ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Saturday January 26th, 2019
Posted by Wendy Wagner on 01/26/19 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 is CONSIDERABLE on slopes above 1,500′. Human triggered slab avalanches between 2 to 5 feet thick are likely due to new snow sitting on a weak foundation. These avalanches could be very large, dangerous and unsurviveable. They also could be triggered from the bottom, side or on top of a slope. Cornices have grown, may break farther back than expected and could trigger an avalanche below. The avalanche danger is MODERATE below 1,500 where an avalanche running into this zone is possible.  

*HEADS UP!  Today is the first sunny day after a storm and the new snow is not expected to have bonded well at the higher elevations. Cautious route-finding and conservative decision-making are essential today.  A  SPECIAL AVALANCHE BULLETIN  has been issued though the National Weather Service.  

SUMMIT LAKE / JOHNSON PASS:    Although much less snow fell, strong wind, a poor snowpack structure and recent avalanche activity points to dangerous avalanche conditions existing in this region as well.  

3. Considerable
Alpine
/ Above 2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
3. Considerable
Treeline
/ 1,000'-2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
2. Moderate
Below Treeline
/ Below 1,000'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1
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.
  • TYPE
    Storm Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

After three days of stormy weather, winds and precipitation quieted down yesterday and skies began to clear. Many avalanches occurred naturally during the storm, which dropped 3-5′ of snow at the upper elevations in Girdwood Valley and ~2′ at Turnagain Pass. The peak in natural activity was Thursday night and yesterday morning. Debris from large avalanches in the Alpine made its way to sea level in the Girdwood and Portage Valley areas. Turnagain Pass saw relatively little natural avalanche activity. Visibility was a challenge, but from the road the only signs of recent avalanches were in the steeper slide paths on the south end of Seattle Ridge. 

The main concern this weekend is a layer of buried surface hoar that is sitting under the 2-5′ of new storm snow at the mid-upper elevations. We have very little information about how reactive this layer is, but we do know it existed on all aspects and elevations before the storms. Wide propagation seen in crowns in Portage Valley yesterday is proof that avalanches triggered by people this weekend could also propagate across slopes. The only good news avalanche-wise is rain fell to 1,500′ and up to 2,500′ in areas. This wetted the pack and has now frozen, stabilizing the lower and some mid elevations. However, the key today will be knowing at what elevation the surface crust starts to disappear, this will be the elevation that triggering a slab avalanche becomes likely.

What to keep in mind if heading into the backcountry:

1-  The higher in elevation one travels, the more likely it will be to trigger an avalanche
2-  Strong winds redistributed the snow and slabs could be over 5′ thick in places as well as allow a person or snowmachine onto them before releasing
3-  Remotely triggering a large slab is possible (this means triggering an avalanche from the bottom, side or on top of a slope)
4-  Whumpfing and shooting cracks are sure signs the buried surface hoar is failing and a Red Flag to stay out of avalanche terrain
5-  No obvious signs of instability may be present either before a slope is triggered

Today is a day to be conservative, as the potential size of an avalanche is too big to manage. Triggering a slide onto another person or group is also possible. If choosing to avoid this hazard, steering clear of avalanche terrain (slopes steeper than 30 degrees with nothing steep above you) is a good option. 

 

CNFAIC Intern Nikki Champion snapped this photo yesterday of crews clearing avalanche debris off the railroad along the Seward Highway. One of the many signs of the avalanche cycle that just ended yesterday morning.

 

Snow pit on Tincan, from 1/24 between storms. This pit shows the buried surface hoar we are most concerned about. It’s the (1/21) layer and sat just over a foot below the surface at the time – another foot of snow has fallen since.

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

The warm sticky snow last week has built up cornices. These are likely to still be tender and could be teetering on the brink of failure. Give them a wide berth above and limit exposure underneath them. Any cornice fall is not only dangerous itself, but is likely to trigger a slab avalanche below. 

Mountain Weather

Yesterday:   Mostly to partly cloudy skies were over the area as the last wave in a three-part storm series exited. Light rain fell below 1,500-2,000′ and light snow flurries above this added only a trace to a couple inches of new snow to the higher elevations. Ridgetop winds decreased dramatically in the morning and have been in the 10-20mph range from the SE over the past 22-hours. Temperatures were in the mid 30’s F at mid and lower elevations, while ridgetops were in the 20’sF.  

Today:   Mostly sunny skies are forecast with light easterly ridgetop winds (5-15mph). Temperatures have cooled overnight and expected to remain near 30F at sea level, 20’s F at the mid elevations and in the teens along the higher peaks.  

Tomorrow:  Partly cloudy skies with a chance for snow flurries later in the day as a low pressure develops south of Kodiak. Models are showing south and easterly winds pick up to the 30’s mph along ridgetops. This next system may bring a few inches of snow Sunday night into Monday.  

 *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   trace   0.1   59  
Summit Lake (1400′) 30   0   0   20  
Alyeska Mid (1700′) 31    1 0.14   44  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   SE   16   61  
Seattle Ridge (2400′) 27   *N/A   *N/A     *N/A