ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday January 31st, 2019
Posted by Heather Thamm on 01/31/19 at 7:00 am.
The Bottom Line
Considerable Avalanche Danger
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Today is one of those days where the likelihood of a human triggered avalanche fits into MODERATE, but practicing the travel advice for  CONSIDERABLE  danger is more appropriate for our tricky avalanche conditions. Human triggered slabs 2-4′ thick are possible above 2000’on slopes steeper than 35 degrees. It may also be possible to trigger a slab remotely from below a slope or along a ridgeline. Any sign of active wind loading today could increase the likelihood for triggering a large slab or initiating a natural cornice fall. Cautious route finding and conservative decision-making will be essential for a safe day in the mountains.

The avalanche danger is  MODERATE  between 1000′ and 2000′ where an avalanche from above could run. Below 1000′ where a firm surface crust exists the avalanche danger is  LOW.  

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 slopes steeper than 35 degrees. Any sign of wind transport could initiate natural avalanches.

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

As we move away from a storm that ended Tuesday and into a sunny day be aware of the temptation to push into steeper terrain. This is a situation where the weather is rapidly improving, but snowpack is slow to follow. Triggering a large high consequence avalanche remains a real possibility due to a reactive layer of buried surface hoar 2-4’ below the surface. Recent natural avalanches occured this week and numerous remote triggered avalanches last weekend were initiated on this layer in Seattle Creek drainage.

What makes this buried surface hoar layer different than previous layers this season? This layer is widespread, large in size, and has been found intact under the snow along some ridgeline. This is why remote triggered avalanches are possible. Strong winds have created variable slab thicknesses across any given slope. This means trigger spots, thinner areas of the slab, will be impossible to identify until the weight of a person or snowmachine collapses the weak layer. An additional challenge exists across the region due to varying storm totals. In Girdwood, where more snow fell, average slab thickness is estimated around ~3’ thick. In Turnagain Pass the average slab depth is ~2’ and in Summit Lake its 1-1.5’ thick.

This type of avalanche problem is tricky to navigate and requires a good deal of skepticism when making decisions. Please keep mind:

  1. The larger the slope the bigger the consequences.
  2. Previous tracks on a slope do not mean its safe. The 3rd, 4th or 8th person could find a trigger spot.
  3. “Whumpfing” may not be present until committed to a steep slope and a large avalanche releases.
  4. Be aware of other groups above or below you. The more people in any given area = more triggers.

Winds/Wind Slabs: Ridgetop winds are expected to be 5-20mph from the Northwest. Whittier and Portage pass will see stronger outflow “gap winds” today. Remember any active wind loading and blowing snow could form tender wind slabs or overload a deeper more dangerous problem. Wind loading is an obvious sign to avoid all avalanche terrain. 

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. A storm that ended on Tuesday triggered numerous large avalanches including a notable cornice triggered avalanche on Spirit Walker. If Northwest winds pick up today in this area, natural avalanche will be possible. Also keep in mind that an avalanche triggered in these zones could step down into deeper weak layers.

Recent natural avalanche in the Twenty Mile drainage on a SW aspect. Note the wind sculpted snow texture on the slope adjacent that didn’t avalanche.

 

 

Although a little tricky to make out in this picture. There are three separate avalanches with large crowns. It is suspected these were triggered by natural cornice fall near or after the last storm. 

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

Large cornices are present along many ridgelines across the region. These ridgtop hazards can be difficult to see and can break further onto a ridge than expected. Triggering a cornice could initiate a large avalanche on the slope below. Keep in mind any active wind loading could trigger natural cornice fall. Avoid traveling on or underneath these unpredictable hazards. 

Sun: We are just starting to see minor solar radiation happening on steep Southeast aspects in the form of small wet-loose point releases. Today radiation is not expected to increase the avalanche danger, but it is an additional factor in the unpredictable nature of cornices. 

 

Mountain Weather

Yesterday: Skies were overcast in the morning becoming partly cloudy with some sun in the afternoon. No precipitation was recorded. Temperatures were in the mid-30F’s near sea level. Temps at the Turnagain SNOTEL reached a day-time high of 33F before dropping into the mid-20F’s overnight. Light ridgetop winds switched from an Easterly direction to a NW direction overnight.

Today: Clear and sunny skies are expected for the day with some valley fog in the morning. Temperatures will be in the mid-20F near sea level and low-20Fs near ridge tops. Light Northwest ridgetop winds 5-20mph are expected with stronger NW winds in Whittier and Portage Pass. No precipitation is expected.

Tomorrow: Clear skies are forecasted most of the day. Temperatures should remain cooler, mid 20Fs to upper teens F. Light Northwest winds will continue. The next chance for precipitation is Saturday into Sunday.

*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′) 30   0   0   56  
Summit Lake (1400′) 25   0   0   23  
Alyeska Mid (1700′) 30   0   0   45  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22    ENE 5   18  
Seattle Ridge (2400′) 28   *N/A    *N/A   *N/A