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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 13th, 2017 - 7:00AM
Expires
Thu, December 14th, 2017 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The  avalanche danger remains HIGH  due to heavy rain, wet snowfall and strong winds.  A very potent warm and wet storm cycle has saturated the snowpack below 3000′ where triggering a wet slab avalanche will be likely today. In the Alpine triggering a storm snow avalanche 2-4 feet thick is also very likely.  Travel in avalanche terrain is NOT recommended.  

Below 1,000′, where little snow exists, there is a  CONSIDERABLE  avalanche danger for debris running into steep or channeled terrain.

**Summit Lake area also has elevated avalanche danger and the weekly summary can be found HERE.

Special Announcements

Elevated avalanche danger remains throughout Southcentral, AK due to continued stormy weather. If you are headed to Hatcher Pass check out  hpavalanche.org  and for Valdez,  valdezavalanchecenter.org.  Also, there have been reports of unstable snow and human/natural triggered avalanches near Petersville and  the Dutch Hills  area.

Scholarship  Applications due on Dec 15th.  The Friends of the CNFAIC have two scholarships dedicated to avalanche education for skiers, snowmachiners and all user groups. The funds generated to make these possible are in celebration of Rob Hammel and Amy Downing, their love and passion for the mountains, and to help others stay safe.  

Wed, December 13th, 2017
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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

Above freezing temperatures, strong winds and periods of heavy rain have impacted our forecast zone over the last four days causing a widespread avalanche cycle in the mid elevation band. In the last 24 hours Turnagain Pass received 0.9” of rain and Girdwood received 1.56” of rain, with the heaviest precip happening between 1am and 6am this morning. On Monday dozens of wet slab avalanches were observed throughout Turnagain Pass on all aspects below 3000’. Rain/snow line has been hovering between 2000’ and 3000’ and snow totals along the road are quickly shrinking. 

 Dec.10 – Dec.13 Storm Totals (Rain to 2500′)

  • Turnagain Pass (Center Ridge Snotel) = 4.0” of Rain
  • Girdwood (Alyeska midway station) = 6.0” of Rain

The avalanches seen on Monday released near the ground on old weak faceted snow. The most interesting (and scary) avalanches occurred on the West facing terrain of Tincan below 2500’, an area many folks go when the avalanche danger is elevated. This speaks to the powerful nature of a saturated snowpack and indicates that even small terrain features are suspect. The snowpack below 2500’ is isothermal, and triggering a small slope from below or getting caught in a terrain trap would be unsurvivable. Until the snowpack has a chance to drain and form a crust the avalanche danger remains high and travel in avalanche terrain is not recommended. 

Photo taken on Tuesday, Dec.11 during a period of good visiblity. Recent avalanche on West facing terrain of Tincan at 2500′. Several more avalanches also occured in the lower elevations on small terrain features and on Todd’s run. Check out the observation HERE.  

 

Photo taken on Tuesday, Dec.11 during a period of good visiblity. Recent avalanche activity on SW aspect of Eddies. Several more avalanches occured lookers left on more West facing slopes. Check out the observation HERE. 

 

 

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

Wet and heavy snow has been falling in the Alpine, which becomes drier with elevation. Based on total rain at the mid-elevations stations an estimated 4-5 feet of snow has accumulated in the alpine this week. New snow and strong Easterly winds have been loading a weak pre-existing snowpack and as of this morning very little information is known about avalanche activity in the upper elevations. Storm snow avalanches that include storm slabs, wind slabs and cornice breaks all can be expected today. There is also potential for a larger more dangerous avalanche to release on a weak foundation near the ground. This size avalanche could run into the lower elevations in steep or channelled terrain. Evidence of such avalanches have occurred in big terrain along the Seward Highway like Pyramid’s West face and other indicator paths. As of Tuesday, the last time we could see into the Alpine, many of the upper elevation zones of Turnagain Pass remained intact. Again, the message is simple for today, avoid avalanche terrain until the snowpack has time to adjust to its new load.


Most of the snow has melted below 1000′ with the exception of some larger avalanche paths where debris can been seen, like the West face of Pyramid, lookers right side of speed limit sign.  

Weather
Wed, December 13th, 2017

Yesterday another   0.9 € of rain fell in Turnagain Pass and 1.56 € of rain fell in Girdwood. Temperatures remained above freezing, averaging 35F at 1000′. Easterly ridgetop winds averaged in the 20’s with gusts in the 60’s (mph). Temps warmed in the evening and Seattle Ridge weather station was reporting 31.5F at 2400′, estimating rain/snow line to be around 2000′. The most intense precipitation occurred this morning with half the total precip amounts falling between 1am to 6am.  

This morning as a strong front passes over Southcentral and weakens heavy rain will transition to showers with another .5 € of rain expected throughout the day. Temps are expected to cool slightly be this evening bringing rain/snow line 1500′. Easterly ridge top winds will start to decrease in the morning to 10-20mph. Scattered rain showers are possible overnight.  

A strong South to North storm pattern that has impacted our region with warm temps and numerous storms is expected to transition to more of Southwesterly flow in the comming days. There is talk of temperatures starting to fall back towards freezing level with another series of storms on tap for the weekend starting Friday, hopefully in the form of snow instead of rain.   At this point there is uncertainty on timing and intensity and what form of precip will be falling from the sky.  

** Sunburst weather station is not working as of 12/11/17 due loss of battery power.

Snow along the highway is quickly shrinking. Tincan trailhead creek has open water and the Southern end of Seattle Ridge dirt and veggitation are visibile.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35   0   0.9   24  
Summit Lake (1400′) 37   0   0.3    9
Alyeska Mid (1700′) 34   0    1.56 17  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) *n/a   *n/a     *n/a     *n/a    
Seattle Ridge (2400′) 29   SE   23   65  
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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