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Issued
Sun, December 22nd, 2019 - 7:00AM
Expires
Mon, December 23rd, 2019 - 7:00AM
Forecaster
Ryan Van Luit
Avalanche risk The Bottom Line

The avalanche danger will be LOW today and is expected to rise to MODERATE tonight in the Alpine as new snow and wind impacts the forecast area.  Ridgetop winds are expected to increase and transport snow this afternoon, which may begin to form small fresh wind slabs in leeward terrain.  Until then, watch for lingering wind slabs and give cornices a wide berth. Although unlikely, at the high elevations above 3,000′, we are still concerned a layer of weak faceted snow near the ground could produce a larger avalanche if someone hits just the right thin spot.

Sun, December 22nd, 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
  • 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

An increase in wind with a few inches of snow (to sea level!) is in the forecast beginning this afternoon. Winds should start picking up around noon, but are not expected to be strong enough to transport snow till around sunset. A few snow flurries could fall today, but the heavier snowfall is expected overnight. The increase in avalanche danger will be directly related to the amount of wind and snowfall.

In the high Alpine, above 3,000′, there is still concern over a weak layer of snow sitting near the base of the snowpack. This is greater where the overall snowpack is shallower- towards the southern end of Turnagain Pass, in Summit Lake and in the Crow Pass terrain north of Girdwood.  The weak layer is buried anywhere from 1 – 6+ feet deep due to the extensive variable wind during the early to mid-Dec storm cycles. There is still a lingering concern that a person could trigger a large avalanche at the high elevations above 3,000′.

NORMAL CAUTION avalanche issues during the daylight hours, before the winds and snow arrive:

Cornice falls:  Large cornices have formed in the alpine along some ridges and gullies.  A ‘cornice crevasse’ was found yesterday, check out the photo below. Give cornices a wide berth and limit exposure under them.

Lingering and possible new wind slabs:  Watch for lingering older wind slabs that may not be well bonded with the snow underneath. As winds are predicted for this afternoon, new wind slabs could form, yet this may not materialize until after sunset.  Small wind slab failure can easily force a person off their feet down the fall line. Use caution when traveling above higher consequence terrain (such as cliff bands and rocky areas).

Loose snow sluffs:  The surface snow is loose in steep terrain.  Remain mindful when traveling above cliffs and rock bands.

Large cornice crevasse was found and pictured by Adrian Beebee along the Magnum Ridgeline. These cracks happen when the cornice peels away from the ridge yet has not fallen or may be supported by the slope below. Size was estimated at 6′ wide and 12′ deep – yipes!!

Looking forward to a new layer of snow tonight and into the week:
Our current snow surface is now covered with feathers of surface hoar – it appears to widespread through the region. This is a notorious weak layer once it becomes buried…

Solstice Surface Hoar:  The past few days of cold clear weather produced this crop of surface hoar on Tincan with Sunburst in the distance – possibly a future weak layer. Thank you to Troy Tempel for the photo!

Additional Concern
  • Glide Avalanches
    Glide Avalanches
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.
More info at Avalanche.org

Keep your eyes peeled for glide cracks in the terrain that you are traveling in. Steer clear and avoid lingering in the runout of this unpredictable hazard. There is a chance we may see some more glide cracks open and release as temperatures slowly rise.

Weather
Sun, December 22nd, 2019

Yesterday:  Clear to overcast skies with patchy valley fog in places. Ridgetop winds were light (5-10mph) and westerly. Temperatures were in the single digits along ridgelines and valley bottoms.

Today:  Partly cloudy to cloudy skies remain in Southcentral with intermittent snow by late this afternoon increasing to steady snow overnight.  Snow accumulation of 2-4″ at 1000′ by tomorrow morning is expected with possibly double this at the higher terrain.  Ridgetop winds are forecast to increase throughout the day from the east from 15-35mph.  Temperatures will slowly rise to the teens in valley bottoms and near 20F along ridgelines.

Tomorrow:  Light snowfall will continue through Monday and into the rest of the week. Tomorrow’s amounts look to be in the 1-4″ range. Ridgetop winds look to back off for Monday before another pulse of snow and wind head in for Christmas Eve.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 8 0 0 28
Summit Lake (1400′) 4 0 0 7
Alyeska Mid (1700′) 10 0 0 15

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8 W 10 23
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Seattle Ridge weather station is not reporting, we hope to have it back online later today.

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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.