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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, April 10th, 2015 - 7:00AM
Expires
Sat, April 11th, 2015 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The danger is CONSIDERABLE in the Alpine.   Slabs 2-3′ thick will be sensitive to human triggers and could release naturally with daytime warming and occasional sunshine on all aspects.   These slabs have the potential to step down to layers buried more deeply on North aspects and be in the 3-5′ range.  

The danger is MODERATE in the Treeline elevations, where slabs in the 1′ range will be sensitive to human triggers on steep wind loaded slopes.

Loose snow avalanches, both dry and wet, will be likely on steep slopes at all elevations.   Cornices will also be sensitive and are yet another reason for conservative decision making today.

Significant loading over the past 24 hours has created unstable conditions.   Because of this it will be critical to avoid slopes over 35 º steep in the Alpine.   Avoidance of runout zones when traveling in the mid elevations will also be necessary as avalanches have the potential to be high in volume and run through multiple elevation bands.

Special Announcements

Beginning next week, we will be issuing advisories 5 days a week.   Advisories will be posted at 7 am on all days of the week except Mondays and Wednesdays.   The final advisory of the season will be posted on Thursday April 30th.

Fri, April 10th, 2015
Alpine
Above 2,500'
3 - Considerable
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

Snowfall that was intense at times yesterday has created slabs that will be tender today.  Slab depths will range anywhere from a few inches to 2 feet thick.  In the Alpine these slabs sit on a slick crust on South facing slopes and a variety of surfaces on North aspects.  Regardless of what these slabs are sitting on, they are dense, thick and very sensitive to human triggers.  Brief periods of sunshine will increase their sensitivity today.  While we found the new snow well bonded to underlying surfaces in our limited observations yesterday, all slopes 35º and steeper should be treated with suspicion today.  These slabs will be thickest, over 2 feet, where they have a wind component, and be found on leeward slopes around Turnagain Pass.

Cornices have been growing steadily over the last week.  Yesterday was no exception.  Expect cornices to be very sensitive today.  Give these behemoths a very wide berth along ridgelines and avoid traveling underneath.

Conservative terrain selection will be key today, as rapid loading over the past 24 hours has created unstable conditions.  Avoidance of terrain 35º and over and being in the runout of terrain from above will be critical.

Avalanche Problem 2
  • 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

The majority of slab activity should be in the 2-3’ range today.  However, there is potential for slabs to step down into more deeply buried weak layers, especially on North facing slopes in the Alpine.  These layers are anywhere from 3-5’ down.  Should avalanches step down into these layers expect volume to be very large and high enough to make consequences of being caught very high.  This is not an issue to be taken lightly.  A significant load has been put onto these layers and it will take time for those layers to adjust. 

Travel advice around this issue is simple for today: avoid North facing slopes in the Alpine.

Additional Concern
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

Both dry and wet loose avalanches are likely on steep slopes today at all elevations today.  Snow below the recent storm slabs was wet yesterday in the lower elevations.  Because of this, the potential for higher volume wet loose avalanches exists in the lower elevations and will be more likely during daytime heating and sunshine today.  Volume will generally be medium to high and will increase on sustained slopes that allow for entrainment.

Weather
Fri, April 10th, 2015

Yesterday brought significant snow, wind and mild temps to the area.   Snowfall totals over the last 24 hours:
Turnagain SNOTEL: 18 € snow/1.4 € H20
Alyeska Mid: 8 € snow/.8 € H20
Summit Lake: 1″ snow/.1″ H20
Rain/snow line was around the 500′ mark for most of the day.   Winds were moderate out of the East.

Today expect lingering snow showers and cloudy skies as a large area of low pressure continues to spin in the Gulf of Alaska.   Snowfall amounts will be light, in the 1-2 € range.   Ridgetop winds will be out of the East at 5-10 mph.   Temperatures at 1,000′ will climb into the mid 30s F.

The extended outlook is showing a very gradual change though the weekend.   Expect clouds and showery conditions to linger as low pressure in the Gulf slowly moves East.

*Seattle ridge wind sensor is rimed.   Average is 6am-4am.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 18 1.4 77
Summit Lake (1400′) 33 1 .1 12
Alyeska Mid (1700′) 31 8 .8 48

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21 ENE 27 58
Seattle Ridge (2400′) 25 * 13* 50*
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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.