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Thu, December 18th, 2014 - 7:00AM
Fri, December 19th, 2014 - 7:00AM
Graham Predeger
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE today in the upper elevations of the forecast area.   This includes terrain above 2,500′ where recent storm snow has been deposited on a weak layer/ bed surface combination.   With our current snowpack structure, human triggered avalanches in the 2-5′ range are likely today.   Below 2,500′ the avalanche danger is MODERATE primarily due to the potential of avalanches initiating higher up and running into this lower elevation band.  

Cautious route finding and conservative decision-making will be essential today to avoid slopes greater than 35 degrees.   This includes run out zones and terrain connected to steep, avalanche prone slopes.

Special Announcements

Join CNFAIC forecaster John Fitzgerald (Fitz) tonight for the 3rd installment of our Fireside Chat series.   Fitz will be presenting on Human factors and decision-making in the backcountry tonight at the Glacier Ranger Station in Girdwood, 6:30PM.

Thu, December 18th, 2014
Above 2,500'
3 - Considerable
Avalanche risk
2 - Moderate
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

We had little trouble finding a layer of buried surface hoar in our snowpits yesterday.  It is present and reactive, as expected above roughly 2,700’ where the snowpack dries out significantly.  This current set up consisting of a stout crust (bed surface) capped by surface hoar (weak layer) and buried by a fresh slab (strong snow) is sort of a ‘perfect storm’ for avalanches.  We have seen this over the last 48 hours with several natural avalanche observations coming in from across the region.  The commonality with this most recent avalanche cycle is that slides are initiating in the dry snow at upper elevations and very likely running on this layer of buried surface hoar.  Today the message is simple: avoid terrain steeper than 35 degrees, and make conservative travel decisions.  This includes the terrain adjacent to or directly under avalanche slopes as the nature of this weak layer/ bed surface combo can allow for wide propagation and triggering avalanches remotely, including from below.

In the lower elevations (below ~2,500’) where the snowpack is moist, the slab appears to be gaining strength over time and as temperatures drop.  The primary concern in this elevation band is one of exposure from alpine start zones.

                    Quick snapshot of the bed surface/ weaklayer/ slab combo found in the mid to upper elevations of Turnagain pass.  This snowpit is at 2,800′ on the west ridge of Sunburst (common approach).


Thu, December 18th, 2014

The most recent storm (Saturday €“ Tuesday) brought another slug of unseasonable warm temperatures into our region with rain at sea level and the snow line fluctuating between 1,500 €“ 2,000′.   Yesterday was an attempt for the weather to return to something closer to €˜normal’ with the snow line dropping to around 500′ in the eastern Turnagain arm region.   Locals in Girdwood even called in to report sighting of snowflakes hitting the valley floor (though not sticking)!   With temperatures fluctuating around 34 degrees at sea level it was still mostly rain yesterday.

In Turnagain pass (1,000′) the temperature was right at 32 degrees during the daylight hours with skies producing off and on snow showers and 1-2 € of accumulation.   Winds yesterday were consistently from the east in the 10-20mph range and a few gusts into the 30’s.

Today’s weather looks to be on the quiet side as we are in between storms and should be feeling the effects of a short wave ridge of high pressure.   Temperatures at a1,000′ will be in the low 30’s with perhaps just a trace of snow falling.   Expect winds to be in the single digits today at ridgetop locations, shifting slightly to a northeasterly direction with the approach of our next low pressure system.

We’ll begin to feel the effects of a powerful low-pressure system spinning into the Gulf of Alaska late tonight and into tomorrow.   Our rain/ snow line should hold around the 500′ level with snow falling through Turnagain pass.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30   2    .1  29
Summit Lake (1400′)  28 0    0  4
Alyeska Mid (1700′)  31  5 .4   19  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 E    12  34
Seattle Ridge (2400′)  25 N/A   N/A   N/A  
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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.