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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 24th, 2017 - 7:00AM
Expires
Sat, February 25th, 2017 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1,500′ on all aspects where triggering a 2-3′ slab avalanche is possible on slopes steeper than  35 degrees. In addition, there are a handful of other avalanche concerns including fresh shallow wind slabs later today with an uptick in winds, cornice falls and sluffs. Also, there is a glide crack opening up on Seattle Ridge, above popular terrain just to the South of the up-track (these can release at any time without warning).

Below 1,500′ there is a  LOW  danger where triggering an avalanche is unlikely due to a snowpack consisting of hard crusts.

In Summit Lake, Girdwood, and near Johnson Pass triggering a deeper more dangerous avalanche near the ground is still possible, but will be hard to trigger.  

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Fri, February 24th, 2017
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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
  • 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

Relatively calm weather has been over the region for a few days now. This has allowed for folks to push further into the mountains with no reports of avalanche activity. Unfortunately, poor snowpack structure is still a concern as a thin layer of buried surface hoar sits 2-3′ below the surface. Triggering an avalanche breaking on this layer is becoming tougher by the day as the pack is taking its time in adjusting. However, the slab is large enough that the consequences could be high. 

The most likely place to find one of these larger slides is on steep slopes with thin areas and rock bands. Trigger points are often these thin areas, near rocks and zones that may have been scoured slightly by winds. Also, these slabs can break above you, and release after several tracks are on a slope. Be aware that snow pits and stability tests may not be representative of the actual slope you are trying to assess. A helpful way to think about this problem is to consider the consequences of a slope if it slides and identify terrain traps (gullies, cliffs, or trees below).

Photo below is of a snow pit yesterday in the Seattle Creek drainage, we found the weak layer of buried surface hoar, but it was strong and took considerable force to make it fail. This is a sign the weak layer is adjusting.

 

 

GLIDE AVALANCHES:
There are a few glide cracks opening up on Seattle Ridge. One of these is just to the South of the motorized up-track and looks like it could release any day now. Keep an eye out for these cracks and limit any exposure underneath them.

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

Ridgetop winds are expected to increase slightly today from the South, but then shift to the NW and pick up into the moderate range later this afternoon (15-20mph averages). If the winds do ramp up later in the day, watch for fresh wind slabs to begin forming. Anytime a fresh wind slab in encountered the general rule is, expect it to be touchy. Any slabs forming today should be on the shallow side, 6″ to a foot. Also, keep in mid, this NW flow is known to be channeled through Turnagain Pass in way that Southerly winds impact the East side of the highway corridor. 

Cornices are large, as seen in the photo below, give these an extra wide berth.

Additional Concern
  • 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

Various layers of faceted snow sits at the bottom of the pack in certain areas. In the Summit Lake zone and some areas in Girdwood Valley and Johnson Pass depth hoar has been found. Similar to the problem above, these layers will be very tough to trigger, but a possibility remains in places with this structure. The more likely scenario is where an avalanche occurring in the upper layers of the pack has the potential to step down and release the entire snowpack. 

Weather
Fri, February 24th, 2017

Overcast skies filled the region yesterday with light Southerly winds along the ridgetops. Temperatures at the upper elevations warmed up during the day into the upper 20’s F and continued warming overnight into the low 30’sF. In the valley bottoms, temperatures are cooler, in the teens and low 20’sF.  

For today, a quick moving frontal band will bring cloud cover and chance for 1-3″ of snow to favored zones. Ridgetop winds are expected to bump up into the 10mph range with strong gusts from the South then back to the NW and increase to 15-20mph this evening. Temperatures will remain mild, in the 20’sF at the upper elevations and in the low 30’sF at sea level.

For the weekend, we are expecting partly cloudy skies with a chance for some snow flurries.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   0   0   67  
Summit Lake (1400′) 25   0   0   33  
Alyeska Mid (1700′) 26   0   0   63  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 28   SW   5   15  
Seattle Ridge (2400′) 27   SE   6   18  
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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.