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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, December 8th, 2023 - 7:00AM
Expires
Sat, December 9th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′ today. A fast-moving storm brought 4-6″ new snow with strong winds overnight, and a person might be able to trigger an avalanche up to a foot deep where the winds drifted that snow into sensitive slabs. Skies are looking to clear up this afternoon, so if you find yourself in the higher elevations be sure to take the time to identify and avoid slopes that have dense, wind-drifted snow on the surface. The danger remains LOW below 1000′.

 

SEWARD/LOST LAKE/SNUG HARBOR: These southern areas received around a foot of snow overnight, with strong winds continuing to blow this morning. This active weather makes natural and human triggered avalanches likely, and extra caution is advised in these southern zones.

Special Announcements

Headed to the Chugach State Park, Summit or Seward this weekend? Be sure to check back this afternoon for our Weekend Avalanche Outlook products, new this season, which we will be posting Friday evenings through the season.

Fri, December 8th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Sat, December 9th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Sat, December 9th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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.
Recent Avalanches

The last known avalanches occurred on Eddie’s ridge during Thursday’s outflow event. These were on the smaller size, up to a foot deep, and failed naturally.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

For the seventh day in a row, wind slabs remain our main avalanche concern. A quick-moving storm brought around 4-6″ low-density snow to Girdwood, Turnagain Pass, and the Summit Lake area last night, with 8-10″ in Portage and Placer, and 12″ to Seward. Unfortunately, that snow was accompanied by strong easterly winds overnight, including a period yesterday evening where speeds averaged 36 mph with gusts to 59 mph at the Sunburst ridgetop weather station. Although the winds are calming down and the clouds are breaking up during the day, we are expecting the wind slabs that formed last night and early this morning to remain reactive during the day today.

It’s looking like it will be a great day to get out, and might even be sunny by this afternoon. If you plan on taking advantage of this weather window before things get stormy again later this weekend, be on the lookout for slopes that have dense, wind-drifted snow on the surface. Keep in mind, a reactive wind slab will not always be a pencil-hard slab; it may just be slightly stiffer than the snow you’ll find on a sheltered slope. If you notice cracks shooting out from your skis or snowmachine, take that as a serious sign that you’ve found a slope where you can trigger an avalanche. Wind slabs can be a little easier to assess as you travel than some other types of avalanche problems. You can gain a lot of info by testing conditions on small terrain features, or taking a minute to step off the beaten path to do a quick hand pit and see if you find the telltale sign of stiff snow on top of soft snow. With a wind slab problem, it works out pretty well that the best skiing and riding will be in sheltered terrain, which will also be the most stable conditions.

The wind was going to work moving massive amounts of snow on Pyramid yesterday afternoon. Photo taken from Girdwood looking south. 12.08.2023

Small test slopes like this one can be incredibly useful in assessing a wind slab problem. This was from Mary’s field day in the Front Range last week, but you can expect to find similar conditions across the advisory area today. 11.28.2023

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

There are two deeper weak layers in the snowpack that we are keeping an eye on. The first is a layer of faceted snow that is developing right above the crust that formed during the Thanksgiving storm, and the second is a layer of facets right at the bottom of the snowpack. It’s been three weeks since we saw an avalanche fail on the facets at the ground, and we have yet to see any avalanches failing at the Thanksgiving crust. However, we continue to get mixed results on that crust layer in our test pits, and we don’t want it to catch anyone off guard. If you’re planning on getting into bigger terrain, it would be a good idea to suss out that interface first.

Weather
Fri, December 8th, 2023

Yesterday: Clouds built through the day, with increasing winds in the afternoon. All of the exciting action happened overnight, with 4-6” snow in Girdwood, Turnagain Pass, and Summit; 8-10” in Portage and Placer, and 12” in Seward. Winds peaked overnight with sustained speeds of 20-35 mph and gusts of 40-60 mph out of the east, and have since switched westerly at 10-20 mph.

Today: We might see another inch of snow this morning before this round passes, with clouds breaking up later in the day an a good chance of sun this afternoon. Temperatures will stay cold today, with highs in the single digits F and overnight temperatures hovering in the single digits to low teens F. Winds will back down during the day, from around 10-20 mph out of the west this morning to 5-10 mph out of the south this afternoon.

Tomorrow: Skies should be partly cloudy tomorrow with a chance for some light snow showers that might bring an inch of snow. Temperatures will stay in the single digits to low teens F during the day, dropping to the single digits above and below zero overnight. Winds should be light out of the west to northwest. Head’s up Seward- it’s looing like you could see another 8-10” snow tonight into tomorrow, with stronger winds during the day.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20 5 0.4 52
Summit Lake (1400′) 18 6 0.4 27
Alyeska Mid (1700′) 20 6 0.33 36
Bear Valley – Portage (132′) 20 8 0.7

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11 E-W 12 59
Seattle Ridge (2400′) 15 E-W 8 25
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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.