ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Sunday February 3rd, 2019
Posted by Wendy Wagner on 02/03/19 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 expected to rise to CONSIDERABLE  today in areas above treeline receiving new snow and wind. Expect fresh wind slabs to be easy to trigger and relatively shallow, up to a foot thick. The more concerning issue is, newly wind loaded slopes are overloading a weak layer of buried surface hoar 1-3′ deep in the snowpack. Due to this poor set-up, large and unsurviveable human triggered slab avalanches remain possible on slopes over 2,000′.  

SUMMIT LAKE / JOHNSON PASS:    Human triggered slab avalanches 1-3′ thick remain possible above treeline as a variety of weak layers exist in the snowpack. Wind loaded slopes are the most suspect for triggering an avalanche.  

LOST LAKE:   We have had reports that avalanches are being easily triggered on wind loaded slopes in the Lost Lake area. New snow and wind is expected to increase the avalanche danger in these areas near Seward today.  

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.

Special Announcements

  • Headed to Hatcher Pass? Make sure and check HPAC’s advisory  HERE!
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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

Another round of snow and wind is on the doorstep today and into tonight. Turnagain only looks to receive a few inches out of this and Girdwood/Portage areas up to 6″ or more. What this will do to the snowpack is add an incremental load, which in our current snowpack set-up is significant. We did not receive any reports of avalanches being triggered yesterday, however on Friday (Feb 1st) several large slab avalanches were triggered. All of them failed on a weak layer of buried surface hoar 1-3′ below the surface. They are summarized in yesterday’s avalanche forecast if you missed it.

The concerning thing is, these avalanches are hard slabs, propagating across entire slopes and proving to still be reactive. They have been predominantly triggered remotely from or near ridgelines. Although the avalanches triggered so far have been in the Seattle Ridge zone, this weak layer is known to be widespread in the region. With another load of wind deposited snow today, these hard slabs on buried surface hoar are being pushed more toward their tipping point. The size and scope of this avalanche problem is nothing to mess with. Cautious route-finding is essential and steering clear of avalanche terrain above the trees is a good way to avoid the hazard. No signs of instability are likely to be present before a slope releases and therefore, we have to remember a bad layer is lurking below our feet.

 

Jr’s Run avalanche, triggered remotely from the ridge on February 1st (you can see snowmachine tracks along the ridge if looking close). Photo taken February 2nd. Although this looks like three separate slides, these were all triggered close to the same time from the ridge. Interesting how the middle portion remains intact.

 

Widowmaker avalanches, triggered February 1st. The ‘Looker’s right’ avalanche was triggered remotely from the ridge an hour or so after the main path ran. The main path was triggered near the crown by a snowboarder and that crown wraps around, out of view to the left side of photo. Very impressive, and scary, propagation.

  

 

The thin gray line the arrow points to is the weak layer under the surface we can’t forget about. 

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 are picking up this morning from the east and expected to blow in the 25-35mph range today. With loose surface snow and 2-4″ of new snow available to transport, wind slabs on leeward slopes should be expected. These are expected be fairly shallow, a foot or less in thickness, yet could be easy to trigger. 

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

It appears that glide cracks are opening again and a glide avalanche released in the Summit zone recently. Heads up to look for glide cracks and limit exposure under them!

Glide avalanche on a southerly slope just north of Manitoba. First noticed Feb 1st. Photo: Patrick Machacek

Mountain Weather

Yesterday:   Partly cloudy skies and sunshine filled the region yesterday. Ridgetop winds were light and variable during the day and have switched to the east overnight, increasing to the teens with gusts to 30mph. Temperatures were civil, in the 20’sF at sea level and valley bottoms with the teens along ridgelines.  

Today:   A southwest flow is ushering moisture up Cook Inlet, bringing snow to the western Kenai mountains and setting the stage for significant snow in the Hatcher Pass area. This flow direction typically leaves Girdwood and Turnagain out of the fun. We can expect cloudy skies with 2-4″ of snow today and another 2-4″ tonight. Ridgetop winds will be 25-35mph from the east with stronger gusts. Temperatures look to rise to near 32F at sea level, bringing a rain/snow mix, while ridgetops should remain in the teens.

Tomorrow:   A brief break between fronts may clear skies tomorrow during  the daylight hours before warmer air and precipitation heads in later in the day and into Tuesday. An active pattern seems to be in place for most of the coming week.  

*The Seattle Ridge 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′) 21   0   0   57  
Summit Lake (1400′) 19   0   0   23  
Alyeska Mid (1700′) 21   3   0.14   47  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17   variable   8 30 from east  
Seattle Ridge (2400′) 18   *N/A   *N/A     *N/A