ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Monday February 4th, 2019
Posted by Wendy Wagner on 02/04/19 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 a ‘scary’ MODERATE  on slopes above 2,000′. A worriesome weak layer of buried surface hoar is lurking 2-3′ below the surface and it remains possible to trigger a large and dangerous hard slab avalanche; similar to those triggered last Friday.  On the surface, there are more obvious avalanche problems associated with yesterday’s snowfall. These are fresh wind slabs, up to a foot thick, and loose snow sluffs. Additionally, cornices are growing and remember to give them a very wide berth.  

SUMMIT LAKE / JOHNSON PASS:    Roughly 4-6″ of new snow has fallen with wind in this zone. A variety of weak layers exist in the snowpack and human triggered slab avalanches 1-3′ thick remain possible on slopes with recent wind loading.  

LOST LAKE:   New snow and wind yesterday in the Lost Lake and Seward area is expected to have increase the avalanche danger.  

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
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.

Special Announcements

  • New snow and wind has created dangerous avalanche conditions regionally. Hatcher Pass Avalanche Center has updated their advisory, please see that  HERE!  
  • GET  your tickets now! SNOWBALL is this Thursday – February 7th:  This annual fundraiser supports CNFAIC and Alaska Avalanche School with a silent auction and a raffle. Come celebrate the winter season with the funk band  Superfrequency  at the 49th State Theater in Anchorage!!
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

New snow and wind loading yesterday has added weight onto a snowpack with a troublesome buried weak layer that sits 2-3′ deep. This is our primary concern today despite the surface instabilities of fresh winds slabs and sluffs in the 4-8″ of new snow. It was only three days ago that three very large and dangerous avalanches were triggered by people along Seattle Ridge. This problem layer, which is buried surface hoar, was responsible for the avalanches. That layer has not gone away and although it has become more stubborn to trigger with time, every new load can cause it to become more reactive. This is the ‘scary’ part of the situation as there are a lot of unknowns. 

What to keep in mind if skies clear enough for travel above treeline:
   1-  Around 2-3 feet under your feet or snowmachine sits a weak layer
   2-  The weak layer may or may not be reactive, this is the tricky part
   3-  If an avalanche is triggered in this deeper weak layer, it can be very large, propagate across the entire slope and fill the valley floor with debris
   4-  No obvious signs of instability are likely to be seen before a slope releases
   5-  Remotely triggering a slab from the ridge, sides or below is possible
   5-  Safe travel protocol and assessing the consequences if the slope slides, the larger the terrain the larger the potential avalanche

To complicate matters, other areas such as the Girdwood Valley have seen other weak layers  of facets associated with crusts. These are now roughly 2-3′ below the surface. All this said, it’s important to keep in mind that the snowpack harbors buried weak layers. 

 

Photo of the crown of the Widowmaker avalanche from Feb. 1st. It’s a bit tough to see, but the bed surface is covered in 1cm feathers of buried surface hoar that was the responsible weak layer.

Avalanche Problem 2
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

Ridgetop winds associated with yesterday’s snowfall were moderate from the east. This is the perfect recipe for building wind slabs along ridgelines. Furthermore, a new batch of surface hoar grew last weekend and these slabs could be more touchy than expected if sitting on the surface hoar. Watch for cracking in the surface snow and stiffer snow over softer snow. Although wind slabs are likely to be shallow, they could be more dangerous if they were to step-down and trigger a large slab that breaks in the buried surface hoar discussed above. 

Loose snow sluffs:  Sluffs on steep slopes within the new snow should be expected.

Cornices:  We had a report of a large cornice fall in upper Seattle Creek drainage over the weekend. Heads up to give these an extra wide berth as they continue to grow. 

Additional Concern
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
  • TYPE
    Glide Avalanches

Glide cracks are opening again. We know of one glide avalanche that has released recently in the Summit zone just north of Manitoba. Heads up to look for glide cracks and limit exposure under them! 

Note the glide cracks in Seattle Creek’s Main Bowl (1st Bowl) on the slopes near the top of the photo. This photo was taken on Feb. 2nd, near the trigger point of the very large Widowmaker avalanche, which occurred Friday, Feb. 1st. 

Mountain Weather

Yesterday:    While heavy snow fell near and north of Anchorage yesterday, only light snowfall was seen in our forecast regions of Girdwood, Turnagain and Summit Lake. Roughly 4-5″ of snow fell at the mid elevations and up to 8″ at the higher elevations. The Girdwood Valley saw closer to 8″ at the mid elevations. Ridgetop winds averaged in the 20’smph with stronger gusts from the east over the course of the day and have decreased to the 5-10mph overnight.

Today:   A break in weather is expected today with a chance for clearing skies and valley fog. Cloud cover and light snow showers are expected to pick back up tonight as another front pushes moisture in from the southwest.   Ridgetop winds are expected to remain light, 5-15mph, from the east today before increasing to the 20-30mph range tonight. Temperatures are slowing climbing into the 20’sF at the upper elevations, where they will remain, and should hover near 30F at sea level.  

Tomorrow:   Light snow showers are expected to continue through tomorrow with accumulations in the 2-6″ range at the mid elevations.  

*The Seattle Ridge anemometer (wind sensor) was destroyed. We have a replacement on the way and it should be operational by mid February.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26   4   0.2   60  
Summit Lake (1400′) 26   5   0.4   27  
Alyeska Mid (1700′) 26   5   0.37   52  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18   NE   12   36  
Seattle Ridge (2400′) 23   *N/A   *N/A     *N/A