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Turnagain Pass Avalanche Forecast RSS

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, February 2nd, 2020 - 7:00AM
Expires
Mon, February 3rd, 2020 - 7:00AM
Forecaster
Ryan Van Luit
The Bottom Line

The avalanche danger is CONSIDERABLE today above 1,000′.  Naturally occurring wind slab avalanches are possible and human triggered wind slabs are likely on steep slopes with previous or active wind loading. If triggered, these wind slabs have the potential to overload buried weak layers and initiate a larger slab avalanche. In areas out of the wind, the possibility remains for a person to trigger a slab avalanche on these buried weak layers.

The NWS has issued a Winter Weather Advisory that begins at 6pm this evening and extends through tomorrow morning. The advisory calls for an increase in wind, low visibility and new snow  – 1-3″ at Turnagain pass and Girdwood, and up to 8″ in the Portage valley.  Avalanche danger is expected to remain at CONSIDERABLE overnight.

SUMMIT LAKE and LOST LAKE:  Very strong winds are expected  this evening south of the forecast zone and naturally occurring large wind slab avalanches on leeward facing terrain are likely.

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Sun, February 2nd, 2020
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
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
Considerable (3)
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

Yesterday two skier triggered avalanches were reported.  In one incident a skier triggered but was not caught in a a small wind slab avalanche in the trees at Tincan.  The avalanche was estimated at 50-60′ wide with a crown depth up to 9″ thick, and ran approximately 50-60′ vertically.

Another slab avalanche was reported near Girdwood.  The rider and partners avoided the runout.  The slab depth was estimated to range from 8”-24” thick and ran 60′ down the slope.

We’re grateful for the positive outcomes, and to the groups for sharing their information.

Skier triggered avalanche near Girdwood.  2.1.20 . Photo: Anon.

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

Our loose snow from earlier in the week moved around in many places from yesterdays northwesterly outflow winds.  This trend of wind transported snow will continue as a storm is forecast for this evening.  Although the outflow winds were from the northwest, and the impending storm is calling for southeast winds, it can be difficult to predict which slopes have formed wind slabs.  Where windslabs exist on steeper slopes, it could be easy for a human to trigger an avalanche.  Small steep wind effected slopes are still suspect.  Because of this, it’s essential to look for clues where wind slabs are forming, or have already formed.  If you see the winds depositing snow onto the lee of ridges or gullies, that’s a clear indicator a slab could be forming.  Where visibility is low or you can’t see your entire route, remain aware of the snow beneath you –  you could be entering a wind slab if you feel stiffening of the snowpack, see shooting cracks, or sense hollow drum-like sound in the snow.  Any of these signs are cause for reconsidering your route.

Snow transporting along Seattle Ridge.  2.1.20 . Photo: A Dahl

 

Cornices:  Cornices continue to form and could be touchy to human trigger as they build with transporting snow.  As always, give cornices a wide margin.

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

Our snowpack structure contains weaknesses formed before the storms earlier this week.  The recent incremental loading from new snow and wind transported snow managed to naturally trigger these weaknesses earlier in the week.  Although the snowpack is adjusting to this new load, it remains possible for a human to trigger these layers 2-3′ down in the snowpack.  These layers could be easier to trigger in shallow areas and near rocky outcrops.  If a wind slab is triggered, it could step down to trigger one of these layers deeper within the snowpack.

Loose Snow Avalanches:  On steep slopes with unconsolidated snow, watch for easily triggered loose snow sluffs.

Weather
Sun, February 2nd, 2020

Yesterday:  Mostly clear skies with cold northwesterly winds. These outflow winds blew generally in the 20-30mph range with areas toward Seward seeing higher speeds. Temperatures along ridgelines were in the -10 to 0°F range while lower elevations remained mostly in the single digits.

Today: A strong low-pressure system heads into Southcentral Alaska today and will bring cloud cover, strong southeast winds, and a chance for some snowfall this afternoon into the evening.  Temperatures are predicted to reach up to 16°F with southeast wind 5 to 15 mph increasing to 30 to 40 mph in the afternoon.  Accumulations range from 1-3″ in the Girdwood and Turnagain pass areas, and up to 8″ in the Portage valley.

Tomorrow:  Mostly cloudy skies with a high near 25°F and low around 21°F. Light snow is expected throughout the day. Winds will be out of the east ranging from 30 to 35 mph, shifting to north winds 5-10 mph in the evening.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 7 0 0 59
Summit Lake (1400′) -1 0 0 20
Alyeska Mid (1700′) 5 0 0 55

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) -1 W 9 24
Seattle Ridge (2400′) 4 VAR 8 22
Observations
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Riding Areas
Updated Mon, October 26th, 2020

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
Skookum Drainage
Closed
Turnagain Pass
Closed
Twentymile
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
Summit Lake
Closed

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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.