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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 18th, 2013 - 7:00AM
Expires
Thu, December 19th, 2013 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

The avalanche danger today is generally LOW with pockets of MODERATE danger where human triggered avalanches are possible in steeper terrain. Furthermore, if todays approaching storm moves into our region earlier than forecasted, ample winds will act to increase the avalanche danger.

Special Announcements

Join us on Thursday night 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!

Wed, December 18th, 2013
Alpine
Above 2,500'
2 - Moderate
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. 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
  • 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 snow from Dec. 13/14th continued its settling process as relatively warmer temperatures (above 0 F) today will promote bonding, allowing more of a consistent or interconnected slab across a given slope.  It is unclear still if warmer temps today will do much to affix last weekend’s storm snow to our December drizzle crust from 12/7.  We have had several reports of small, skier triggered soft slabs running on this crust over the last few days.

The crust is easy to find as it proves supportable to a skier and is widespread throughout the Turnagain zone.  Given time it will continue to break down and become absorbed into our mid-winter snowpack but right now we are paying particular attention to how any new load of snow will react on this bed surface.  Continue to pay attention to steeper, rocky terrain or convex rollovers where you may pull out a soft slab pocket today.

Moreover let’s not lose site of what lies below.  The lower half of our snowpack consists of very weak, faceted snow.  I suspect a big enough trigger such as a snowmachine may be able to penetrate this rotten layer and produce an avalanche.

 

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

Our surface still consists of low density, loose snow that is ripe for wind transport with the coming storm (see mountain weather below).  New surface hoar has also formed over the last couple of cold and clear nights to cap our snowpack.  If winds pick up early enough in the day today, we could see wind slabs forming below ridges and on cross-loaded slopes prior to nightfall.

Weather
Wed, December 18th, 2013

Yesterday temperatures were stubborn to break single digits.   Winds were generally light form the north and northwest allowing valley fog to stay in place for much of the day in low lying areas while anyone in the upper elevations was rewarded with clear skies and a very low (on the horizon) sun.

Expect today to be a day of transition as mid and high level clouds move in to our area ushering in the arrival of what looks to be a quick, yet potent blast of winter weather.  A strengthening low-pressure system to our south will arrive this afternoon/ evening packing SE winds and ample snow.   There is a Blizzard Warning in effect for 6PM this evening.   The peak intensity of this storm looks to hit the eastern Turnagain Arm region overnight and if the system lines up favorably, we could wake up tomorrow with 12-18 € of new snow.   The exact track of this low will play a big role in wind direction and specifically what areas are favored with snowfall, but in short precip will be in the form of snow across our region and winds will be moderate to strong overnight, creating those blizzard conditions.  

Observations
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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.