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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, February 21st, 2015 - 7:00AM
Expires
Sun, February 22nd, 2015 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche danger will start out at CONSIDERABLE and climb to HIGH in the Alpine by the end of the day.   The same progression will occur at the Treeline elevations moving from MODERATE to CONSIDERABLE by the evening.   The danger will be MODERATE below treeline.   Avalanches occurring in the higher elevations have the potential to run through multiple elevation bands today.

New snow and wind will create slabs up to 1′ deep in the upper elevations.   These slabs on their own will be enough to injure or bury a person.   The potential also exists for these newer slabs to step down into deeper weak layers, creating the potential for large avalanches.

Travel in or below avalanche terrain is not recommended today.

Sat, February 21st, 2015
Alpine
Above 2,500'
4 - High
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
  • 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

A modest amount of snow combined with winds will add stress to a snowpack that is hanging in the balance.  Fresh slabs up to 1’ thick on steep leeward slopes in the Alpine have the potential to release on their own.  Given the warm temps and high level of moisture in the new snow, many of these slabs will “stick” well to underlying surfaces.  Because of this it will take steeper slopes, over 35 degrees, to get slabs to move.  This concern will be more pronounced where winds are actively loading slopes.  These slabs also have the potential to step down into deeper layers in the snowpack.  

Snow that feels stiff, sounds hollow or produces shooting cracks and is connected to steeper terrain should be avoided today.

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

A layer of weak faceted snow sits anywhere from 6” to 3’ below the surface.  This layer continues to be reactive in tests.  Average slab depths in the Alpine prior to this current storm were 2 feet.  Adding into the equation another slab makes this avalanche concern a higher priority in terms of consequences.  While it may be more difficult for humans to trigger a slab at this depth, the potential still exists.  This layer has also shown high propagation potential.  Avalanches occuring at this layer have the potential to pull out across large areas.

Because of this potential travel in avalanche terrain is not recommended today.

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

Rain will weaken the snow surface today.  Should the rain/snow line creep above the 2,000’ level, there is potential for wet slab avalanche activity.  Below 2,000’ wet loose avalanches are possible in steep terrain.  Volume will be generally low.  Pay attention to the terrain below you, as terrain traps such as cliffs, gullies, and trees will increase the consequences of being caught in relatively small wet loose avalanches today.

Weather
Sat, February 21st, 2015

Yesterday saw the beginning of a surge of moisture spreading across the forecast area.   Warm, wet and windy was the general theme.   Ridgetop temperatures were in the high 20s F and low 40s F at sea level.   Precipitation began by late morning with the rain/snow line hovering around the 2,000′ level.   Below this line rain fell with the Girdwood Valley receiving .5 € and Turnagain Pass picking up .3 €.   Snowfall amounts above the rain/snow line range from 3-5 €.   Ridgetop winds have been moderate with Seattle Ridge blowing in the 30-40 mph range.  

Southerly flow will continue to impact the area today with more of the same.   Precipitation amounts will be around .5 € of H20, with snowfall in the 3-5 € range.   The rain/snow line will be around the 2,000′ mark.   Winds will be out of the East at 25-35 mph.   Temperatures will climb to 40 F at 1,000′.

The extended outlook is showing a continuation of this warm, wet and windy pattern through the weekend.   Intensity will diminish by tonight, but showery conditions will persist through Sunday and into the early part of next week.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33 2/rain .2 43
Summit Lake (1400′) 37 rain .1 8
Alyeska Mid (1700′) 35 rain .5 24

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27 n/a n/a n/a
Seattle Ridge (2400′) 29 n/a 30 67
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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.