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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 14th, 2019 - 7:00AM
Expires
Sun, December 15th, 2019 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE above 1,000′. At the mid-elevations, glide avalanches continue to release. Avoid being under any crack forming in the snowpack as these are very unpredictable avalanches that release spontaneously. At the high elevations, new snow and strong wind over the last two days have created storm snow avalanche issues. This includes wind slabs and storm slabs (around 1 to 2 feet thick) and cornice falls, any of which could easily be triggered by a person.

*Careful snowpack evaluation is necessary for travel into the high elevation zones with dry snow.

Special Announcements

Headed to Hatcher Pass? Don’t forget to check hpavalanche.org and their Facebook page!

Join CNFAIC Forecaster Aleph Johnston-Bloom at Blue & Gold Boardshop Monday, Dec 16th, 7:00-8:30 for a FREE evening avalanche discussion on patterns in Alaskan avalanche accidents with practical takeaways to use this season. There will also be an avalanche gear demo outside in the snow (weather permitting).

Sat, December 14th, 2019
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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. Dangerous avalanche conditions. 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

Despite very socked in weather and poor visibility yesterday, we do know of several glide avalanches that released in the Girdwood Valley. We are very suspect many others released in the forecast area as well but just could not see them.

Glide avalanche on the lower portion of Raggedtop Mtn, south facing slope. Released around 12/12.

 

Yesterday’s storm is slowly moving out this morning and we should see a break in cloud cover and precipitation before another front heads in this evening. The most exciting news however… is the snow line dropped last night! Snow is falling at 1,000′ this morning and may add an inch or two before skies clear.

Turnagain Pass DOT RWIS station snow stake at 1,000′. 

Avalanche Problem 1
  • Glide Avalanches
    Glide Avalanches
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

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 glide cycle we are currently in is expected to continue today with our cooling temperatures. It may seem a bit counter intuitive, but glide cracks tend to release during cooling trends as well as warm periods. It will be interesting to see what happens today.

Glide avalanches are completely unpredictable and not triggered by people or explosives. The only way to manage this problem is to avoid, or limit, any exposure under glide cracks. These cracks look like big brown frowns- keep your eye out for them.

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

Snowfall in the Alpine zones yesterday added another 8 inches to a foot of new snow. This is on top of 6 – 10″ of new snow from the day before (Thursday). Strong easterly winds were associated with the snowfall and though decreasing slightly today, should still be blowing in the 15-30mph range along ridgelines from the east. This said, don’t be surprised to find wind slab avalanches up to 2 feet or more thick.

If traveling to the higher elevations in search of dry snow today be very wary of finding and triggering a wind slab avalanche. Things to be on the lookout for:

  • Watch for wind loading patterns and areas with wind deposited snow.
  • Feel for stiffer snow over softer snow.
  • Is there any cracking in the snow around you? Hollow feeling snow?
  • Quick hand pits can be a good way to assess the top foot or so of snow. Remember, the weak layer may be lower than this and could be missed.

Storm slab avalanches:  Areas out of the wind could harbor slab avalanches that are simply made up of the storm snow that has not bonded yet with the older snow surface. Again being aware of any red flags in areas with new snow.

Cornices:  Give cornices a wide berth! They likely have grown and could be easy to knock off.

Additional Concern
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.
More info at Avalanche.org

In the Summit Lake area and other shallow snowpack zones that have seen less new snow, we are concerned about weak faceted snow that sits under the new snow from the past week. This is at the upper elevations, above 2,500′, where the snow is dryer. Before the storm cycles, faceted snow existed at the base of the snowpack and other layers of facets/crusts in areas. How these layers have adjusted over the past week of stormy weather and increasing snow load is something we are watching and something to pay attention to if headed to these areas.

Weather
Sat, December 14th, 2019

Yesterday:  Cloudy skies and heavy rain was seen in the morning below 2,500′ with snow above. Light rain continued overnight along with cooling temperatures, which brought snow down to 1,000′ (finally!). Almost an inch of water fell at Turnagain and Girdwood over the past 24-hours, adding up to a foot of new snow above 2,500′ and around 1-2″ of snow at 1,000′ from overnight’s cooler weather. Ridgetop winds remained strong yesterday averaging 30-40mph from the east.

Today:  A break in storms today should bring partly cloudy to clear skies before another front moves in this evening. Scattered snow showers may add another .2-.4 ” of rain at sea level and 2-4″ of snow from 1,000′ and above. Ridgetop winds look to decrease slightly to the 15-30 mph range from the east. Temperatures should remain on the cooler side, near 32F at 1,000′ and 25F along ridgelines.

Tomorrow:  Another round of precipitation and wind will move through tomorrow. The rain/snow line is forecast to bump back up slightly to 1,500′ and possibly 2,000′. We could see an inch or so of rain at sea level with another foot or so of snow at the high elevations. Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33 4 0.9 21
Summit Lake (1400′) 33 1 0.2 7
Alyeska Mid (1700′) 32 4 0.75 13

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24 NE 23 78
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Seattle Ridge weather station is down and as soon as the weather clears we will get it up and running!

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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.