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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 27th, 2015 - 7:00AM
Expires
Sat, February 28th, 2015 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

Above 2500′ the avalanche danger is  MODERATE  today where a poor snowpack structure will make it possible to trigger an avalanche 1-2′ deep.   Likely trigger points will be mid-slope on steep rollovers and unsupported convexities.   Steep, southerly aspects will also warrant special attention today as any localized, rapid solar warming could be enough to wake up a buried layer of facets.

 Below 2500′ the avalanche danger is LOW, where a supportable melt/ freeze crust has formed, locking up the shallow snowpack that does exist.

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Fri, February 27th, 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. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. 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
  • 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

Information from the field yesterday validates the fact that a poor structure (slab/ weak layer/ bed surface) is persisting and a fracture, if initiated has the potential to propagate across a slope.  The most reactive snow, and where you are more likely to trigger an avalanche appears to be in the mid-elevation band between 2400’ and 2900’.  This is where my partners and I experienced large and frequent collapsing yesterday; prompting us to change our travel plans and seek out mellower terrain.  Obvious red flags such as whumphing, recent avalanches and shooting cracks should not be taken lightly as this persistent weak layer is keeping us on the higher end of moderate danger right now.

Above 3,000’ our layer of concern is comprised of smaller, less advanced buried facets still overlain by a slab.  This is more difficult to see in a snowpit and whumphing and shooting cracks may not be so obvious until a slope avalanches, making safe travel protocol of the utmost importance.  Identify features of concern such as unsupported convexities and mid-slope roll overs.  Avoid these areas today or at the least expose only one person at a time and have a well thought out escape route and safe zone if a slope does slide.

A further concern on south facing slopes today has to do with direct sun and inverted (warmer at ridgetops) temperatures.  Rapid warming today could be enough to initiate an avalanche on steep, southerly terrain particularly near rock out crops or shallow spots in the slab as seen below.

Natural avalanche observed yesterday on the periphery of the advisory area near Grandview.  SSW aspect at about 4,000′.

Weather
Fri, February 27th, 2015

Calm winds and clear skies prevailed in the backcountry yesterday with temperatures increasing to the low 30’s at ridgetops by days end as high cirrus clouds moved in.  Winds are back online (light out of the west) at the Sunburst weather station after a successful anemometer replacement mission yesterday!

Temperatures are slightly inverted this morning with ridgetops readings in the low to mid-30’s while Center ridge (1880′) is reading 24 degrees at 6am.  Expect this air to mix and the inversion to lessen as the sun begins heating up the earth’s surface.  Winds are expected to be in the 8-20mph range shifting to the southeast as clouds stream into our region this afternoon and evening.  

We may see a trace of snow overnight and up to a couple inches by tomorrow night but the upslope nature of this weak surface low looks to favor Anchorage more than the eastern Turnagain arm.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25 0   0   40  
Summit Lake (1400′) 20   0   0   6  
Alyeska Mid (1700′) 24   0   0   23  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 30   *W    *6  *14
Seattle Ridge (2400′)  26 N/A    3  12

*Sunburst wind info is an average from 2pm to 6am.

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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.