Turnagain Pass RSS

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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 22nd, 2022 - 7:00AM
Expires
Fri, December 23rd, 2022 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

MODERATE avalanche danger remains above 1,000′. New wind slab avalanches could be found and triggered in the higher elevations due to increasing easterly winds over the past 24 hours. These could be 1-2′ deep and forming near ridgelines. Watch for active wind loading. Additionally, triggering a larger slab avalanche that breaks in weak snow 2-4′ deep is possible. These larger slabs are most concerning in Girdwood Valley and the south end of Turnagain Pass. The danger is LOW below 1,000′.

SUMMIT LAKE:  The snowpack in the Summit Lake area is generally thinner and weaker than Turnagain Pass. Strong winds may create wind slabs that could overload buried weak layers, creating a lager avalanche. Extra caution is warranted in this area.

Special Announcements

Chugach State Park: A large avalanche was seen from a distance above Eagle River yesterday. There have been a number of reports of an unstable snowpack in the CSP over the past week.

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Thu, December 22nd, 2022
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. Dangerous avalanche conditions. 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

We did not get any reports of wind slab avalanches yesterday, despite the uptick in easterly winds. The last known avalanches in our forecast zone were immediately following the Dec 14-15 storm. Multiple large avalanches failed in weak faceted snow near the Thanksgiving crust, with the largest avalanches in the Girdwood Valley and on the south end of Turnagain Pass.

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

Although a few inches of light snow is falling this morning, which should tapper off before noon, the main news is the winds. Ridgetop winds have been blowing steadily now for 24 hours from an easterly direction. Speeds are 10-20mph with gusts as high as 40mph at our weather stations. One report from the field yesterday noted the winds were just strong enough to transport some snow, but not quite enough to really load slopes. That said, this may not be the case everywhere.

For today, watching for active wind loading will again be our main clue as to where wind slabs could be forming. Slabs could be anywhere from 1-2′ thick, forming in smaller pockets or catchment zones, and on the softer side due to the lower wind speeds. Along with watching for the winds, feeling for stiffer snow over softer snow and noting any cracks that shoot out from you will be prime clues you’ve found a wind slab.

Unfortunately we have no photos or videos this morning from John Sykes who was at Summit Lake gathering snowpack information yesterday. He was delayed, with many others, behind the road closure near Portage until late last night; thoughts to all involved. Look for his report tomorrow and big thanks to John and Eliot Pearce for passing on their observations from Eddies yesterday.


Ski penetration on the ridge near treeline at Eddie’s. Note the wind effect. John Pearce, 12.21.22.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • 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.

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

Below any new wind slabs, buried 2-4′ in the snowpack, are various layers of faceted snow that have formed both above and below the Thanksgiving crust. These layers caused several noteworthy collapses at Turnagain as well as large avalanches in Girdwood and the south end of Turnagain Pass during the tail end of the last storm, a week ago now.

As time goes on, it’s becoming more unlikely a person could trigger an avalanche that fails in these deeper layers. Furthermore, we have not heard of anyone getting a collapse for several days. All good signs. But, it’s still prudent to remember these layers exist and be especially suspect of them in areas that have seen little or no traffic. It is still early season and there could be surprises out there. One of the many reasons to always practice good travel techniques such as exposing only one person at a time and watching our partners.

Weather
Thu, December 22nd, 2022

Yesterday:   Partly cloudy skies were over the region yesterday. Ridgetop winds blew 10-20mph with gusts in the high 30’s from the east. Temperatures warmed through the day into the 20’s F at most locations.

Today:  Light snowfall this morning then skies should clear by midday. Around 2-4″ of total snow is expected in most areas. Ridgetop easterly winds should continue to blow 10-20mph with stronger gusts until this evening. Temperatures have warmed substantially, mid 30’sF at sea level and into the 20’s in the mountains.

Tomorrow:  Mostly clear skies with light ridgetop winds are expected tomorrow with temperatures cooling back into the teens. The next storm cycle is still looking to move in right around Christmas day. Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 24 2-3 0.2 39
Summit Lake (1400′) 21 1-2 0.1 30
Alyeska Mid (1700′) 23 3 0.24 40
Bear Valley – Portage (132′) 24 3 0.3

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14 NE 16 39
Seattle Ridge (2400′) 19 SE 10 26
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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.