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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, April 11th, 2017 - 7:00AM
Expires
Wed, April 12th, 2017 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

With sunny skies and warm temperatures forecast today, the avalanche danger is expected to increase to  CONSIDERABLE  this afternoon above 1000′ due to daytime warming. Large and dangerous slab avalanches 2-6+ feet deep remain possible to trigger on slopes steeper than 35 degrees that have not avalanched.  Daytime warming will increase the likelihood of triggering one of these monsters in the afternoon.  Additionally, triggering a wet loose avalanche or cornice will also be more likely in the afternoon. Careful snowpack evaluation, cautious route-finding and conservative decision-making are essential.  

Below 1000′ a  MODERATE  avalanche danger exists where triggering a wet avalanche will be possible during the heat of the day.  Watch for changing conditions.

Hiking in Portage Valley and on summer trails around the Advisory area (including the Turnagain Arm Trail a.k.a the bike path).  Avalanches occurring at the higher elevations in this last storm sent large amounts of debris into valley bottoms and covered snow-free hiking trails. Continue to avoid trails that cross under avalanche paths such as Byron Glacier or portions of the Trail of Blue Ice and Crow Pass. The Turnagain Arm Trail between Bird and Girdwood remains  CLOSED for the winter.

Summit Lake:  An active avalanche cycle occurred in Summit Lake last week and  elevated caution is advised.  Read the  Saturday Summit Summary  HERE  and observations from the last few days  HERE.

Tue, April 11th, 2017
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

It has been 5 days since the end of a 10-day storm cycle that brought 3-6′ of heavy dense snow and rain to the region. This snow fell on a widespread layer of buried surface hoar and unfortunately this layer continues to inhibit good bonding between the storm snow and the old March surface. As days pass, this layer of buried surface hoar is becoming more difficult to trigger. Furthermore, the fact that is sits over 3′ deep on many slopes also makes it more difficult to trigger. However, this should not lure us into thinking a slope is safe. Keep in mind thin spots in the slab, which is where an avalanche is most likely to be triggered, exist on all slopes and are tough to see and avoid – making this deep slab problem very tricky. Things to remember if heading out today:

1-  No signs of instability may exist before a large slab is triggered
2-  Slabs can be triggered remotely from below!
3-  Several tracks may be on a slope before someone finds the thin spot
4-  Daytime warming will increase chances for triggering a slab
5-  All aspects are suspect 
6-  These slabs are large and dangerous and nothing to mess with
7-  Low angle slopes continue to be your friend, with nothing steeper above you

*More details have come if from the several snowmachine triggered avalanches in the Seattle Creek drainage over the weekend. Please see the excellent write up and photos sent in to us from the close call where a snowmachiner was able to ride up and off a very large slab avalanche in Jr’s Bowl. 
 

Large snowmachinge triggered slab avalanche in Jr’s Bowl (2nd Bowl) on Saturday, April 8th. Photos, Cory Runa.

Crown was up to 10-12′ thick –  very large slab near the ridgeline where winds deposited snow during the 10-day storm cycle.

 

Large avalanche that was presumably triggered by a snowmachine on either April 7th or 8th on the NW aspect of Main Bowl (1st Bowl). Photo taken by Cory Runa on April 8th. Because of it’s safe and easy access, this is the avalanche that we closely investigated yesterday – flank profile and video below.

 

 

–

Avalanche Problem 2
  • 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

Springtime is upon us and daytime warming below 2,000′ will again soften the surface crusts at the lower elevations and on Southerly upper elevations. While the upper elevation temperatures have remained relatively steady, in the mid 20’sF, the lower elevations below 2,000′ are cooling overnight to ~30F then warming into the 40-45F range. By late afternoon watch for the snow to become saturated and unsupportable, this is the sign to move to cooler aspects. Wet loose and wet slab avalanches are still possible at these lower elevations.

Additional Concern
  • 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

Cornices are large and likely hanging close to their tipping point. Direct sunshine, a person, or a group of people on top these could be enough to cause one to break. They can be extremely dangerous on their own, but with our current snowpack, they could trigger a large slab avalanche below, making a big problem even bigger. Give cornices a wide berth from above and limit exposure under them from below. 

Weather
Tue, April 11th, 2017

Mostly cloudy skies coupled with instability showers along with some patches of blue sky were over the region yesterday. Over the past 24 hours, ridgetop winds have been light from an Easterly direction in the 5-10mph range and only a trace an inch of new snow has fallen. Temperatures have remained in the mid 20’s F along ridgetops and warmed into in the low 40’sF below 2,000′.

Today, we should see sunshine in most areas. Ridgetop winds are expected to be light, 5-10 mph, and from a Northerly direction. No measureable precipitation is expected. Temperatures should be similar to yesterday, rising to the low 40’s at the lower elevations below 2,000′ and remain in the mid 20’s F along the ridgetops.  

A warmer air mass begins to push in overnight and beginning tomorrow, warm temperatures, sunny skies and light winds should rule the weather for the remainder of the week.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36   trace   0   72  
Summit Lake (1400′) 35   0   0   25  
Alyeska Mid (1700′) 34   trace   0.03   67  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24   NE    10 22  
Seattle Ridge (2400′) 28   SE   7   19  
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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.
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