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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, January 20th, 2020 - 7:00AM
Expires
Tue, January 21st, 2020 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

There is a MODERATE avalanche danger today as we usher in the first winter storm since New Year’s day.  6-8″ inches of new snow overnight has added .4” of water weight over what was a very weak surface.  Storm slabs and fast moving loose snow sluffs will have the potential to be problematic and will gain volume and momentum once initiated, particularly in steep and channeled terrain.  Ice climbers take note as these may initiate naturally far above a climb.

SUMMIT LAKE: This region is just out of our advisory area to the south. The snowpack is shallow, composed of several weak layers, and experienced more wind effect and wind slab development. Extra caution is advised for triggering a slab avalanche.

Mon, January 20th, 2020
Alpine
Above 2,500'
2 - Moderate
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.
Recent Avalanches

A skier triggered about a 12″ deep wind slab over facets on the far south end of Turnagain Pass yesterday.  Overall snowpack was quite shallow and likely more similar to the Summit Lake region.

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

Finally some reprieve today in the form of new snow after a cold, dry start to 2020.  A surface low moved into the region late yesterday afternoon providing patient powder-seekers 6-8″ of low density snow overnight and moderately warmer temperatures.  New snow fell mostly on a weak, faceted surface and has not had a chance to bond.  We may see storm slabs forming today given (relatively) warmer temperatures and moderate wind in the alpine.  Any winds today will build fresh, shallow wind slabs given this low density snow and underlying loose facets.  A small slab initiated today may gain mass quickly and could entrain a significant amount of loose snow in steep, channeled terrain with potential to run to the valley floor.  Ice climbers in Portage Valley and elsewhere need to be particularly cognizant of this avalanche problem today and into the near future.

Close up of weak, faceted surface snow found on Sunburst, 1/18.  New snow today will not easily bond to yesterday’s (faceted) surface.  photo: Heather Thamm

Loose snow sluffs: Our storm snow last night and today is likely to exacerbate the loose snow sluffing problem by adding volume to an already impressive layer of loose faceted snow near the surface.  This will prove most dangerous in steep, channeled terrain as observers have reported sluffs running fast and far.  A sluff today may gain enough volume to be problematic for a skier above a terrain trap.

For your reading pleasure, here’s an interesting article on sluff managment techniques from the late 90’s written by some of the Valdez heli ski scene pioneers.

 

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

Very weak surface snow from the January cold and clear spell and a crust/ facet combo from NYE have persistent slabs still on our radar.  An observer yesterday triggered a wind slab over facets in the alpine on the far south end of Turnagain Pass.  The water weight of this most recent storm is unlikely to tip the balance for a natural avalanche cycle but it isn’t out of the question to see human-triggered avalanches on an older persistent weak layer today given the .4” of water we’ve just added to our generally weak snowpack.

Weather
Mon, January 20th, 2020

Yesterday: Mostly clear skies in the morning gave way to diminishing light and obscured peaks by about 4pm.  Light snow began falling around 5:30p with winds picking up into the 20’s at some ridgetop locations around midnight last night.  Temps at 1,000′ were around 0°F and warming to the mid- teens at higher elevations.  We’ve picked up 6-8″ of low density snow overnight, likely more in the alpine.

Today: Another 4-6″ of snow is expected throughout the day at all elevations, tapering off this evening.  Winds should stay mostly light from the southeast with temperatures topping out in the 20’s at 1,000′.

Tomorrow:  As the current surface low moves out of our area tonight and tomorrow, snow will taper off as another dry air mass sets up over mainland Alaska.  This will be a return to ‘cold and clear’ at least in the short term.

 

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 17 8-10 .4 47
Summit Lake (1400′) 7 1-3 .1 17
Alyeska Mid (1700′) 11 10 .4 44

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 ENE 10 39
Seattle Ridge (2400′) 20 ENE 3 12
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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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