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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 9th, 2019 - 7:00AM
Expires
Tue, December 10th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
High Avalanche Danger
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
Avalanche risk The Bottom Line

Due to strong winds, heavy snow and rain the avalanche danger is HIGH at all elevation bands. Natural avalanches are likely. Travel in avalanche terrain is not recommended. This includes low elevation terrain that is in the runout of slopes above.

*Today is not the day to hike Byron Glacier trail.

Special Announcements

CHUGACH STATE PARK – avalanche burial near Flattop:  Saturday, a solo hiker was fully buried and then rescued by another hiker in the area, over an hour later. Please see link for information on this near miss. We are very grateful to the hiker who responded and effected a successful rescue and very thankful the buried hiker is ok.

**Many summer trails cross avalanche terrain and please remember, avalanche training is necessary for traveling these trails during the snowy months.

Tonight!! Join CNFAIC Forecaster Aleph Johnston-Bloom at Blue & Gold Boardshop Monday, Dec 9th, 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).

Mon, December 9th, 2019
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
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

What to say… Call it the Hawaiian Sucker Punch, an atmospheric river or in the mouth of a firehose, we are in line getting heavy precipitation and strong winds across the forecast region today. Expect natural avalanches today. Up high the existing snowpack is getting a heavy snow load or rain. Rain is also saturating any snow left at lower elevations and the winds are cranking. This is great day for holiday shopping while wearing water wings. Travel in avalanche terrain is not recommended. This includes summer hiking trails in the runout of avalanche paths above. An inch of water has already fallen (2″ in Portage) since late last night with 100 mph winds.  Another 2.5 inches is forecast today and the high winds will continue. Expect a smorgasbord of avalanche issues including storm slab avalanches, wind slabs, wet loose and wet slab avalanches and cornice fall.

Atmospheric river bringing warm air and moisture from Hawaii!

Natural wind slab avalanche in Hippy Bowl observed yesterday, likely triggered by the winds on Saturday. 

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

The high elevation snowpack, over 3,000′, remains a concern due to a layer of faceted weak snow over a hard crust near the bottom of the snowpack. With heavy new snow and very strong winds today we are curious to see if this weak layer becomes overloaded and results in larger avalanches that break near the ground. This is yet another reason travel is not recommended today.

Weather
Mon, December 9th, 2019

Yesterday: Skies were partly cloudy in the morning becoming mostly cloudy by late afternoon. Temperatures were in the mid 30Fs to mid 20Fs. Winds were light and easterly in the morning and increased in the afternoon. Overnight easterly winds ramped up with peak gusts over 100 mph. Precipitation started in ernest just before midnight with an inch of water falling before 6 am this morning. Temperatures climbed into the low 30Fs at upper elevations and mid 40s at sea level.

Today: Cloudy skies and heavy rain and snow throughout the day, 2.5 inches of water/3′ of snow is forecast depending on elevation. Rain/snowline looks to be around 2500′ but may climb over 3000′. Winds are easterly 30-50 mph with gusts into the 100s. These should slowly diminish this evening. Overnight precipitation will continue but intensity will decrease. Another half an inch of water is forecast to fall and temperatures will remain steady.

Tomorrow: Partly cloudy with temperatures in the 30Fs. Winds will be light and easterly. Clouds will build again in the evening as the next system approaches. The pattern looks to be unsettled through the week with temperatures slowly coming down and a chance of snow increasing.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35 1 0.9 20
Summit Lake (1400′) 34 1 0.3 9
Alyeska Mid (1700′) 35 0.5 1.0 20

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25 NE 30 122
Seattle Ridge (2400′) NA* SE* 27* 51*

*Seattle Ridge temperature sensor is not functioning. A new temperature sensor is arriving soon and we hope to get it up on the next clear day. The winds came online at midnight.

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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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