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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, December 1st, 2023 - 7:00AM
Expires
Sat, December 2nd, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE at elevations above 1,000′. Today is the first day after a storm and the most likely time for people to trigger avalanches. These types of avalanches are storm slabs up to 3 feet thick or wind slabs from 1-6 feet thick. Due to the size, any avalanche triggered today will likely be dangerous and unmanageable. Letting the snowpack adjust and sticking to the lower angle slopes (30 degrees and less), without steep slope above you, is a good way to avoid these avalanche issues.

At elevations below 1,000′, the danger is MODERATE for overhead hazard. Meaning triggering an avalanche is unlikely, but debris running into this zone from above is possible.

Special Announcements

AKDOT & PF:  There will be intermittent traffic delays today, Dec 1, on the Seward Highway for Avalanche Hazard Reduction work from mileposts 88.5 to 83 on the Seward Highway, south of Girdwood. Expect delays up to 45 minutes from 9:00 am to 11:00am. Updates will be posted at 511.alaska.gov.

New weekend outlook products:  Starting this evening, we will begin issuing Weekend Avalanche Outlooks for Chugach State Park, the Summit Lake/Central Kenai zone, and the Seward/Southern Kenai zone. These products will be published Friday at 5pm and will provide avalanche information for three new areas.

Fri, December 1st, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sat, December 2nd, 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 2nd, 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

Another stormy day made it hard to see the extent of avalanche activity in the backcountry yesterday. However, we did get a couple reports of small pockets of 2 feet deep storm slabs that people were able to trigger on small slopes and rollovers in the trees.

Additionally, a large avalanche was seen in motion near Girdwood; seen from the Seward Highway running almost to sea level.

Andrew noticed this debris in motion yesterday afternoon as he was driving between the Portage Curve and Girdwood. 11.30.23.

 

An example of a storm slab in terrain that is not steep enough to slide, except for a small rollover. Photo by Kakiko Ramos-Leon, 11.30.23.

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

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

It seems the word is out. After a potent 2-day storm deposited 2 – 3 feet of snow, skies are clearing and temperatures are cooling today and through the weekend. The stoke is high but caution is warranted as we head into the mountains.

First, ridgetop easterly winds continued to blow last night. Sunburst reported gusts at 60mph just after sunset and this morning winds are 15-20mph gusting near 30. The snowfall ended right around midnight. That said, today we are less than 24 hours from the end of the storm and in the prime window for human triggered avalanches.

How quickly the snowpack will stabilize is the question. Typically it takes 1-2 days without any funny weak layers. So as we gather more information, our message is to err on the conservative side with our terrain choices, sticking to the lower slope angles and smaller slopes. Keep your eyes peeled for evidence of recent avalanches, this is a great clue as to how the storm snow bonded to the old Thanksgiving crust. *But keep in mind the absence of recent avalanches may just mean they were covered back up at the end of the storm. Once again there are two main types of avalanches we are most concerned with:

Storm Slabs:  On slopes over 30 degrees, and especially on the steeper slopes (40 deg and above), it could still be easy to trigger an avalanche composed of the new snow (2-3 feet deep). This would be a large avalanche and certainly unmanageable and dangerous. Shooting cracks and whumpfing may be present, but not necessarily. With such a thick layer of storm snow, these signs can be a bit hidden. The new snow fell on a crust that formed after the Thanksgiving rains and we’ll be watching for how well it bonds over the foreseeable future.

Wind Slabs:  We can bet the higher terrain and exposed slopes above the trees harbor all kinds of wind slabs. They could be anywhere from a foot deep to 6 feet deep or more in places. They may or may not be easy to trigger today, but because of their depth could allow a person onto the slope before releasing. Watch for the classic wind slab signs, stiffer snow over softer snow and cracks that shoot out from you.

Cornices:  The ridgelines are likely to look a bit different after the storm. Watch for overhanging snow and remember these can break further back than expected.

 

Storm day conditions at Turnagain. 11.30.23.

 

Snowpack at the lower elevations of Turnagain Pass. No weak layers found here, just heavy moist snow. Jon Davis, 11.30.23.

Weather
Fri, December 1st, 2023

Yesterday:  The last pulse of a 2 day storm moved through yesterday. Around 12″ of snow fell during the day at Turnagain Pass, Portage Valley, and Girdwood. The rain/snow line fluctuated between 100 and 800′ through the day. Ridgetop winds were strong, gusting near 60mph at times with sustained winds 20-30mph from the east.

Today:  Clearing skies, cooling temperatures, and gusty easterly ridgetop winds are forecast today (10-20mph with gusts to 40, quieting in the afternoon). No precipitation is expected. Temperatures should sit in the 20’sF at most locations.

Tomorrow:  Mostly clear skies with some high clouds are expected through the weekend. Temperatures will continue dropping, into the low teens by Sunday, with light and variable ridgetop winds. Excellent wintertime weather.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 12 1.1 59
Summit Lake (1400′) 29 2 0.2 23
Alyeska Mid (1700′) 31 9 0.93 41
Bear Valley – Portage (132′) 36 rain 1.4

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 NE 22 60
Seattle Ridge (2400′) 26 N/A* N/A* N/A*

*Seattle Ridge anemometer is rimed over from the warm storm.

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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.