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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, March 20th, 2016 - 7:00AM
Expires
Mon, March 21st, 2016 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger will be CONSIDERABLE today at all elevations and aspects above 1,000′ due to new snow and wind. Slab avalanches 8-12+” thick are likely to be triggered on slopes steeper than 30 degrees. These may also occur naturally along with cornice falls. Glide avalanches remain a concern as well and cracks will be obscured by the recent snow and wind.  

Travel in avalanche terrain will take expert level snowpack assessment and very cautious route-finding. Safer places to recreate will be on mellow slopes, with nothing steep above you, or in the flats.

**Areas on the periphery, such as Portage Valley, that may receive more snow than forecast will have a HIGH avalanche danger. If you find yourself in a zone with over a foot of new snow travel in avalanche terrain is not recommended.  If you are headed to the Summit Lake area don’t forget to check  Summit Lake Summary  here.

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Sun, March 20th, 2016
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

After a week of mostly quiet weather, yesterday ushered in the first – of what looks like a couple – storm systems slated to head our way. Over the past 24-hours the mountains surrounding Turnagain Pass have seen between 8 and 12″ of snow. The storm started out cold with snow to sea level but warm air is arriving and the rain/snow line is now around 1,000′. The cold start to this storm created low density snow and sluffing was the only form of instability seen yesterday. In fact, we had a report of natural sluffs releasing off the Easterly steep slopes near the Byron Glacier trail in Portage Valley.

STORM SLAB:
The low density snow from yesterday is a thing of the past, with a rising rain/snow line and warm temperatures. The 2-3″ of snow that fell last night is expected to be heavy and the additional
 6-10″ of snow for today will be even heaver as the rain/snow line is forecast to rise to 2,000′. This is a text-book ‘upside-down’ storm event. This means heavier snow is falling on lighter snow creating a ‘storm slab’ (slab over a weak layer). The weak layer in this case being the light snow from yesterday and slab thickness will depend on how much snow has fallen since last night and through today. This could be anywhere between 5 and 12″. These storm slabs are expected to be on the shallow side at Turnagain Pass but could be much larger in places favored by the storm. 

WIND SLAB:
With winds picking up to the 20-40mph range we can expect wind slabs to be forming on leeward slopes. These have the potential to release naturally. They will be thicker than the storm slab, in the 1-2′ range and likely easy to trigger. Travel on wind-loaded slopes is not recommended.

CORNICES:
Cornices will be building today and could fall off naturally. Steer clear of slopes, even mellow slopes, under cornices.

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Glide cracks continue to be a concern as they may release and avalanche without warning throughout the forecast region. Keep in mind cracks may be buried by the new snow and will be difficult to see. These cracks cover all aspects within the mid elevation band (between 1000-3000’) and remain a significant threat to popular terrain. As long as glide cracks continue to open up, move and release, we will stress the importance of avoiding them.

Photo below is of a glide crack from the Silvertip drainage on Friday, March 18th – credit Billy Finley.
 

Weather
Sun, March 20th, 2016

Yesterday, a region-wide snowfall event occurred over most of Southcentral Alaska with the Turnagain Pass area seeing 8-10″ by the late afternoon. Temperatures were cold enough for snow to sea level but have warmed overnight, as of 6am the rain/snow line has risen to ~1,000′ with an additional 2-3″ of snow. Ridgetop winds were light yesterday but have risen overnight as well, averaging 20-25mph with gusts in the 50’s from the East.

Today, the storm will continue. We are expecting another 6-10″ of snow (heavy snow) with a rain/snow line rising has high as 2,000′ (.8″ of rain at the lower elevations). Ridgetop winds should remain strong out of the East averaging 25-40mph. Temperatures are expected to climb to 37F at 1,000′ and the upper 20’sF along the ridgetops.  

For tonight, this storm should slowly move out – we could see another 3-5″ at the upper elevations with continued warm temperatures. Monday and Tuesday will be a slight break in weather (light snowfall and breezy winds) before another very large system looks to move in for Wednesday.
 

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   8   0.5   139  
Summit Lake (1400′) 24   7   0.5   50  
Alyeska Mid (1700′) 27   6   N/A   113  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   NE   17   56  
Seattle Ridge (2400′) 22   SE   15   52  
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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.