

LOST LAKE:    This zone is out of the advisory area, but recent reports show the potential for triggering slab avalanches 2-3′ thick on older weak layers under the storm snow.  If headed to this area pay attention for signs of instability like collapsing and recent avalanches.

OVERHEAD HAZARDS – ROOF AVALANCHES:   Roofs should start shedding snow with warming  temperatures. Also watch for large chunks of snow falling from tree branches. Please use caution and be aware of these overhead hazards as temperatures warm.  

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

PARKING CONCERNS along Placer Valley and Turnagain Pass

The Chugach National Forest has opened most of their riding zones, including Placer Valley and Snow River yesterday. These openings  happened to coincide with an unexpected 4 foot storm.  The USFS and AK DOT are requesting both motorized and non-motorized users to avoid parking on the highway where  lots are still being cleared. Parking on the road and riding around the motorized lot will prevent plow operators from being able to clear the lots and delay the process through a busy holiday period. This is also a serious safety issue. Should you go to Turnagain Pass today, consider heading south where parking is not an issue.

Hatcher Pass avalanche conditions:  If you’re thinking of heading to Hatcher Pass be aware of dangerous avalanche conditions. Read the Hatcher Pass Avalanche Center Saturday morning forecast  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

The solstice sleeper storm of 2018, which dropped an impressive 4-5′ (yes feet) of very low-density snow from December 19 to 20, has settled an impressive 20″ in the past two days. Light snowfall overnight as added another 3-4″ to the pack and another 2-4″ is expected today – and winter begins! In the Turnagain Pass area all signs are pointing to good bonding between the new and old snow. Reports yesterday spoke of “No obvious signs of current instability“. Although there were several natural avalanches during the storm (photos below), no avalanche activity has occurred, that we know of, in the past two days. The catch is, winds picked up yesterday, continued overnight and are forecast to remain – this will be the main concern for today.

WIND SLABS:  Sustained easterly winds along ridgelines and peaks are likely having their way with all the new loose snow. Add to this warming temperatures to help stick the snow together, and we are set up for a classic wind slab problem. Keep your eyes peeled for areas winds have deposited snow, pillowed surfaces and stiffer wind packed snow. Shooting cracks are likely in this case and triggering a wind slab from below, while ascending, is also likely. These issues should be easily seen if we are looking for them. 

LOOSE SNOW AVALANCHES:  Triggering a loose snow avalanche could be large and unmanageable in steep terrain with so much new snow. Below 1,000′ we could see damp or wet sluffs as temperatures reach above freezing.

SNOW IMMERSION SUFFOCATION:  Getting caught in a tree well, stuck head down in a depression and other means of snow immersion are all possible with so much snow. Watch your partners and be aware of this hazard that claims lives in the US each year.

 

 

 Natural ‘storm slab’ avalanche occurring near the end of the storm on Tincan at treeline. Photo: Aleph Johnston-Bloom.

 

 Another shot of natural ‘storm slab’ avalanches occurring near the end of the storm on Tincan’s CFR ridge near treeline. Photo: Aleph Johnston-Bloom.

 

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

Areas south of Turnagain Pass, such as Johnson Pass, Lynx drainage, Twin Peaks/Silver Tip, the Summit Lake zone and possibly Lost Lake are all very suspect for harboring buried weak layers under the storm snow. Triggering a large and dangerous avalanche cannot be ruled out and is considered possible in these zones. Weak layers consist of a facet/crust combination between 2,000-3,000′. The addition of 4′ of new snow clearly overloaded these and several natural avalanches were seen Thursday on the far northern end of Summit Pass. Look out for whumpfing, cracking, and recent avalanches if you head to these areas. 

Natural avalanche that may have released in buried weak layers. This is across the street from Twin Peaks at 62 mile. South facing terrain.

 

Heads up – Lost Lake zone harbors buried weak layers – be aware of triggering a slab 2-3′ thick. This picture was taken Thursday and the timing of this avalanche is unknown and may be a natural avalanche from a different day. Photo and observation courtesy of Iron (iii) Oxide.

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 opening as we speak all over the mountains. Notable locations are Tincan’s SW face, Sunburst’s SW face under the weather station, Gold Pan area (behind Cornbiscuit/Magnum) and Johnson Pass. These cracks can release at any moment. They are not associated with human triggers and the best way to manage the hazard is to avoid being on or beneath slopes with cracks. 

Crazy cracks… Yesterday cracks were found opening in the 4′ of new settling snow. These are not glide cracks, but similar. There is not an avalanche hazard associated with them, but they could trip you up.

Mountain Weather

Yesterday:  Partly cloudy skies were over the region yesterday before clouds moved back in along with snowfall overnight.  Roughly 3″ of snow has fallen overnight at Turnagain Pass and 4-5″ in the Girdwood Valley. Ridgetop winds have been sustained in the 20-30mph range with gusts in the 40’s from the east. Temperatures have been on a slow rise and sit in the 20’sF along ridgelines and at 32F at 1,000′.  

Today:   Cloudy skies and snow showers will continue in the mountains. An additional 2-4″ of snow is expected to fall above 500′ with a rain/snow mix below and possibly light rain at sea level. Ridgetop easterly winds should remain in the 20-30mph range with gusts up to 50mph. Temperatures are expected to stop climbing and sit in the mid 20’s along ridgelines and 32F at 1,000′ (sea level temps will be in the mid to upper 30’sF).

Tomorrow:   A break in stormy weather is expected along with clearing skies for both Sunday and Monday. Temperatures look to cool down slightly Sunday along with a decrease in winds.  

*Seattle Ridge weather station is rimed over and not recording any data.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   3   0.2   67  
Summit Lake (1400′) 26   trace   0.1   16  
Alyeska Mid (1700′) 29   4   0.3   38  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19   NE   22   47  
Seattle Ridge (2400′) *N/A     *N/A     *N/A     *N/A