ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday December 6th, 2018
Posted by Heather Thamm on 12/06/18 at 7:00 am.
Avalanche risk
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

The avalanche danger remains  MODERATE  above 2500′ in the Alpine where triggering a wind slab in steep terrain will be possible.  Additionally, triggering a  slab avalanche 1-3′ thick, releasing on buried surface hoar, is possible though trending toward a lower likelihood.

SUMMIT LAKE: A very shallow snowpack exists, but triggering a slab large enough to bury a person is possible in the upper elevations.

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Treeline
/ 1,000'-2,500'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
0. No Rating
Below Treeline
/ Below 1,000'
Avalanche risk

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Avalanche Problem 1
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.
  • TYPE
    Wind Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Overnight moderate Easterly ridgetop winds were accompanied by 2-3” of new snow in the upper elevations. Be aware of fresh shallow wind slabs on steep leeward features. Earlier in the week moderate winds re-distributed the snow in the alpine and older wind slabs may be hard and hidden. Pillowed snow, cracking and hollow, drum-like sounds are clues wind slabs may be reactive. Yesterday on Tenderfoot observers noted recent wind slab formation on top of weak faceted snow. Wind slabs in the Summit Lake area may be more reactive due to a weaker foundation. As always evaluate the terrain for consequences and avoid terrain traps like gullies, rocks, trees below. Also keep in mind very little snow exists below 2000’.  

Wind drifted snow on Sunburst from Monday (12-3-18)

Avalanche Problem 2
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.
  • TYPE
    Persistent Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

In the upper elevations we are keeping tabs on a thin layer of buried surface hoar sitting 1-3′ below the surface. This layer has been documented in a number of snowpits above 2500’ over the last week. In many places this layer is laying down and unreactive, where in other places it may be upright and still able to propagate. Overnight 3” of snow fell above 2500’ in Turnagain Pass and Girdwood. This isn’t enough weight to be concerned about natural activity, but it could be adding some stress to the snowpack. As always use safe travel protocol and obvious clues like ‘whumpfing’ may not be present.

*Also be aware of a shallow snowpack with weak faceted snow near the ground in the Summit Lake area. Persistent slabs may be more reactive in this zone and could be large enough to bury a person on a large connected slope.

Surface hoar found 2′ below the surface on Sunburst on 12-3-18

 

In this area there wasn’t much of a slab, but be aware of facets under wind drifted snow in Summit Lake. This structure was found in most hand pits above 2500′ on Tenderfoot. 

Mountain Weather

Yesterday: Temperatures increased from mid-20F’s to mid-30F’s in the mid-elevations. Light rain started falling below 1000′ yesterday and 2-3 € of new snow fell in the upper elevation overnight. In Portage where precip totals are typically higher 0.5 € of rain fell near Bear Valley. Ridgetop winds were moderate from the East 15-35mph overnight.    

Today: Temperatures will remain in the mid to upper-30F’s near sea level and rain snowline is expected to be around 1300′. Precipitation below this elevation will fall as light rain (up to .15 € of water) and another 1-2 € of snow is possible in upper elevations. Easterly ridgetop winds have already started to decrease into the 5-15mph range, but will increase again this evening.

Tomorrow: Get ready as a fast moving system moves into Prince William Sound and with it another round of strong winds, warm temps and a period of heavy rain for our region. Expect above freezing temps in the lower to mid-elevations with rain/snow line around 1500′. Easterly winds will build overnight into sustained 30-40’s mph ridgetop winds with gusts up to 50’s mph.

* Warm temps and winds have finally freed up the Seattle Ridge Anemometer. Wind data is from 12pm – 6am.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35   3   0.3   16  
Summit Lake (1400′) 26   trace   0.1   3  
Alyeska Mid (1700′) 35   2   0.25   *N/A  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27   ENE   12   38  
Seattle Ridge (2400′) 31   ESE   12   34