ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday December 20th, 2018
Posted by Heather Thamm on 12/20/18 at 7:00 am.
The Bottom Line
High Avalanche Danger
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Travel is not recommended today  in avalanche terrain due to HIGH avalanche danger at all elevations. Large natural avalanches 3-5+’ thick are very likely in the upper elevations due to heavy snow at all elevations. A Special Avalanche Warning has been issued for the mountains surrounding Turnagain Pass, Portage Valley and Girdwood Valley. This also includes Johnson Pass and Lynx Creek drainage.

PORTAGE: In Portage Valley where 2-4′ of snow has fallen in the upper elevations avalanches could run their full length to valley bottoms. Avoid being near any avalanche paths.

SUMMIT LAKE: In Summit Lake a very weak and shallow snowpack exists and human triggered avalanche 2+’ thick are likely in avalanche terrain.  

4. High
Alpine
/ Above 2,500'
Avalanche risk
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
4. High
Treeline
/ 1,000'-2,500'
Avalanche risk
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
4. High
Below Treeline
/ Below 1,000'
Avalanche risk
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Special Announcements

Many riding areas on the Chugach National Forest have opened near Seward. Be aware of increased avalanche danger today. Very little info exists about the snowpack in these zones. Turnagain Pass is being assessed. An up-to-date riding status is available at the bottom of this page.

Due to an unexpected amount of snow in the last 24 hours and up to a foot more today, please be aware that parking areas will not be plowed until roads are cleared. Please yield to DOT plow drivers and let them focus on the road.  

If you’re thinking of heading to Hatcher Pass be aware of dangerous avalanche conditions in this zone. Check out the Hatcher Pass Avalanche Center mid-week summary HERE.  

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

Today we recommend avoiding the backcountry due to HIGH avalanche danger. Heavy snow has been falling over the last 24 hours and is expected to continue through early afternoon. The estimated snowfall for Turnagain Pass is 4-5’ and more snow is expected through early afternoon. Snowfall rates have been in the 6”/hour (0.2 – 0.3” SWE/hr) overnight at the Turnagain Pass SNOTEL and 1-2”/hour in Portage and Girdwood. This is an unprecedented amount of snow in a very short period of time and natural avalanches could run far distances and very fast. Avoid all avalanche terrain including slopes steeper than 30 degrees and stay far away from any runout zones. In Portage Valley where channeled terrain runs to sea level avoid any trails with steep slopes above, like Byron Glacier trail. As an alternative to the backcountry, consider going to Alyeska!!! Santa’s ski for free today.

 24-hour Precip:

  • Turnagain Pass SNOTEL: estimated 4-5 feet (3.4” SWE)
  • Portage Valley: 22” (1.03” SWE)
  • Girdwood – Alyeska Midway: 20” (1.04” SWE)
  • Summit Lake: 10” (0.4” SWE)

The snow stake at Turnagain Pass DOT lot shows ~ 3-4 feet of new snow in the last 24 hours. 

 

Crowns from natural storm snow avalanches were seen yesterday in the Girdwood Valley at around 2500′. In this zone there was 13″ new snow mid-afternoon.  

Avalanche Problem 2
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

In thinner snowpack zones such as Summit Lake we are tracking buried layers of facets, crusts and buried surface hoar that sit 2-3′ under the snow surface. 10” of snow has fallen over night in Summit Lake and 6-12” is possible today. This new snow and moderate ridgetop winds may be adding more stress to an already weak snowpack. Triggering a slab near the ground is likely in avalanche terrain. The Lynx Creek and Johnson Pass zone have little to no snowpack data and often have a similar snowpack to Summit Lake. These zones are now open to snowmachining and this area likely received 2-3’ of snow in the last 24 hours. Today is not the day to push into Johnson and Lynx. HIGH avalanche danger is expected in this zone similar to Turnagain Pass.

Mountain Weather

Yesterday: Heavy snowfall started around 8am and has continued through this morning. An estimated 4-5′ has fallen at Turnagain Pass (3.4 € SWE.) The Portage Rail Road Weather station is reporting 22 € at sea level (1.03 € SWE) and Alyeska midway is similar. Temperatures have remained in the mid to low 20F’s in the lower elevations and in the teens F at ridgetops. Ridgetop winds have been from the East 15- 30mph with some higher gust.  

Today: A band of moisture and low pressure continue to pump snow through the Eastern Turnagain Arm region (North of Moose Pass to Girdwood.) The weather service has issued a winter storm warning with an additional 6-16″ is possible through early afternoon. Temperatures are expected to remain in the low 20F’s to low teens (F). Ridgetop winds will be light to moderate.

Tomorrow: An active weather pattern will continue tomorrow into the weekend with more snow showers in the forecast. Temperatures are expected to be in the 20F’s with light to moderate winds.  

*Seattle Ridge weather station is rimed over and not recording any data.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20   46″ 3.4 85
Summit Lake (1400′) 16   10″   0.4   23  
Alyeska Mid (1700′) 20   20″    1.04 40  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13   ENE   14   37  
Seattle Ridge (2400′)  *N/A  *N/A    *N/A    *N/A