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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
Sat, February 1st, 2020 - 7:00AM
Expires
Sun, February 2nd, 2020 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger is expected to rise to CONSIDERABLE today above 1,000′ due to strong northwesterly winds forecast to impact the region. Naturally occurring wind slab avalanches are possible and human triggered wind slabs are likely on steep slopes with active wind loading. Wind slabs also 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.

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

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Sat, February 1st, 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

Updated @ 9am: We had a report of a snowmachine triggered slab avalanche near the Johnson Pass area yesterday. Reported as ‘a slab on a smallish slope near the Johnson pass trail’. Heads up in areas like this with steep avalanche terrain above you.

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

Cold Arctic air is spilling in from the north this morning and along with clearing out the skies, it’s also expected to wreak some havoc with the snow. As of 6am this morning, the northwest ridgetop winds have bumped up to the teens with gusts in the 20’s at the Sunburst weather station. They are expected to reach averages near 20-30mph with stronger gusts and even stronger winds are expected south of Turnagain Pass. Although the snow surface was affected by Wednesday night’s winds, there is still plenty of loose snow available for transport. This flow direction can funnel through Crow Pass near Girdwood and the Kenai mountains in interesting patterns, loading different aspects on the same pieces of terrain, and can also split around and spare certain zones.

For anyone headed out today, keep a close eye out for where the winds are transporting snow, building cornices and loading slopes. Plumes are likely to be visible if the winds verify. As always, feel for stiff snow over softer snow and any cracks that shoot out from you. These are signs you’ve likely found a wind slab. Avoid being under cornices, these could form in unusual places. Additionally, buried weak layers hide 2-3+’ below the surface and freshly wind loaded slopes could overload these, creating a much larger avalanche. It’s a day to avoid any slope that has recent or active wind loading.

The animation below is from windy.com and shows the general forecast wind pattern along ridgetops, around 5,000′ in elevation. Blue is light wind and orange/red is strong (color legend is on the bottom of the image). Note: weather model data is useful for a general pattern and not always accurate for a specific point.

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

As we saw yesterday with the brief period of visibility, several avalanches occurred during the winds on Wednesday night and many of these were suspect to have released on the old weak snow that developed during the first part of January. Even in areas out of today’s winds, don’t forget there are facets and buried surface hoar hiding 2-3+’ under us. As the 2-3′ of low-density snow from the past week settles and stiffens, these layers are still something that could catch us off guard and triggering a persistent slab avalanche remains possible. Sticking to lower angle slopes, with nothing steeper above us and the flats are good ways to manage this risk.

This photo was shown yesterday and is taken of the northerly aspect of Sunburst. Note the wide propagation of the slab avalanche that released sometime Wednesday night (1.29.20) in conjunction with the bump in easterly winds. 

Loose Snow Avalanches:  On steep slopes that are spared the wind and in the lower elevations, watch for easily triggered sluffs.

 

Weather
Sat, February 1st, 2020

Yesterday:  Overcast skies were over the region with light snowfall in Girdwood and Portage Valley adding 1-2″ of new snow. Ridgetop winds were light from the west (0-5 mph). Temperatures were cold, single digits at all elevations, with the exception of sea level areas which remained in the upper teens.

Today:  Mostly clear skies are expected today along with very cold northwesterly winds. These outflow winds are forecast to blow generally in the 20-30mph range with areas toward Seward seeing higher speeds. Temperatures will continue to drop along ridgelines to the -10 to 0°F range while lower elevations remain just above zero.

Tomorrow:  A strong low-pressure system heads into Southcentral Alaska tomorrow and will bring cloud cover, a switch in wind direction to the east and a chance for some snowfall.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 11 trace trace 60
Summit Lake (1400′) 6 0 0 21
Alyeska Mid (1700′) 10 2 0.1 57

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 4 W 5 22
Seattle Ridge (2400′) 5 *N/A *N/A *N/A

*Seattle Ridge anemometer (wind sensor) is rimed over and not operating.

Observations
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Riding Areas
Updated Fri, May 01st, 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
Closed as of May 1. Thanks for a fun, safe season!
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.