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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 19th, 2013 - 7:00AM
Expires
Fri, December 20th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Pockets of  CONSIDERABLE  avalanche danger may be found today.  Watch for areas with deeper storm snow and wind driven slabs.  Last night’s storm is clearing today, but residual avalanche danger will linger as our weak base adjusts to the new load.  

During the fading daylight yesterday afternoon a couple small natural avalanches were noted along the Seward Highway.  This was at the very beginning of the stormy weather.  

Special Announcements

Join us tonight at  Alaska Mining and Diving Supply for an Avalanche Safety Technology Workshop  where we’ll be discussing and demoing some of the latest avalanche rescue technology including Avalanche Airbag packs and three antenna beacons.   As always, any CNFAIC workshop is FREE to the public!

Thu, December 19th, 2013
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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
  • 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

There was a lot of low density snow available for wind transport before this storm, and all the new storm snow came in cold.  Storm totals are in the range of 9-12 inches, with wind slabs probably reaching twice that depth.  Water content of the storm snow was around 0.6-0.9 inches (Snow Water Equivalent).

Wind was predominately from the East and Southeast at our Turnagain Pass ridgetop stations.  Expect the majority of wind loading to be on West aspects with crossloading on North and South.  

This cold storm should be compared to the one we got over the weekend.  In that storm we had a little more snow, but we saw a number of skier triggered avalanches.  We can expect the same today, with possible warning signs like whoomphing (collapsing) and shooting cracks.  Conservative decisions are justified today as the snowpack adjusts to the increased stress. 

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

Storm slabs and wind slabs will be the evident surface layers, but the weak interfaces that may fail today could be older and deeper.  The freezing rain crust was one of the most reactive weak layers from last weekend.  It may show itself again today.  Below that rain crust we still have well developed facets on the ground.  Fortunately, digging a full depth pit to look at all those layers is easy with our shallow snowpack.  

Weather
Thu, December 19th, 2013

Yesterday temperatures started a dramatic rise.  Portage had one weather station increase 56 degrees in the last 24 hours, from -25 F yesterday to 31 F this morning.  Ridgetop stations continue to read colder temperatures  – less that 20 degrees through the snowfall.

Snowfall and wind are the big news for us today.  Mid elevations in Girdwood and Turnagain Pass have about 9 inches of fresh snow overnight.  Wind was strong during the snowfall, reaching gusts to 64 mph on Sunburst and 56 mph on Seattle ridge.  

The blizzard warning for yesterday has expired, and the snowfall and wind are diminishing.  Some clearing this afternoon is expected.

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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.