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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
Thu, February 27th, 2020 - 7:00AM
Expires
Fri, February 28th, 2020 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger is a ‘scary MODERATE‘ for the chance a person could trigger a very large and dangerous slab avalanche. These slabs are 3-6′ thick and sitting on weak faceted snow. They could be triggered on, near or even below a slope. Although the likelihood of triggering is slowly decreasing, the consequences could be deadly, it is a tricky situation. Continued cautious mind sets and conservative terrain choices are necessary to limit this hazard.

Other avalanche issues include, lingering wind slabs on steep wind loaded slopes and cornice breaks along ridgelines. If the sun shines, watch for roller balls and loose snow sluffs with daytime heating.

SUMMIT LAKE TO SEWARD REGION:  The likelihood for triggering a large slab avalanche is higher in this area due to a weaker snowpack structure. Extra caution is advised.

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Thu, February 27th, 2020
Alpine
Above 2,500'
Moderate (2)
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
Moderate (2)
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

The sun made a quick appearance yesterday afternoon and initiated some roller balls and small loose snow sluffs on steep southerly aspects.

Loose snow sluffs on the southerly face of Pk 4940 (south end of Seattle Ridge), viewed from near the Johnson Pass trail head. 

Avalanche Problem 1
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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

With clearing skies, light winds, some great powder conditions to be had, and some smaller avalanche issues in the top of the snowpack, it might be easy to forget there is a very large and deadly avalanche problem to deal with. The facets from January, that are now buried 3-6′ deep and have been responsible for countless large slab avalanches, continue to be our main concern. It was three days ago two large slabs were remotely triggered from 1,000′ away in the Grandview area and this type of scenario is still possible today and into the weekend. Areas with a shallower snowpack, such as the Summit Lake area, have a higher likelihood for triggering.

Why is this so tricky? It’s because the snowpack can ‘appear generally stable’. In this situation, it’s easy to focus on the current surface instabilities, listed below, and forget about what’s several feet below us. Additionally, triggering these large slab avalanches is becoming more difficult because (1) the thickness of the slab makes it harder to affect the weak layers, and (2) time is allowing the snowpack to slowly adjust to the weight of the slab. Due to the size and deadly nature of a potential large avalanche, along with the lowering changes a person will trigger one, we are in a ‘scary MODERATE’ avalanche hazard.

Current surface instabilities:

Wind slabs:  Lingering wind slabs are still possible to trigger on steep wind loaded slopes and in steep rocky terrain. These were formed either in Tuesday night’s 8″ of new snow or hidden below that snow and formed during Sunday’s strong outflow wind event. Either way, even a small wind slab triggered in the wrong place can send us somewhere unforgiving.

Sun Effect:  Solar heating in the afternoon may cause loose snow sluffs again today on steep southerly aspects.

Cornices:  As always, give cornices a very wide berth and limit exposure under them.

Keep in mind, any one of the above issues on the surface has the potential to ‘step down’ and trigger a large slab. It’s also good to remember there has been very little traffic since the big storm last week. As we venture further into the hills during this clear sky period we need to consider a number of things:

  • Areas that have seen little to no traffic this year are more likely to have a large slab release.
  • Signs of instability are not likely to be present before a slope releases
  • It could be the 10th person on the slope before it avalanches
  • Remote triggering a slab from below, the side or on top is possible
  • Consider the consequences if the slope slides, will debris take you into a terrain trap or fan out?
  • This is difficult low probability / high consequence situation and our guard has to stay up

*If you’d rather leave these issues behind, sticking to slopes 30° and less, with nothing steeper above you, is a great way to enjoy the excellent powder.

 

Large slab that was remotely triggered on Monday, Feb 24th, in the Grandview area. 

Weather
Thu, February 27th, 2020

Yesterday:  Cloudy skies gave way to large patches of sunshine in the afternoon. No precipitation was recorded in the past 24-hours. Ridgetop winds were light and easterly with some moderate gusts in the afternoon up to 22mph. Temperatures were generally in the 20’s°F, yet did warm during the daytime sun and hit the mid 30’s°F at mid and lower elevations briefly.

Today:  Partly cloudy skies with valley fog in areas, inland skies could be fairly blue. An inversion has setup overnight with cold single digit temperatures in valley bottoms and 20’s°F in the higher terrain. Winds just shifted to the NW this morning and are expected to be light, in the 5-10mph range.

Tomorrow:  Mostly sunny skies with some potential for valley fog. The inversion is likely to still be present with valley bottoms in the single digits and warmer air along ridgelines. Ridgetop winds look to remain northwesterly and could kick up to the 10-20mph range. A weak storm front moves in late Saturday and looks to give us another few inches of snow over the weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27 0 0 80
Summit Lake (1400′) 18 0 0 30
Alyeska Mid (1700′) 22 0 0 86

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 NE 8 22
Seattle Ridge (2400′) 26 *N/A *N/A *N/A

*Seattle Ridge anemometer (wind sensor) is heavily rimed and not reporting.

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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
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Turnagain Pass
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Closed as of May 1. Thanks for a fun, safe season!
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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.