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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, March 19th, 2015 - 7:00AM
Expires
Fri, March 20th, 2015 - 7:00AM
Forecaster
Graham Predeger
The Bottom Line

There continues to be a CONSIDERABLE avalanche danger at and above treeline today.   Slab avalanches 2′ thick or more are likely and will require cautious route finding and conservative decision-making.   Avoid steep, wind-loaded slopes in the alpine and abrupt rollovers at and below treeline today.

The pre Saint Paddy’s day storm was the biggest of the season thus far and came in both upside down and resting on a facet/ crust combo (see photo below) in many areas.   Careful terrain management will be essential today as the old surface continues to adjust to this most recent load.

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Thu, March 19th, 2015
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
No Rating (0)
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
No Rating (0)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

A layer of feeble, faceted snow that made up last week’s surface continued to show signs of activity yesterday both in our snowpits and on our tour up Tin Can.  With 20- 24” of storm snow deposited upon this layer on Monday, natural avalanches are becoming less common but the poor structure of our snowpack is still proving ripe for human-triggered avalanches in the 2’ range, particularly near treeline.  Above about 3,000’ slabs may be thicker than 2’ as moderate to strong winds blowing from the NE loaded many leeward slopes yesterday.

The snow below tree line is quite heavy and still very active.  Extra caution is warranted as you travel through areas such as Tin Can trees or Eddies where you may encounter short, steep rollovers ending in terrain traps.  Small slopes are holding enough snow right now to bury a person.  Watch your slope angles (keep it sub 35*) and run out zones.  This will be extra important as you encounter other groups in the backcountry today vying for similar terrain.

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
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.
More info at Avalanche.org

Winds were moderate to strong from the northeast yesterday with visible plumes of snow erupting off of ridges.  In addition to loading south and west aspects the wind all but erased any signs of tracks or crown faces from Tuesday.  Given the structure discussed above, Monday’s storm combined with stiff winds yesterday are likely to have formed slabs 2’ or greater in the alpine today.  A stiff, drum-like surface, shooting cracks or whumphing will be indicators that you are dealing with a potentially unstable wind slab in the alpine.  Again, effectively managing your terrain and avoiding steep slopes will be key today.

Weather
Thu, March 19th, 2015

Steady rain around Girdwood and the Turnagain arm gave way to partial clearing on Turnagain pass mid-day yesterday.   The rain snow line was observed to be around 1500′ with temperatures in the mid to high 30’s at Turnagain pass (1,000′).   Winds were moderate to strong at times, gusting into the 50’s and 60’s from the northeast (see photo below).   Snow accumulation around the Pass was minimal (~2 €) with new snow struggling to keep up with the impressive settlement rates of Monday’s storm

Today we can expect temperatures again to be in the mid to high 30’s at 1,000′ and winds from the northeast, backing off slightly ahead of an approaching short-lived high pressure system that will begin to set up tonight.   A few showers this morning may add an inch or two of snow but the general theme for the day will be one of clearing.

We’ll start the weekend with clear skies tomorrow and then clouds building back in by Saturday as a weak north Pacific low moves into the eastern Gulf of Alaska.

Sticky surface snow and strong winds lead to wind-initiated roller balls at treeline on Tin Can.  photo: John Sykes

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  36 2   .2    55
Summit Lake (1400′) 37    0 0    13
Alyeska Mid (1700′) 35 0 .24 36  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27    NE 24    67
Seattle Ridge (2400′)  29  n/a 17    73
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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
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Closed
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Skookum Drainage
Closed
Turnagain Pass
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Closed as of May 1. Thanks for a fun, safe season!
Twentymile
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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.