Turnagain Pass Avalanche Forecast RSS

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, April 17th, 2021 - 7:00AM
Expires
Sun, April 18th, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

We have issued a SPECIAL AVALANCHE BULLETIN for the mountains surrounding Girdwood, Portage Valley, Turnagain Pass and the Kenai mountains to Seward. This bulletin is in effect through tomorrow night.

The avalanche danger is MODERATE this morning and expected to rise to HIGH again on east, south and west facing slopes this afternoon as intense sunshine and very warm temperatures melt surface crusts and destabilize the snowpack. Natural wet loose and wet slab avalanches will become likely during the heat of the day on slopes with direct sunshine. Human triggered slab avalanches are likely on sunny slopes (east, south and west aspects) with moist or wet surface snow. Northern aspects, shaded slopes, will have a MODERATE danger where triggering a slab avalanche 1-4′ deep, composed of last week’s storm snow, is possible.

PORTAGE VALLEY, CROW PASS: There is potential for avalanche debris, from a slide occurring above, to run to low elevation terrain. Avoiding summer trails that run through avalanche runout zones, such as Byron Glacier and Crow Pass Trails, is recommended through this weekend.

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Sat, April 17th, 2021
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
High (4)
Avalanche risk
Below Treeline
Below 1,000'
High (4)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
High (4)
Treeline
1,000'-2,500'
Avalanche risk
High (4)
Below Treeline
Below 1,000'
Avalanche risk
High (4)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

Widespread natural wet loose and wet slab activity was seen yesterday along the mountains around Turnagain Arm, Girdwood, and the Kenai Mountains around Moose Pass and Seward. These avalanches occurred on easterly, southerly and westerly aspects during the heat of the afternoon and evening hours. A large avalanche off south facing Goat Mountain was caught on camera in Girdwood (video below). Take a look at the recent observations as well for images from the natural cycle last Wednesday into Thursday.

There were also three known human triggered avalanches on west and southwest slopes on the backside of Seattle Ridge. These were snowmachine triggered storm slabs (16-24″ deep) that were becoming moist and more reactive during the heat of the day. No one was caught.

Goat Mountain natural avalanche caught on camera, thanks to Tarryn Megan for the video! 4.16.21.

 


Snowmachine triggered slab avalanche in -3 Bowl on the backside of Seattle Ridge. SW aspect at 3,000′. Rider was not caught. Slab was 16-24″ deep and composed of last week’s new snow. Failed around 2-3pm on an old sun crust. 4.16.21

 

Snowmachine triggered slab avalanches on the backside of Seattle Ridge (-3 Bowl) yesterday. Failed between 4-5pm. They were on a westerly aspect around 3,300′ and composed of last week’s storm snow. 4.16.21. Photo: Chris Yelverton.

Avalanche Problem 1
  • Spring Conditions
    Spring Conditions
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Spring Conditions
Warmth has a tricky effect on snow. On the one hand it speeds up the stabilization of the snowpack (reduces the chance of slab avalanches). But a SUDDEN rise of temperature increases the chance of slab avalanches considerably. When this warm period is followed by cooling down, then the chance of slab avalanches reduces. Even more so: the more often the temperature changes, the more stable the snowpack becomes when looking at slab avalanches. Once the temperature becomes too warm we have to deal with wet snow avalanches.

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

The springtime transition is starting. The snowpack saw a shock to the system yesterday with rapid warming and that will only intensify today. As of 6am this morning ridgetop temperatures are already in the mid 30’sF. The high pressure in place is creating a subsidence inversion, which is warming the ambient air at the high elevations. Couple this with 15 hours of solar heating and light winds …. and we have avalanche problems.

Regardless of elevation, any slope in direct sunshine that has not avalanched, will be likely to do so today; especially steep rocky terrain. These sunlit slopes are expected to experience significant surface warming and melting of any crusts that have formed overnight. Once the melting takes place and the surface turns to wet snow, all bets are off and natural avalanches become likely and human triggered avalanches very likely. This will begin first with east aspects, then south, then west as the sun makes its way across the sky. Today is a day to avoid being on, or under, any slope or steep terrain baking in the sun. Be aware of your exit routes. If surface crusts are present in the morning keeping the snowpack in check, those crusts may not last long before turning into a mushy mess, signaling the potential for natural wet snow avalanches.

Wet snow avalanches today may be either wet loose or wet slab, or a combination. They may begin to gouge down into lower layers as water makes its way deeper in the pack today. They may start small and if they are in steep sustained terrain, they could entrain enough volume and momentum to run to valley bottoms. A reminder, that even a small wet sluff can be dangerous as wet or moist snow is much more difficult to escape from than dry snow. There could also be some natural slab avalanches induced by the warming, or triggered by a wet loose avalanche that still harbor a fair amount of dry snow. That all said, the shock to the snowpack will be ongoing today.

Human triggered slab avalanches: Triggering a slab that is still mostly dry, or moist, is likely again today on east, south and west aspects that have not seen a lot of melting below the surface yet. Similar to yesterday, avoid slopes over 30 degrees that are seeing direct sunshine and warming.

Glide avalanches: We have seen a few of these release over the past few days (Skookum and Summit Lake) and are expecting more to start releasing with the warm conditions. Watch for and limit any exposure under glide cracks.

 

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

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

On shaded aspects that are not seeing direct sunshine and surface melting, there are still avalanche concerns. The new snow from last week is essentially a dense slab anywhere from 3-4′ thick in the mountains around Girdwood, and ~2′ thick at Turnagain Pass. Some wind effect and stubborn wind slabs were seen yesterday on the top of this slab. How well the storm slab has bonded to the old hard wind affected snow underneath is a bit of a question. With such warm ambient temperatures, even out of the sun, these slabs may still be reactive and keeping our guard up, looking for signs of cracking or whumpfing, will be key for assessing north aspects.

There are still older weak layers in the pack under the new snow. This is most predominant in areas such as Summit Lake with a shallower snowpack. As we move forward and watch what this spring transition does, we should keep in mind these layers exist and avalanches could step down on northerly slopes as well as southerly.

Cornices:  These grew during last week’s storm and are becoming more tender with the springtime warming. Be sure and steer well away from them along ridgelines and limit exposure under them.


Cornice break triggered by a snowmachiner. Smaller cornice over a moat, but you get the idea. These have not only grown last week, but are becoming less stable with the springtime warming. 4.16.21.

Weather
Sat, April 17th, 2021

Yesterday:  A high pressure built in yesterday allowing for sunny skies, a light easterly breeze along ridgetopes (~5mph), and warm springtime temperatures. Temperatures climbed to the 40’sF at the mid elevations and mid 30’sF in the upper elevations.

Today:  The high pressure over the region is strengthening. Overnight low’s have remained warm in the high elevations, ~35F,  and have only dropped to ~30F in valley bottoms. Continued sunny skies are expected today with light easterly ridgetop winds (~5-8mph). Daytime warming should increase temperatures to the low 40’sF in the high elevations, near 50F in the mid elevations and the mid 50’s at sea level.

Tomorrow:  Sunny skies and unseasonable warm temperatures are forecast to continue for Sunday and into Monday. Warm ambient air, in the mid 30’sF, should remain in the high elevations, limiting over night refreezing. Ridgetop easterly winds however, look to increase to ~10mph Sunday and further, up to ~20mph, on Monday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 38 0 0 110
Summit Lake (1400′) 37 0 0 43
Alyeska Mid (1700′) 38 0 0 126

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 33 NE 7 13
Seattle Ridge (2400′) 36 var 2 3
Observations
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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.