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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 7th, 2023 - 7:00AM
Expires
Fri, December 8th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE on slopes at the mid and upper elevations (above 1,000′). Both lingering wind slabs from yesterday’s winds along with fresh wind slabs that could form this afternoon will be the main issue. A snow squall is forecast to bring 4-8″ of snow this evening along with east winds beginning sometime after noon. Wind slabs should range from 6-12″ thick and found on various aspects near ridgelines and gullies.

Special Announcements

Chugach State Park:  Strong NW winds were blowing snow off the ridgelines yesterday along Anchorage’s Front Range. Watch for freshly formed wind slabs.

Become a Member in December! The Friends of the Chugach Avalanche Center is a non-profit, which means we need your help to keep our avalanche center running. This is our most crucial time of year for fundraising, so if you haven’t yet, please consider becoming a member. Did you know that nearly half of our Forest Service avalanche specialists’ positions are funded by community donations to the Friends, as well as 100% of the new Chugach State Park Avy Specialist’s position? Everyone who donates will be entered to win some awesome prizes (like Dynafit bindings, a Voile splitboard, and more!) at our Girdwood Brewery Forecaster Chat on January 19.

Motorized event tomorrow night – Friday in Anchorage:  Join Alaska Safe Riders and professional snowmobilers Dan Adams, Matt Entz, and Dustin Pancheri for an Avalanche Awareness Open House to prepare for your next mountain adventure.  All the details can be found here!

Thu, December 7th, 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
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
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
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

Strong NW Outflow winds occurred yesterday in many areas of Southcentral. Although much of Turnagain Pass looked to escape these winds, a group on Eddies noted the unique terrain channeling effect that can occur during a NW wind event. This is a band of southerly winds (yes southerly) that forms along the east side of Turnagain Pass (non-moto side). The southerly winds formed wind slabs, wind crusts, and a few shallow wind slab avalanches on west to north aspects yesterday. See their report HERE.

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

As of this morning, winds have quieted and temperatures have dropped; Turnagain Pass parking lots sit around 10F currently. The break in weather will be short lived as a quick hitting snow storm is forecast for this afternoon. Between 2 and 9pm 4-8″ of snow is expected with moderate easterly winds (10-20mph with gusts near 30). Snow should fall to sea level even though temperatures will rise a bit with the weather.

Wind Slabs:  Lingering wind slabs formed yesterday and any new wind slabs that form this afternoon will be the main concern. They are likely to be relatively shallow, up to a foot deep. Watch for these on all aspects in exposed areas in the trees, and above the trees. The usual signs will be stiff snow over softer snow, cracks that shoot out from your machine, board, or skis. Keep in mind, new snow can cover up the older slabs making them a bit harder to see.

In the mountains after dark:  The peak of the storm is forecast around 6pm (around 2 hours after sunset). So if anyone is out after dark, new snow sluffs in steep terrain may be seen in places that receive upwards of 6″ of new snow.

 

This NWS graphic shows 24 hour snowfall, yet for this storm it looks like it all happens between 2 and 9pm today, Thursday. These images can be found HERE. 

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

If you have been reading this far into the forecast, you’ll know we are paying close attention to the Thanksgiving crust (buried 2-4′ deep). Crusts can develop weak snow around them over time and produce large avalanches down the road. How this one will evolve is still a question. We do not not know of any avalanches on it yet, but snowpits are starting to show signs it may be a future problem.

Additionally, at the upper reaches of the forecast zone (above 3,500′) a layer of facets at the ground, now buried 6-8′ deep, may still be weak enough to produce a large avalanche if a person finds the perfect thin trigger spot. This is becoming more and more unlikely with time but something to consider in high alpine steep and rocky zones.

Weather
Thu, December 7th, 2023

Yesterday:  High clouds were over the region yesterday with no precipitation. Ridgetop winds were out of the northwest and blowing 5-15mph. However, a strong outflow wind event (NW winds) was seen along the Western Chugach Mtns and Southern Kenai.

Today:  Increasing clouds, cold temperatures, and a burst of snow and wind this afternoon. This morning winds are light and temperatures sit between 0-15F. However, a front is forecast to pass through this afternoon/evening that will bring 10-20mph east winds and 4-8″ of snowfall (to sea level). Snow should start falling just after noon and peak around 5-6pm.

Tomorrow:  Clearing skies, light winds, and chilly temperatures are expected tomorrow, Friday, before clouds and another chance of snow pushes in on Saturday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18 0 0 48
Summit Lake (1400′) 15 0 0 21
Alyeska Mid (1700′) 20 0 0 31
Bear Valley – Portage (132′)
Grouse Creek (700′) 28 0 0

 

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11 NW 4 14
Seattle Ridge (2400′) 16 NW 8 18
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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.