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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, March 7th, 2015 - 7:00AM
Expires
Sun, March 8th, 2015 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche danger is generally LOW and will rise to MODERATE in the Alpine late in the day.   Fresh shallow slabs up to 10 € in depth could be triggered on steep leeward slopes.   Low volume sluffing is also possible in steep terrain in the higher elevations today.

The danger is LOW in the Treeline elevations, where avalanches are unlikely.

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Sat, March 7th, 2015
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Dangerous avalanche conditions. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
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.
More info at Avalanche.org

Issues directly related to new storm snow will be at the forefront of our avalanche concerns today.

4” of snow overnight combined with another 3-4” today will help to build small slabs in the Alpine.  These slabs will be thicker on leeward slopes and require terrain at least 35 degrees in steepness to trigger.  Bonding of these slabs will be generally good as the current storm progression is going from warm to cold, laying down “right side up” slabs.  The exception to this will be in isolated pockets on steep leeward slopes.  It is in this terrain where wind slabs could become as thick as 10” by the end of the day.  Avoid slabs in this terrain that sound hollow, are “punchy”, or produce shooting cracks.

Low volume sluffing will be possible in very steep terrain (over 40 degrees).  These should be slow moving and easy to avoid.  Sluffs will be dry above 3,000’ and gradually more damp as you lose elevation.  Heightened awareness of sluffing will be important when traveling above or towards terrain traps such as gullies, cliffs and trees.

Avalanche Problem 2
  • 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

Weak layers buried 1-2’ deep seem to be dormant for the most part around Turnagain Pass.  More uncertainty exists in the Girdwood Valley where we have less information about this layer.  Yesterday my partner and I had a good look at the late January facet layer in steep Alpine terrain and found it to be unreactive.  Testing over the past week has been consistent with what we found.  New snow will not be enough of a load to awake this layer today.  However, the possibility still exists to trigger slabs in the 2 foot range in isolated pockets in very steep (over 40 degrees) terrain.  Focused assessment of this layer will be important if you find yourself venturing into steep Alpine terrain.

Weather
Sat, March 7th, 2015

Warm air continued its chokehold on the region over the past 24 hours.   Temperatures have begun to dip slightly in the early morning hours with freezing levels around 1,000′.   Moisture associated with a large system moving in from the Southwest has brought light amounts of snow with the Center Ridge SNOTEL recording 4 €/.4 H20 and Alyeska Midway station with 3.7 €/.3 €H20.   Ridgetop winds have been light to moderate out of the East.

Today we can expect to squeeze another 3-4 € of snow/.3 € of H20 out of this system.   Ridegtop winds will be out of the Southeast at 15 to 20 mph.   Rain/snow line will be around the 1,000′ level and slowly drop to 500′ by the evening hours.

The extended outlook is showing a gradual clearing and cooling trend.   Precipitation will linger through the weekend with clear skies and daytime highs in the teens by the beginning of next week.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33 4 .4 44
Summit Lake (1400′) 36 trace .1 7
Alyeska Mid (1700′) 34 4 .3 25

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 26 E 10 31
Seattle Ridge (2400′) 27 n/a 16 32
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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.