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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, December 18th, 2016 - 7:00AM
Expires
Mon, December 19th, 2016 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

Strong winds along with several inches of new snow is expected to bump the avalanche danger to CONSIDERABLE. This will be  in the Alpine elevations that see over 5″ of snow accumulation. In these areas, fresh wind slabs up a foot thick may release naturally. Terrain that sees only 2-4″ of snow, this could include the Turnagain Pass area, will remain at a MODERATE avalanche danger. Any slope over 30 degrees with recent wind deposited snow is suspect for triggering a newly formed wind slab avalanche. Watch for wind slabs along ridgelines as well as lower on the slopes due to cross-loading.

Safer (and likely more enjoyable) areas to recreate today will be in treed zones where the wind has not affected the snow.  

***Portage Valley ice climbers and hikers: A MODERATE danger exists below 1.000′ where there is potential for avalanche debris, from a slide occurring above, to run through gullies. Avoiding gullies and runout zones is recommended.

Special Announcements

What are you doing Tuesday night?? Come to the  Blue & Gold Boardshop for a CNFAIC presentation on the ‘Turnagain Pass snowpack’!! This will be an interesting discussion considering the interesting snowpack! Details on link.

Motorized use:  The Chugach National Forest continues to monitor snow coverage daily for motorized openings. A 36 € base is a general number the Forest uses to determine adequate coverage – this could be more or less depending on snowpack density. Thanks for your patience and don’t let up your snow dance €¦.

Sun, December 18th, 2016
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
  • 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

More blow than snow…? It looks like this may be the case for the storm that will be quickly moving through today. As of 6am this morning, Easterly ridgetop winds have jumped up and are averaging in the 30’s mph. Snowfall has just begun in Girdwood and a trace has fallen on Turnagain Pass. If you are thinking of heading out today, expect quite windy conditions and increasing avalanche hazard. 

We are expecting anywhere from 2-6+” of new snow today. In favored areas, with over ~5″ of new snow, we can expect the strong winds to form slabs quick enough that they release naturally. In areas with lower snow amounts, new wind slabs should be relatively shallow (6-10″). Slabs could also be a bit stubborn to trigger – keep this in mind as they could allow you to get out onto them before they release. Another thing, slabs may be thicker than expected if formed over Friday’s wind slabs, adding to the consequences. Remember, expect any slab you find to be sitting on weak faceted snow and poor bonding is likely.

What to watch out for:

  • Areas with current wind loading (winds should blow all day)
  • Slabs lower on slopes from cross-loading
  • Stiff snow over softer snow (punchy snow)
  • Cracks shooting from your feet
  • Whumfing sounds (collapsing of the snowpack into the faceted snow underneath)

 

Photo: Wind effect on the Sunburst Ridge yesterday. In the back you can see the cross-loaded Northerly slopes on Magnum. (Thanks to Sam Galoob for sending us this picture and report)

Avalanche Problem 2
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Watch for fresh cornice growth. Small chunks of fresh pieces of cornice are likely to fall today. 

Weather
Sun, December 18th, 2016

During the past 24 hours we have seen partly cloudy skies and no precipitation. Ridgetop winds were light yesterday and are ramping up this morning from an Easterly direction. Temperatures sit in the upper 20’s F at the lower elevations up to treeline and have cooled slightly on the ridgelines to the upper teens.  

Today, we will be on the North edge of a large area of low pressure heading East. This will usher in strong wind and some snowfall. Easterly ridgetop winds are on the rise currently and should peak with averages around 30-40mph and gusts 70-80mph. Snowfall numbers are unfortunately meager with only 3-5″ expected on Turnagain Pass, 1-2″ in Summit Lake and 8-10″ in the Portage Valley. The good news with the low staying so far South is temperatures should remain cool with a rain/snow line around sea level to 200′. Ridgetop temperatures will stay close the 20F with 1,000′ temps in the upper 20’s F.

An unsettled weather pattern will continue into the work week – it’s difficult to tell what kind of snow amounts we will see – stay tuned tomorrow morning!

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26   trace – 1″ 0.1   23  
Summit Lake (1400′) 21   0   0   8  
Alyeska Mid (1700′) 28   0.5″ 0.06   15  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19    NE 15   56  
Seattle Ridge (2400′) 22    SE   12   27  
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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.