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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 10th, 2020 - 7:00AM
Expires
Tue, February 11th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE at all elevations and may rise to CONSIDERABLE in the Alpine this afternoon with increasing outflow winds. Triggering a large and dangerous slab avalanche 2-3′ thick on buried weak layers is possible and these avalanches may be triggered remotely. Watch for blowing snow and wind slabs forming in steep leeward terrain. Evaluate snow and terrain carefully.

Special Announcements

6th Annual Snowball! Valentine’s Day – February 14, 2020. 7-11 pm at 49th State Brewing Co. in Anchorage. I Like Robots will be rockin’ our favorite 80’s tunes! Awesome silent auction and costume contest, so break out your best 80’s fashion and snow gear! $35 tickets on sale HERE. Proceeds support Friends of the Chugach Avalanche Center and the Alaska Avalanche School.

Mon, February 10th, 2020
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
  • 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

The current state of the snowpack could well be described as scary MODERATE. The likelihood of triggering a large dangerous avalanche is decreasing but the consequences remain very real.  The overall snowpack structure is poor, especially in the mid elevation band.  There is slab 2-3+ feet thick over weak layers of snow. Last week there were multiple natural and human triggered avalanches that released on the buried weak snow. Snowpack tests indicate that this set up now might be stubborn to trigger but could still fail. Girdwood to the north side of the Pass received the most snow this weekend and we are pretty close to having what is considered a deep slab issue. Please think about a few important factors when you choose your objective today:

  • Tracks on a slope do not make it safe. It could be the 2nd or 10th person that triggers the slab.
  • Remote triggering is possible. This means triggering a slide from below, the side or from on top
  • Signs of instability may or may not be present before a slab releases. Observers are still noting whumpfs in the mid elevation band.
  • Thin areas in the snowpack or near rock outcropping are likely trigger points.
  • Consider the consequences if the slope does slide. Choose terrain wisely. Is there a terrain trap you could be caught in?

Increasing winds in the afternoon today may add additional load in the Alpine and potentially trigger an avalanche that steps down to a buried weak layer.

Snowpack structure at 2000′ on Seattle Ridge, 2.9.20. Photo: Wendy Wagner. 

Video link HERE. 

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Northwest winds are forecast to increase this afternoon, are expected to reach averages of 15-25 mph with stronger gusts and there is snow available for transport. This flow direction can funnel through Crow Pass near Girdwood and the Kenai mountains in interesting patterns, loading different aspects on the same pieces of terrain, and can also split around and spare certain zones.

For anyone headed out today, keep a close eye out for where the winds are transporting snow, building cornices and loading slopes. Plumes are likely to be visible if the winds verify. As always, feel for stiff snow over softer snow and any cracks that shoot out from you. These are signs you’ve likely found a wind slab. Additionally as mentioned above, buried weak layers hide 2-3+’ below the surface and freshly wind loaded slopes could overload these, creating a much larger avalanche.

Cornices:  Cornices have grown over the past few days and winds today could form new ones. Avoid travel on or underneath them and remember a cornice fall could trigger an avalanche on the slope below.

Weather
Mon, February 10th, 2020

Yesterday: Skies were mostly overcast. Temperatures were in the low 30Fs to mid 20Fs. Winds were southerly and light. Overnight temperatures cooled slightly and winds remained light.

Today: Skies will be mostly sunny. Temperatures will be in the 20Fs to high teens and will drop into the single digits this evening. Winds are shifting to the west/northwest this morning and are forecast to increase this afternoon blowing 10-20 mph with gusts into the 30s. Skies will be mostly clear overnight and winds should decrease early tomorrow morning.

Tomorrow: Mostly sunny skies with temperatures in the teens to low 20Fs. Winds will be calm during the day and then increase into Wednesday. Clouds will build overnight and with snow showers starting early Wednesday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 0 0 56
Summit Lake (1400′) 27 0 0 23
Alyeska Mid (1700′) 28 0 0 59

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21 S 9 39
Seattle Ridge (2400′) 24 SE 5 10
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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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