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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, November 30th, 2019 - 7:00AM
Expires
Sun, December 1st, 2019 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A CONSIDERABLE avalanche danger remains at the higher elevations above treeline. Storm snow avalanches, including wind slabs, are likely to be triggered on slopes over 30 degrees. Additionally, triggering a surprisingly larger, and dangerous, slab avalanche is possible that breaks in old weak snow 1-3+ feet below the snow surface. Heads up once the skies clear and we can travel above treeline!

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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.

Sat, November 30th, 2019
Alpine
Above 2,500'
3 - Considerable
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
  • 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

Winter is hanging on by a thread at Turnagain. Beginning today, we should see slowly cooling temperatures and hopefully a few inches of snow making it close to sea level over the weekend. Despite the low coverage below 2,500′, avalanche issues seem to be very alive and well in the Alpine.

For today, storm snow instabilities such as wind slabs, storm slabs and cornices remain likely to trigger above 2,500′ where dry snow exists. Winds have redistributed much of this high elevation snow. As the storm slowly moves out this weekend, we are still within the 24-48 hour window of a heavy snowfall event and the snowpack needs time to adjust.



Wind loading looking down from the Sunburst Ridge, Friday 11/29.


Scouring and wind loading on Magnum Ridge, also from yesterday.

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

Spooky snowpack in the Alpine. Above 3,000′ there is evidence of a reactive layer of facets sitting on a crust anywhere from 1-3+ feet below the snow surface. Aleph found this concerning layer yesterday on Sunburst at 3,000′. Of note, one of our public observers experienced a large whumpf at 3,200′ in the South Fork of Eagle River on Thanksgiving day (Chugach State Park lands), indicative of a similar snowpack structure that could exist region wide.

**Heads up for travel in the Alpine once the skies clear. If this layer of facets turns out to be widespread, large and deadly avalanches are possible up to 3-5 feet thick. Moving forward, this is enough of a concern that we must use very careful snowpack assessment and terrain management in the upper elevations.

Red flags to watch for:

  • Recent avalanches – Do you see any signs of avalanche activity?
  • Whumpfing (collapsing) of the snowpack?
  • Shooting cracks, likely to be seen near ridgelines where the wind has formed wind slabs?

Location of the large whumpf (collapse) that Aleph experienced yesterday (Sunburst Ridge, 3,000′).

Weather
Sat, November 30th, 2019

Yesterday:  Mostly to partly cloudy skies were seen with light rain below 2,500′ on the northern end of Turnagain Pass. Girdwood picked up .4″ of rain over the past 24 hours at the lower elevations, which equates to around 3-5″ of snow at the higher elevations. No precipitation was recorded at Turnagain Pass or Summit Lake. Winds have been in the teens to 30’s mph along ridgetops from the east. Temperatures remained warm, in the mid to upper 20’s F along the ridgelines and mid to upper 30’s F at 1,000′.

Today:  Mostly to partly cloudy skies are expected with a chance for 1-2″ of snow above 2,000′; a light rain/snow mix is possible at the parking lots at 1,000′. Winds should remain easterly in the 10-25 mph range along the ridgetops. Temperatures will slowly cool to the low 20’s F along the ridgetops and the low 30’s F at 1,000′.

Tomorrow:  Cloudy skies with strong ridgetop easterly winds are forecast as another low pressure moves through. If the stars align and enough cold air gets wrapped in, there could be a chance for several inches of snow to fall close to sea level. Stay tuned! This appears to be our last shot at snow before a cold clear period with northerly winds set in for the mid-week.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34 0 0 13
Summit Lake (1400′) 36 0 0 7
Alyeska Mid (1700′) 34 0 0.4 17

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 26 NE 20 52
Seattle Ridge (2400′) NA* NA* NA* NA*

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

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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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