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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, January 27th, 2018 - 7:00AM
Expires
Sun, January 28th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

Today there is CONSIDERABLE  avalanche danger in the Alpine where triggering a storm slab or wind slab 2-3 feet deep remains likely. Loose snow avalanches are also possible in steep terrain. Above 3000′ be aware of older weak layers deep within the snowpack which are becoming very difficult to trigger, but could have high consequences. Evaluate snow conditions as you move up in elevation. Cautious route-finding, and conservative decision making are essential.  

Below 2500′ the avalanche danger is MODERATE where triggering a storm slab 1-2 feet deep or loose snow avalanche is possible. Be aware of runout zones, terrain traps and other people around you.  

If headed to Summit Lake check out teh summary HERE.

Special Announcements
  • TODAY, January 27th, 11:00 am – 12:30 pm:  Join CNFAIC forecasters for a FREE hands-on and in-the-snow avalanche beacon practice!!  This 1.5 hour  informal rescue practice is geared for all user groups. Grab your friends and join us  before hitting the hills!! Great intro or refresher!  Hosted by the Anchorage Snowmobile Club!  At  Turnagain Pass (motorized parking lot) €“ Look for the blue CNFAIC snowmachine trailer.
  • The Chugach National Forest has opened Johnson Pass trail. Access trail thru open gate and park trucks/ trailers along the road toward the trailhead.   Please keep the turnaround (near large signboard) clear for vehicles to TURN AROUND!
Sat, January 27th, 2018
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

A sunny Saturday combined with a new load of snow and recent wind loading is a perfect recipe for a human triggered avalanche. A storm that ended Thursday night dumped around 20+” of cold low-density snow and loaded several weak layers (buried surface hoar and near surface facets.)  Several groups reported “whumpfing” yesterday and one human triggered avalanche occurred in the Alpine of Sunburst where a person breaking trail triggered a soft slab that propagated about 30’ wide without incident. A period of elevated winds (15-50mph) last night has been moving loose snow around and loading leeward features and increasing the strength of the slab. Triggering a storm slab today could be big enough to bury a person, and will depend on the steepness and the size of the terrain you are exposing yourself to. Obvious clues like collapsing and shooting cracks will be evidence the snowpack still needs more time to adjust before venturing into bigger and steeper terrain. Hand pits are a good tool to evaluate the slab and weak layers as you travel. Be aware that you may encounter two types of storm snow problems. 

Wind Slab: Fresh wind slabs formed overnight may be shallow or propagate 2-3’ deep on a widespread layer of buried surface and near surface facets. This problem will be more likely on wind loaded features below ridge lines or in cross loaded gullies. This includes the starting zone of the back bowls of Seattle Ridge and the popular SW facing terrain along on the East side of the road.  Pay attention to places where the snow is becoming stiffer and more supportable and avoid steep wind loaded features and unsupported slopes with the classic wind-pillowed shape.   

Storm Slab: Yesterday’s storm snow was loose and poorly bonded and didn’t want to stick together or act as a slab except where it was more consolidated.  Overnight the slab has settled around 5” at Center Ridge Weather Station. This could be enough cohesion to allow the new snow to act more as a slab, even where the slab is very soft. Triggering a storm slab could run farther and faster than expected, which is the nature of cold snow.   Be aware of other groups of people in the same area and practice safe travel protocols. 

Photo on left by Kathy Still of a small slab triggered Sunburst near 3600′ and a close-up photo by Chad Saetre of the crown (~24″ deep). Your observations are welcomed and apprecated!!!

 

 

Note the very large buried surface hoar in a pit at 2700′. This layer was buried with a few inches of snow a few days before the storm is widespread at all elevations. 

 

Avalanche Problem 2
  • Dry Loose
    Dry Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Loose snow avalanches ‘sluffing’ is possible on steep terrain features protected from wind. Similar to a storm slab, this dry loose snow could run faster and farther than expected. 

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

Triggering a deep slab is becoming difficult, but is still possible above 3000′ where a hard slab (3-8 feet thick) is sitting on a variety of weak layers in the mid pack (including buried surface hoar) and old November facets near the ground. The most likely trigger spots are in thin areas in the snow cover, often near rocks, or where the slope rolls over. The Southern end of Turnagain Pass to Johnson Pass is more suspect due to a thinner snowpack where there is a potential for more trigger spots. Remember, this is a ‘low probability, high consequence’ situation. This issue can simply be avoided by sticking to terrain below 3000’ and is secondary to triggering a storm slab avalanche today. Choosing low-consequence terrain in the Alpine is recommended. 

*** There is very little info in the Johnson Pass and Lynx Creek zone. If you’re headed to either of these areas and you observe any signs of instability: recent avalanches, shooting cracks, or whumpfing please send us an observations HERE. 

Weather
Sat, January 27th, 2018

Yesterday skies were partly cloudy with upper elevations temps in the high teens (F) and valley temps in the upper 20F’s. Ridge top winds were light from the East increasing yesterday evening, 15-30mph, with gusts in the 40’s and 50s mph.  

Today ridge top winds are expected to be light and variable. Skies could range from partly cloudy to mostly sunny and no new precipitation is expected. Temperatures will remain in the high teens (F) at ridge tops and mid 20F’s at lower elevations.  

Cooler temps will continue to influence the weather through the weekend.   Northerly winds are in the forecast for Sunday evening and may be strong at times. There is still talk in the long term forecast for a pattern change mid to end of next week with a possible warm-up with a Southerly flow.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   0   0   71  
Summit Lake (1400′) 20   0   0   21  
Alyeska Mid (1700′) 25   0   0   55  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19   ENE   15   57  
Seattle Ridge (2400′) 22   SE   13   43  
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.
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