ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Wednesday January 30th, 2019
Posted by Heather Thamm on 01/30/19 at 7:00 am.
The Bottom Line
Considerable Avalanche Danger
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

The avalanche danger remains  CONSIDERABLE  on slopes above 2,000′. Human triggered slabs 2-4′ thick are likely on slopes steeper than 35 degrees. These slabs can be remotely triggered from below or along a ridgeline. Triggering a lingering wind slab or large cornice is possible today and has the potential to initiate a larger avalanche below. Cautious route finding and conservative decision-making will be essential for a safe day in the mountains.  MODERATE avalanche danger exists in the treeline zone where triggering a slab 1-2′ thick is possible.

SUMMIT LAKE / JOHNSON PASS:    Poor snowpack structure and multiple weak layers in the snowpack exist in this zone. Triggering a slab 1-3′ deep is likely in the Alpine on recently wind-loaded slopes.

3. Considerable
Alpine
/ Above 2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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Avalanche Problem 1
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

Strong winds and new snow over the last two days has added additional weight to an already stressed out snowpack. A layer of buried surface hoar sits 1-3’ below the surface in Turnagain Pass and 2-5’ deep in Girdwood Valley. This layer has been very reactive this week. Yesterday debris from recent natural avalanches was seen on the SE facing slopes of Seattle Ridge. This past weekend nine remote triggered avalanches occurred in Seattle Creek drainage and many were triggered from the ridge above. 

What makes this buried surface hoar different from previous layers this winter? This particular layer is more widespread, larger in size, and has been found intact along ridge lines. This means a human can triggering a slab remotely and more potential exists for a slab to propagate across an entire slope. Strong winds have created variability within the slab which means there are more trigger spots – thinner areas of the slab where the weight of a person or snow machine could collapse the weak layer. 

South of Turnagain – Johnson Pass/Summit Lake zone: Areas south of Turnagain Pass harbor a thinner, weaker snowpack with multiple weak layers present, including buried surface hoar.  An avalanche triggered in these zones could step down into deeper weak layers.   See video here  of a snowpit in the Silvertip area south of Turnagain Pass, that clearly demonstrates the step down potential. 

Unfortunately this type of avalanche problem is not going away quickly. Choosing low consequence terrain and keeping slope angles below 30 degrees is a safe way to avoid this problem. Also keep in mind:

  1. Whumpfing and shooting cracks may not be present until the entire slope releases.
  2. The bigger the terrain more potential for a larger more connected avalanche. 
  3. Avoid being in the runout zone of steeper terrain above

Recent natural avalanche on SE aspect of Seattle Ridge seen yesterday (1/29/18) just South of the Seattle Ridge Uptrack. 

 

Snowpit on Tincan yesterday (1/29/18) shows the buried surface hoar layer 2′ below the surface. This “MLK BSH”, buried on Martin Luther King Day, is our primary layer of concern across the region. 

Additional Concern
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

WIND SLABS: Wind slabs formed yesterday by strong winds and 6-12” of new snow on Monday may still be reactive on slopes steeper than 35 degrees.  More concerning is the possibility of triggering a smaller wind slab that steps down into the weaker snow below, causing a larger more dangerous avalanche.  

CORNICES: Large cornices are present along many ridgelines across the region. These ridgtop hazards can be tricky to see and can break further onto a ridge than expected. Triggering a cornice could initiate a large avalanche on the slope below. Watch for other groups and avoid traveling on or underneath these unpredictable hazards. 

Mountain Weather

Yesterday: Skies were overcast with a mix of rain and snow across the region. Rain/snow line was near 1000′. Portage Valley received an 1 € of rain at lower elevations, an estimated 12 € of snow in the upper elevations. Alyeska midway station recorded .33 € SWE (1-2 € wet snow) and Turnagain Pass DOT RWIS station recorded 0.1 € SWE (trace of wet snow along the road.) Ridge top winds were 15-30mph from the East.

Today: Overcast skies and with a chance for 1-2 € of snow. East ridge top winds 15-30mph will decrease to Light and shift to the North by this evening. Above freezing temperatures at sea level will dip into the upper-20Fs tonight. Temperatures at ridgetops should decrease into the mid to low-20F’s.

Tomorrow: Mostly sunny skies are in the forecast tomorrow. Moderate Northwest outflow winds will build in the morning 15-30mph along ridge tops. Temperatures will be in the mid-20F’s. No precipitation is expected.

 *Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed.   We are currently working to replace it.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34   trace   0 59  
Summit Lake (1400′) 34   0 0   23  
Alyeska Mid (1700′) 33   2   .34   47  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25   ENE   15   46  
Seattle Ridge (2400′) 29   *N/A   *N/A   *N/A