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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 2nd, 2019 - 7:00AM
Expires
Tue, December 3rd, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

A MODERATE avalanche danger exists in the Turnagain Pass region above 2,500′.  Triggering a small fresh wind slab along the higher ridgelines is possible today.  Additionally, it may be possible to trigger a much larger avalanche above 3,000′ due to a suspected weak layer 1-3 feet below the snow surface.

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Tuesday, Dec 3rd: Turnagain Pass – Snow, Weather and Avalanches @ Ski AK
6:30pm – 8:00pm. Cost FREE!
Join CNFAIC forecasters for a look under the hood at the avalanche center. We’ll discuss current Turnagain Pass snow and avalanche conditions, how avalanche forecasts are produced and some tips on being your own avalanche forecaster.

Mon, December 2nd, 2019
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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
  • 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

Yesterday upper elevations picked up around 5″ of new snow and as temperatures cooled overnight an inch or so fell to sea level. Winds were easterly and strong enough to blow snow around at ridgetops. Winds shifted to the west/northwest overnight and are forecast to increase. Be on the lookout for shallow wind slabs today and watch for opposite loading patterns with the wind shift. Northwest winds can do some funky things as they are funneled through Turnagain Pass. Drifting, cracking and hollow sounding snow are all indicators of wind slab conditions.

Wind loading on Sunburst 11.29.19. Pay attention to loading patterns today!

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

We continue to be concerned with the snowpack above 3000′. We have very limited snowpack data from the upper elevation slopes that harbor dry snow. The issue is a layer of faceted snow that sits on a very hard melt-freeze crust near the base of the snowpack. The crust is widespread and found in every pit dug by forecasters and/or observers. The faceted layer on the other hand is the question. How widespread is it? We’ve only found it on Sunburst and Tincan so far… yet it is suspected to exist in upper Girdwood Valley and possibly region-wide. With such limited data at this time we are still in the “guilty until proven innocent” mindset moving forward. We will be speaking more to this as the skies clear this week and travel to our upper elevation zones becomes possible.

Suspect snowpack structure Sunburst, 11.29.19

Weather
Mon, December 2nd, 2019

Yesterday: Skies were cloudy and there was light rain/snow showers with upper elevations picking up 5″ of snow in the last 24 hrs. Temperatures were in the mid 30Fs to mid 20Fs. Winds were easterly 10-20mph with gusts into the 30s. Overnight temperatures cooled slightly and brought snow to sea level. Winds became light and westerly.

Today: There is snow in the forecast with temperatures in the high twenties to mid teens, 1-4″ of snow possible to sea level. Winds will be northwesterly 10-20 mph with outflow conditions over the eastern Kenai Peninsula and maybe gusty. Overnight very light snow showers continue with temperatures in the teens and northwest winds.

Tomorrow: Mostly cloudy with a chance of snow and temperatures in the teens and low twenties. Winds will remain northwesterly. There is a drying trend into the week as ridge of high pressure moves over the area. Look for cooler temperatures and some sunshine Wednesday and Thursday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31 4 0.3 15
Summit Lake (1400′) 31 2 0.2 9
Alyeska Mid (1700′) 31 5 0.4 15

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 NE 10 35
Seattle Ridge (2400′) NA* NA* NA* NA*

*Seattle Ridge is not recording temperature and wind stopped recording at 10 pm on 11/28.

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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