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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, March 29th, 2015 - 7:00AM
Expires
Mon, March 30th, 2015 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

There is a  MODERATE avalanche danger today at all elevations and aspects. With 5-7″ of new snow and moderate Easterly winds in the forecast above 2,500′, fresh wind slab avalanches will be possible to trigger on windloaded slopes over 35 degrees. These slabs are expected to be in the 8-12″ thick range. Additionally, wet loose avalanches are possible on steep (> 40 degree) slopes below 2,500′. Lastly, cornices are growing by the day and minimizing time underneath them and avoiding walking on them is key.

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Sun, March 29th, 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. Dangerous avalanche conditions. 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

Today begins the 4th day of stormy weather across the Eastern Turnagain Arm. During this time, Turnagain Pass has seen around 3′ of new snow above 2,500′, wet snow below and rain at the parking lots. The last pulse of moisture was Friday night, adding 6-8″ of new snow. Moderate Easterly winds yesterday loaded leeward slopes and we had a report of one person caught in an 8″ thick slab in the Tincan area, along the ridgeline (just West of Common Bowl). If anyone has information regarding this please pass it on HERE, much appreciated!

Wind slabs will again be the main concern for the upper elevations today. With 5-7″ of new snow and moderate Easterly winds, fresh slabs should be expected and likely in the 8-12″ range. These could also be thicker if a lingering wind slab from yesterday releases. In general, the new snow this week has been stabilizing relatively quickly, however if you catch a slab while or just after it forms, expect it to be reactive and possibly take you for a ride. Feeling for an “upside-down” nature in the top 1-2′ of the snowpack (stiffer snow over softer snow) will be a good clue for sussing out slabs. This can be done with quick hand pits, poking your pole in the snow and walking off the skin track.

Avalanche Problem 2
  • 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

Depending on where the rain/snow line is today will determine where wet avalanches will be expected. There was little natural wet activity yesterday and with temperatures similar, if not a bit cooler, we should see little again today. However, human triggered wet loose avalanches (push-a-lanches) were easy to initiate on steep slopes below 2,000′. Something to keep in mind on exiting through mid-elevation terrain; for example, the lower steep section of Magnum’s West Face.

Small wet loose avalanche triggered in steep mid-elevation terrain (~2,000′).

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices have grown significantly over the last week.  Steer clear of cornices and always know where the cornice begins and the underlying terrain ends.  Pick routes on the way up that minimize time spent beneath these features.

Weather
Sun, March 29th, 2015

Overcast skies and poor visibility were over most of the region yesterday. Light snowfall added just an inch or so to the 8″ of new snow observed at the upper elevations Saturday morning. Ridgetop winds were in the 15-20mph range from the East and Northeast while temperatures were mild, mid 30’s F at 1,000′ and upper 20’s F around 3,000′.

Today, another round of precip is on tap.  The Developmental Eastern Turnagain Arm Forecast is calling for .65″ of water which equates to around  5-7″ of new medium density snow at the upper elevations; we might not see quite this much at Turnagain. The rain/snow line should be ~800-1,000′ with temperatures in the mid 20’s F on the ridgetops. Ridgetop winds are expected to be 20-30 mph with stronger gusts from the East and Northeast.

For the next several days we will continue to be in this unsettled weather pattern. Which in short is a series of small low pressure centers rotating around a broad area of low pressure in the North Pacific. How much precipitation and wind will be associated with each disturbance is hard to predict, but nonetheless, the snowpack is growing above 1,500′! (This pattern can be seen on this cool Navy satellite loop – it shows visible images during the day and infrared during the night.)

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32    1 0.2   60  
Summit Lake (1400′) 35    trace 0.1   11  
Alyeska Mid (1700′) 34    0 0.01   32

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24   E   15    41
Seattle Ridge (2400′) 23   n/a   9    37
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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.