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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, January 13th, 2019 - 7:00AM
Expires
Mon, January 14th, 2019 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE on slopes above 1,000′. Natural avalanches due to strong wind and several inches of new snow are possible in the Alpine and in areas exposed to wind in the Treeline.  Human triggered slab avalanches, up to a foot thick or more, will be likely  and may be triggered remotely.  Pay  attention to new snow amounts and wind effect. Avoid being in runout zones from avalanche paths above you. In addition, glide cracks may release into avalanches; limit/avoid exposure under them.  

SUMMIT LAKE / JOHNSON PASS:    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.

OVERHEAD HAZARDS:   Heads up! Roof avalanches are expected near sea level in Girdwood, Portage and other areas where temperatures are rising to 40 degrees F.

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Sun, January 13th, 2019
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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

The first in a series of storms is over the region today. Snowfall began yesterday and as of this morning, roughly 2-4″ has fallen with up to 5″ or more in the Girdwood and Placer Valleys. An additional 2-4″ is expected over the course of the day. Although snowfall amounts are meager to what some of us may have hoped for, the winds are producing. Ridgetop winds have been blowing strong from the east (20-30mph with gusts to 50) for almost 24-hours now. The other producer is the temperature. Temperatures have risen as much as 40 degrees F in the last 24-hours. The minus single digit conditions felt in valley bottoms are now in the tropical 30’sF. These are all abrupt changes to the snowpack and avalanche danger has risen. 

There was one suspected natural 2′ slab avalanche yesterday in the Johnson Pass/Center Creek area and more of these could occur today. There is very weak snow (sugary facets and surface hoar that formed during our cold/clear period), which will cause any new slab to be very touchy. Watch for shooting cracks and recent avalanches, these are Red Flags that an avalanche will be easy to trigger.

Wind Slabs:  With little new snow to work with, the winds will also be transporting the older loose snow surface into sensitive wind slabs. Slabs are likely to be in the foot thick range in general, but could reach as thick as 2-3′ along the tops of ridgelines. They should range from soft to stiff and could be triggered remotely.

Storm Slabs:  Warming temperatures are causing an upside-down storm, which will create shallow storm slabs in areas outside of the wind. Again, watch for cracking and carefully evaluate the storm snow. 

Cornices:  Cornices are growing and the warmth can help weaken them. Cornice falls are possible and may trigger slabs under them. 

 –

Cracking in the snow on a recently wind loaded rollover at ~2200′ in the Johnson Pass/Center Creek area yesterday 1/12. Photo: Andy Moderow.

  

 

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
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

South of Turnagain – Johnson Pass/Summit Lake zone:  As new snow and 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
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. They 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.
More info at Avalanche.org

Glide cracks are likely to become hard to identify with snow/wind filling them in. Known areas with cracks are the southerly facing slopes on Eddies, Tincan, Sunburst, Magnum, Cornbiscuit, Lipps, Johnson Pass, Girdwood Valley and a few on the easterly slopes on Seattle Ridge. Not to mention the Lynx creek glide extravaganza. Avoiding/limiting time under these features is prudent as they can release into an avalanche at any time and are completely unpredictable. The rapid temperature rise and additional snow load over the next few days may or may not cause an increase in glide activity. 


A large glide crack near the head of Lynx creek drainage (seen yesterday 1/12) threatens a popular area to ride.

Weather
Sun, January 13th, 2019

Yesterday:    Overcast skies, light snow showers and rapidly warming temperatures were over the region. Snowfall picked up overnight with 1-4″ accumulating so far – higher amounts seen in Portage Valley and at Alyeska in Girdwood. Ridgetop winds have been strong from the east, averaging 20-30mph with gusts to 50mph. Temperatures rose from single digits to 40F at sea level, 32F at 1,000′ and into the 20’s F along ridgetops over the past 24-hours.

Today:    Snow showers should remain over the area today adding an additional 1-4′ with another 1-4″ overnight. Again favored areas are Girdwood and Portage Valley. Temperatures are slated to keep climbing and the rain/snow line may reach 1,000′ by this afternoon and up to 1,500′ by tomorrow morning (#thinkcoldthoughts!). Ridgetop winds will remain strong, 20-30mph from an easterly direction with gusts into the 50’s and 60’s mph.  

Tomorrow:   Precipitation should continue through Monday with light rain up to 1,500′ and light snow showers above 1,500′. An active weather pattern bringing additional warm and wet weather is expected for the remainder of the week with a possible surge late Tuesday into Wednesday.  

*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′) 27   2   0.2   53  
Summit Lake (1400′) 21   1-2   0.1   21  
Alyeska Mid (1700′) 26   3   0.27   43  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  18 NE   22   51  
Seattle Ridge (2400′) 24   *N/A *N/A   *N/A  
Observations
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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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