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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, April 20th, 2013 - 7:00AM
Expires
Sun, April 21st, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

A quick look around the mountains of Turnagain Arm should create an accurate general impression of current conditions.  No new snow in over a week.  Tracks in many areas with steep terrain.  Variable surface conditions, ranging from shiny sun crust to smooth powder pockets to windblown stiff slab.  Some minor wet avalanche activity and a few pockets of dry sluff and dry slab are evident in some areas.

Overall, the snowpack is fairly stable.  Pockets of shallow unstable snow can be found, but should be the exception and should be small and manageable in size.  Watch for those unstable pockets in steeper terrain, especially in the far reaches of the forecast area.

Special Announcements

From now until the end of April we will be issuing advisories on the weekends and Tuesday and Thursday  (Sa,Su,Tue,Thur). Keep checking the observations page for information that comes in – during the week especially. And for those getting out, keep the reports coming! The next advisory will be on Thursday, April 18th.

Also, a reminder that Skookum drainage is  closed  to motorized use.

Sat, April 20th, 2013
Alpine
Above 2,500'
1 - Low
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. 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

Last weekend we got a rash of reports of skier triggered avalanches.  There was a good size avalanche in Surprise Bowl in Girdwood with people partially buried, a remotely triggered slab on Silvertip, and small slabs kicked loose in Turnagain Pass.  Through the work week we haven’t had any more reports of similar avalanche activity, but we expect some areas may still behave this way.

With fantastic spring weather and strong temperature swings between day and night the snow at the surface is getting faceted and weaker.  This process breaks up the slab character and makes it more difficult for stiffer snow to connect together in larger plates.  We think the slabs that people found a week ago will be more difficult to find this weekend because of that faceting or degrading of the stiffer pockets.  

Skiers and snowmachiners are putting tracks all over the place right now, including steep terrain.  It’s a good time to explore some of the farther reaches of Alaska, but keep in mind that anomalies may be found.

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

We saw some really large overhanging cornices yesterday, including one that had recently fallen.  This concern increases late in the day as temperatures rise and the cornice structure loses strength.  Avoid spending time below overhanging cornices and watch your exposure at the ridgetops.  

Additional Concern
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

The wet avalanche meltdown still has not begun.  Direct south facing slopes, especially at lower elevations should be suspect late in the day.  As the daytime temperatures heat up the snow there is a corresponding loss of strength and wet avalanches may be initiated.  Temperatures last night did not get as low as they have been, so there may be a slight increase in activity this afternoon.

Weather
Sat, April 20th, 2013

One more day of sunshine before a pattern change…  Tomorrow we have clouds and a chance of precipitation in the forecast.

Nothing major is notable in the weather of the last 24 hours through today.  Overnight temperatures were a little warmer last night – in the low 20s.  This could make it easier for afternoon warming to produce wet avalanche activity.

Wind is light from the southeast.  Afternoon temperatures should reach into the low 40s again today.


Wendy will issue the next advisory tomorrow morning, April 21st.

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.