Turnagain Pass Avalanche Forecast RSS

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Fri, February 19th, 2021 - 7:00AM
Expires
Sat, February 20th, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is expected to rise to CONSIDERABLE above 2,500′ today due to strong northwest winds forecast to impact the region. In this case, wind slab avalanches and cornice falls may occur naturally on upper elevation slopes seeing active wind loading. Human triggered wind slab avalanches around a foot deep will be likely on any slope with new wind deposited snow. In areas without wind effect, the danger is MODERATE where human triggered slab avalanches 1-2′ deep remain possible at all elevations due to buried weak layers.

*Gap winds squeezing through Crow Pass and Portage Pass are expected to be strong. Natural avalanches are possible in these windy areas.

SUMMIT LAKE/LOST LAKE/SNUG:  The central Kenai Mountains often see the brunt of northwest winds such as these. If the winds verify here, a natural wind slab avalanche cycle is possible.

Special Announcements

REGION-WIDE Winds:  Strong northerly winds are impacting many areas around Southcentral, including Anchorage’s Front Range. A reminder that strong winds can increase the avalanche danger quickly on any slope seeing wind loading.

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Fri, February 19th, 2021
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Moderate (2)
Below Treeline
Below 1,000'
Avalanche risk
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

Girdwood Valley, under Notch Mtn (1600-1700′ elevation, WNW facing):  A group of skiers knocked a small cornice chunk onto a small steep slope and triggered this persistent slab avalanche yesterday. It was up to 2′ deep at its max and 100′ wide. This avalanche failed in a weak layer of facets on top of a 3-4″ thick buried melt/freeze crust.

Photo of the persistent slab avalanche described above. 2.18.21.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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.

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 is a day to pay close attention to what the winds are doing and avoid slopes with fresh wind deposited snow. As of 6am this morning, the weather stations are still reporting light to moderate NW winds, however, that could all change quickly through the course of the day. How much snow is available for transport is somewhat variable, anywhere from 4-12″ (plus or minus) depending on your location.

If these winds do verify, we can expect to see plumes off the peaks and the potential for natural wind slabs and cornice falls, especially in areas such as Crow Pass and Portage Valley. Additionally, this will be most likely in the Alpine, above the trees, yet watching for wind loading at the mid elevations will be key as well. New wind slabs could be up to a foot thick and possibly thicker. If found, they are likely to be easy to trigger and could step down to old buried weak layers below. In this case, a larger avalanche is possible. Along with watching for active wind loading, feeling for stiff snow over softer snow and watching for cracks that shoot out from you will be signs you’ve found a wind slab.

This flow direction is tricky for Turnagain Pass. It can funnel through the Pass from the south and load north aspects on the non-motorized side, while at the same time load the SE face of Seattle Ridge. It can also split around the Pass and not affect much of the terrain in the heart of Turnagain at all. You might find you are in an area that is seeing no wind, or not enough to move the surface snow into wind slabs. If this is the case, see below as there are some buried weak layers still causing concern.

 

View from Tincan yesterday that looks SE over 4-6″ of low density snow from the past couple days. 2.18.21.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

Buried in the top 2′ of the snowpack, likely deeper in areas near Portage and Placer Valley, are a couple of weak layers (facets and buried surface hoar). At the lower elevations, below 1,600′ at Turnagain Pass and 2000′ in Girdwood and Placer Valley, a crust sits just under one of these faceted layers. This was the setup in the avalanche triggered under Notch Mtn yesterday. Crusts can accentuate this persistent slab problem by providing a slick bed surface, encourage wider propagation if an avalanche is triggered and inhibit the weak layer from gaining strength.

In short, the reactivity of these weak layers continues to be quite variable around the region; meaning whether they are able to cause an avalanche or not. They may be sitting under old wind slabs and hard to identify. Snowpack tests are pointing to both good and not so good stability, making it difficult to truly assess how stable steep slopes are. As Aleph said yesterday, this uncertainty makes the whole situation tricky and the snowpack not that trustworthy… Simply knowing these weak layers exist, watching for signs of instability and following safe travel protocol can help hedge our bets if choosing to push into steeper terrain. Add to that, digging a representative snowpit and assessing the pack yourself is a great way to get more information for the slope in question.

Weather
Fri, February 19th, 2021
Yesterday:  Mostly cloudy skies were over the region with light and variable winds. Girdwood and Portage/Placer picked up an inch or two of snow early yesterday morning before the sun tried to poke through the clouds. Temperatures were in the 20'sF at the mid and upper elevations. Today:  Despite the sunny skies, a cold northerly wind has developed overnight, which is impacting much of Southcentral. Temperatures have plummeted to the single digits along ridgetops and valley bottoms should see temperatures dip to the teens through the day. The northwest winds are forecast to average 25-35mph along the ridgelines by this afternoon before decreasing late tonight. Tomorrow:  Mostly sunny skies are expected for tomorrow and Sunday along with light and variable winds. Temperatures will remain cold however, in the single digits and even the minus single digits in some valley bottoms. PRECIPITATION 24-hour data (6am - 6am)
Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 28 0 0 116
Summit Lake (1400') 27 0 0 44
Alyeska Mid (1700') 28 2 0.1 115
RIDGETOP 24-hour data (6am - 6am)
Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 18 W 6 21
Seattle Ridge (2400') 20 NW 9 24
Observations
Recent Observations for Turnagain Pass
Date Region Location
05/18/21 Turnagain Observation: Tincan Ridge
04/30/21 Turnagain Observation: Sunburst
04/27/21 Turnagain Observation: Tincan Ridge
04/26/21 Turnagain Observation: Magnum
04/25/21 Turnagain Observation: Sunburst
04/24/21 Turnagain Observation: Airplane obs
04/24/21 Turnagain Observation: Corn biscuit
04/23/21 Turnagain Avalanche: Lynx Ck Drainage
04/23/21 Turnagain Avalanche: Sunburst
04/23/21 Turnagain Observation: Center Ridge Turnagain pass
Riding Areas
Updated Tue, June 01st, 2021

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Placer River
Closed
It is packrafting and jetboat season!
Skookum Drainage
Closed
The Skookum Valley is closed to snowmachines. This closure occurs annually on April 1 as per the CNF Forest Plan.
Turnagain Pass
Closed
Closed as of June 1. 188 day season, that\'s a wrap!
Twentymile
Closed
It is packrafting and jetboat season!
Seward District
Carter Lake
Closed
Closes May 1.
Lost Lake Trail
Closed
Closes May 1.
Primrose Trail
Closed
Closes May 1.
Resurrection Pass Trail
Closed
Closed for the 2020/21 winter season. Will be open for moto use in the 21/22\\\' winter season as per the CNF Forest plan.
Snug Harbor
Closed
Closes May 16th.
South Fork Snow River Corridor
Closed
Closes May 1.
Summit Lake
Closed
Closes May 1.

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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.