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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, December 16th, 2016 - 7:00AM
Expires
Sat, December 17th, 2016 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

Generally the avalanche danger will be LOW today throughout the advisory area.   However, if areas see more than about 6 € of new snow and winds in the 20’s mph range, the danger will rise to MODERATE where a skier could kick off a fresh, shallow wind slab.   This is more likely in areas susceptible to wind, such as the alpine.   New snow at all elevations will not bond well to yesterday’s loose surface, so keep in mind that steep terrain could also produce loose snow avalanches with higher volumes than we’ve seen this season.

*Continue to practice good travel habits; exposing only one person at a time in avalanche terrain, watch your partners, communicate, have an escape route planned and group up in safe zones.  

Special Announcements

The Chugach National Forest is monitoring snow coverage and depths across the Forest daily for a motorized opening.   A 36 € base is a general number the Forest uses to determine an area opener but that could be more or less depending on snowpack density  Thanks for your patience and don’t let up your snow dance €¦.. The forecast looks promising!

Fri, December 16th, 2016
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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

With skiers and riders eagerly anticipating a change in the weather pattern, yesterday afternoon did not disappoint with the first few flakes falling in over two weeks!  Though yesterday’s surface is now buried, let’s not forget the poor foundation we’ve laid during the first half of December.  Effectively we’ve got buried surface hoar (Nov.16th) obscured by about 12” of loose faceted (sugar) snow capped off by more surface hoar now buried by last nights new snow.  It’s unlikely we’ll get enough of a load today to build a slab on this weak foundation, but with an increase in easterly winds, a skier may be able to tickle out a shallow, tender wind slab.  Any avalanche today is expected to be small and manageable.  That being said, be mindful of terrain traps such as cliffs and rocks where the consequences of getting caught up in a small avalanche are compounded.  Light snowfall is expected to continue throughout the day and increase in intensity after dark.  If snowfall rates begin to intensify during daylight hours, the avalanche danger will rise to moderate, making small, human triggered avalanches possible, particularly in the alpine where wind will be more of a factor.

 

Avalanche Problem 2
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

New snow falling today will not bond well to yesterday’s loose surface. Keep in mind that steep terrain may produce ‘facet sluffs’/ loose snow avalanches with more volume than we’ve seen to date this season.

Yesterdays surface: 2-3mm surface hoar, mostly laid down and resting on a 10-12″ layer of weak faceted snow.  This thick layer of facets is likely to act as our next weak layer.

Weather
Fri, December 16th, 2016

The temperature inversion broke yesterday afternoon with temps at sea level rising from single digits to the mid to high 20’sF overnight.   Ridgetops temps were comfortable in the low 20’s for most of the day and winds began to kick up from the ESE last night with the approaching storm.   Skies were mostly clear with high clouds streaming in during the daylight hours yesterday and the first few flakes falling around 5PM.

Storm totals overnight were less than impressive with Summit Lake looking to be the winner with about 3 € new snow.   Turnagain pass and Girdwood are reporting 1-2 € this morning.

Today we can expect a bit of a lull as this current short wave moves out of the area to be replaced by a slightly stronger wave of energy this evening and overnight.   We can expect light snow during the daylight hours (another 1-3 €), ridgetop winds from the SE in the 10-25mph range and temps at 1,000′ in the 26-33 degree range.  

In general we have entered a stormy period, where a parent low in the Bering Sea looks to be supporting unsettled weather across south-central Alaska at least through the weekend.   For the most part temperatures appear to be cold enough to support mostly snow to sea level, though we may see a short period of rain/ snow at sea level overnight tonight before turning back to snow by the weekend.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 24   1-2    .1 21  
Summit Lake (1400′) 19    3 .4   8  
Alyeska Mid (1700′)  25 2   .15   11  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 E 6 22
Seattle Ridge (2400′)  22 SE   12   32  
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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.