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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, April 14th, 2015 - 7:00AM
Expires
Wed, April 15th, 2015 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  in the Alpine and the Treeline elevation bands for wind slab and loose snow avalanches associated with new snow instabilities.  Up to 6″ of snow, along with moderate Easterly wind, is forecast through the day with another 2-6″ overnight. Wind slabs are expected to be 4-8″ thick today and up to 14″ by tomorrow. Additionally, there are a number of weak layers and interfaces buried deeper in the pack on all aspects where a small possibility exists for a large avalanche to be triggered.

AVALANCHE OUTLOOK for tomorrow, Wednesday:

Avalanche conditions are expected to be similar tomorrow as the storm slowly moves out. Light snowfall, adding another 2-4″, is expected along with light Easterly ridgetop winds. Human triggered slab avalanches composed of the storm snow are possible along with widespread sluffs. If the sun comes out on Wednesday, wet loose avalanches on southerly slopes should be expected.

Special Announcements

We are currently issuing advisories 5 days a week until our final advisory on Thursday, April 30th.   Advisories will be posted at 7 am each day except Mondays and Wednesdays.

Tue, April 14th, 2015
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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 waiting for months, it seems winter is finally trying to show up. We have another storm system moving in today which is expected to add 2-6″ of new snow and another 2-6″ tonight – snow should make it to sea level. Avalanche conditions will be mostly related to how much new snow accumulates, however there are some concerning layers deeper in the pack to keep in mind (more on that below).

WIND SLAB and STORM SLAB:

Wind slabs forming on leeward slopes will be fairly shallow (4-8″ thick) today. Even in areas seeing little new snow, these should build due to 6-8″ of existing loose snow available for transport. With continued snowfall overnight, wind slabs could be in the 10-14″ thick range tomorrow – which will be more of a concern. Areas out of the wind, where up to a foot of new snow may fall by tomorrow, could see slabs as well. If you are headed out, monitoring the new snow with quick hand pits and/or using your pole to check for stiffer/denser snow over softer/weaker snow will be good ways to assess slab development.

LOOSE SNOW AVALANCHES (SLUFFS):

Natural and human triggered sluffs are expected in the new snow on all aspects and elevations. When the sun comes out, possibly tomorrow, watch for wet/damp sluffs on Southerly aspects. 

CORNICES:

Cornices will continue to grow and fall naturally with this warm(ish) storm. 

Avalanche Problem 2
  • 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

The mountains are currently in a transition period from winter to summer which means a highly variable snowpack that changes dramatically with aspect and elevation. In short, Northerly slopes have weak faceted snow sitting anywhere from 2-8′ below the surface; this spread is so dramatic due to the snowfall amounts during last week’s storm. The South side of Turnagain Pass has a thinner snowpack and therefore these weak layers are more concerning. On Southerly slopes, a stout crust exists below the storm snow from last week (2-4′ below the surface). Initial bonding was poor and though stronger now, still a concern.

What this all boils down to is: triggering an avalanche that fails deeper in the pack, although not likely, is not out of the question. Remember your safe travel protocol: expose one person at a time, have escape routes planned, watch your partners and know how to effect a rescue if a slide is triggered.

Weather
Tue, April 14th, 2015

Mostly sunny skies and springtime temperatures filled the region yesterday. Winds were light and variable. Temperatures reached the mid 30’s F in the sun mid-day before dropping back into the teens overnight.

Today and tonight, we will see a relatively weak storm roll in as a low-pressure, just South of the Aleutians, moves East. We can expect cloudy skies, 2-6″ of new snow today with an additional 2-6″ tonight. Ridgetop winds will increase from the East to 25-35mph with stronger gusts. Rain/snow line will be right around sea level.

For Wednesday, this low-pressure looks like it might stall out over the Kenai bringing the possibility for widespread instability showers. We could pick up another 2-4″ of snow in some areas while others see sunshine. Ridgetop winds on Wednesday should be light, 5-10mph, from the East.  

Thursday looks to be a break between storms before another low moves in for Friday. Stay tuned on Thursday’s forecast.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 0   0   71  
Summit Lake (1400′) 25   0   0   13  
Alyeska Mid (1700′) 29   0   0   41  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21   var   4   31  
Seattle Ridge (2400′) 25    n/a 7   25  
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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.