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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, January 13th, 2018 - 7:00AM
Expires
Sun, January 14th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

An avalanche warning has been issued for Turnagain Pass, and the avalanche danger remains HIGH at all elevations due to rain, heavy snow, strong winds and above freezing temperatures.   Natural and human triggered wet slab avalanches and storm slabs 2-4 feet deep will be likely today in avalanche terrain and could run to valley bottoms. Travel is not recommended on or near slopes steeper than 30 degrees including runout zones.  

*Portage and Whittier, outside of our advisory zone, has received double the amount of precipitation as Turnagain Pass and another 2 € of rain is expected today. Avalanche activity in these zones may run to valley bottoms.  

For information about the Summit Lake avalanche conditions click HERE.

Special Announcements

An avalanche warning has been issued for the Turnagain Pass advisory zone including Girdwood Valley and will be in affect through Sunday, January 14.  

 **How about an alternative activity? There is a  FREE avalanche rescue clinic at Hatcher Pass today where you can practice your rescue skills with the guidance of professional instructors – clinic runs from 11am – 1:30pm at the Gold Mint parking lot.  

Our advisory page has changed!  Information on the changes can be found  HERE.  

Sat, January 13th, 2018
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
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
  • Wet Slab
    Wet Slab
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Slab
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.
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

Rain on snow is expected below 1500’ and will be adding significant stress to the snowpack in the mid and lower elevation zones. A weak interface (facets and buried surface hoar sitting on a crust) is underneath yesterday’s 20” of storm snow. Add today’s rain and this is a perfect recipe for wet slabs. Natural and human triggered wet slabs 2-3 feet deep are likely on slopes steeper than 30 degrees including both large and small terrain features. Triggering this kind of avalanche will be impossible to escape and could be unsurvivable. It will be very important to avoid terrain traps and being near the runout of larger slopes. 

 

Remote triggered avalanche on Tincan yesterday. This is a good example of a small terrain feature that could be releasing naturally if rain saturates the snowpack today. Photo credit: Matti Silta

 

Avalanche Problem 2
  • 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

Yesterday 20” of new snow was observed in Turnagain Pass and today another 12-20” of heavy snow is expected in the upper elevations above 1500’. Natural slabs 3-4 feet deep will be likely today and human triggered avalanche very likely. Strong Easterly winds in the 40’s with gusts in 90’s are also anticipated. All 6 Red Flag warning signs were observed yesterday at Turnagain Pass. This includes a remote triggered avalanche on Tincan,  large shooting cracks, collapsing “whumpfing sounds”, rapid loading due to heavy snow and strong winds, and warming temperatures. All of this new snow has fallen on surface hoar and near-surface facets and slabs will be easy to trigger. Below 2000’ these weak layers are sitting on a slick crust  and a slab could easily catch you by surprise, even in the protected trees of Tincan or getting your snowmachine stuck under a small steep terrain feature. Strong winds today could trigger a large natural avalanche that could run down to valley bottoms. Maintaining a safe distance from the runout zones of all larger slide paths including Repeat Offender will be important. Remote triggering an avalanche from below or above is also possible. Basically today is a good day to avoid the mountains around Turnagain Pass. In fact this is a good day to head to Hatcher Pass and practice your avalanche rescue skills. Click HERE for details about the free clinic. 

A very large shooting crack on a wind loaded terrain feature was intentially triggered by a snowmachine near the motorized lot yesterday. Windloading has created variable slab depths. Slabs could be as deep as 4-5′ on leeward features. 

 

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

Natural and human triggered avalanches today could step down to deeper layers of the snowpack producing a very large avalanche depending on the size of the slope. A layer of buried surface hoar from the New Year’s holiday has been showing propagation potential in test pits including a pit dug yesterday at 2000′ on Tincan by an avalanche course. In addition, a Deep Persistent Slab problem remains a concern in the upper elevations above 3000’, and serves as one of the many reason to avoid avalanche terrain today. 

Weather
Sat, January 13th, 2018

Yesterday Center Ridge Snotel recorded 0.6 € Snow Water Equivalent (SWE) at 1700′ and the RWIS DOT Wx station recorded (1.13 € SWE) at 1000′. Field observations confirmed about 20 € of new snow since Thursday when the storm began.   Sunburst Wx station showed strong Easterly winds all day averaging in the 40’s mph with a gust to 92mph last night. Rain/snow line started out at sea level yesterday morning and increased to just below 1000′ this morning.  

Today there is .95 € rain (SWE) expected, this translates to another 12-20 € of snow in the upper elevations. Unfortunately rain/snow line will continue to rise to 1500′ possibly higher. Strong winds will continue to blow from the East and average in the 40’s mph with gust in the 80-90’s mph. Peak intensity and warmest temperatures should begin later morning through 8pm this evening.

Tomorrow afternoon another front is expected. There’s some uncertainty as to how much precipitation and wind, but temperatures are expected to remain warm.  

*RWIS DOT weather station at 1000′ on Turnagain Pass recorded 1.13 € SWE. This weather station is located further North where precipitation is often heavier than then further South in Turnagain Pass.  

**Seattle Ridge weather station is covered with rime and stopped recording wind data at 4pm yesterday (1/12/18)

 

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32   *4   *0.6   57  
Summit Lake (1400′) 34   1   0.1   15  
Alyeska Mid (1700′) 32   4   0.92   48  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   ENE   42   92  
Seattle Ridge (2400′) 26   **SE   **24   **46  
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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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