ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday December 21st, 2018
Posted by Heather Thamm on 12/21/18 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 at all elevations for the mountains surrounding Turnagain Pass. Trigging a storm slab 3-4′ deep is likely in avalanche terrain due to a massive storm that ended yesterday morning. Triggering an unmanageable loose snow avalanche that entrains and picks up momentum is also a real concern. Today’s message is simple, let this storm settle and give the plow drivers time and space to clear parking areas. This also includes Girdwood, Portage, Johnson Pass, and Lynx Creek drainages that received 2-3+’ of snow during this last storm.

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

LOST LAKE:  This zone is out of our advisory area, but observations yesterday show the potential for triggering slab avalanches 2-3′ thick on an older weak layer within the snowpack.  If headed to this area pay attention for signs of instability like collapsing and recent avalanches.

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.
3. Considerable
Below Treeline
/ Below 1,000'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Special Announcements

Many riding areas on the Chugach National Forest have opened, but parking is a REAL CONCERN in Turnagain Pass! A Forest order submitted for a Turnagain motorized opening happened to coincide with an unexpected 4ft sleeper storm. The USFS and AK DOT are requesting both motorized and non-motorized users to avoid Turnagain Pass today due to very limited parking. Parking on the road and riding around the motorized lot will prevent plow operators from being able to clear these lots and delay the process through a busy holiday period. This is also serious safety issue. Should you go to Turnagian Pass today, consider heading South of Summit Lake where parking is not an issue.

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

STORM SLABS: A surprise storm that ended yesterday morning dumped 46” (~4’ and 3.4” SWE) of snow in Turnagain Pass to Johnson Pass. This storm arrived with moderate ridgetop winds and very low-density snow. Natural storm slabs and loose snow avalanches were seen across the forecast zone from Girdwood to Portage.  Triggering a storm slab in terrain 35 degrees and steeper today will be an unmanageable hazard. This new snow has been settling, but moderate ridgetop winds may make this new snow more cohesive and slab-like in the alpine. As always it’s important to give all big storms a few days of rest before easing into steeper terrain. In addition we may see 4-8 inches of additional snow today.

LOOSE SNOW AVALANCHES are a very real concern due 4′ of very low-density snow. Triggering a loose snow avalanche could be large and unmanageable. This kind of snow sould easily knock you off your feet and bury you. Avoid all steep terrain features (large and small) over 35 degrees and be extra aware of hidden depressions and creek drainages. Remember that the lower elevations prior to this storm didn’t have much snow coverage. 

Climbing over the second berm into the Center Ridge Parking area. Snow totals in the parking lot were measured at 47″ at 1:30pm yesterday. 

  

 

Climbing out of Center Ridge Parking lot yesterday. In the background is Seattle Ridge. Many of the gullys on Seattle were full of fresh debris and numerous storm slab crown could be seen with binos.   

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

We have been monitoring a facet/crust issue mid-pack between 2000-3000’. Prior to this storm this layer was not showing reactivity and so far there has been no known natural activity on this layer in Turnagain Pass. However, with the addition of 4′ of new snow, uncertainty remains. This is an additional reason to let the snowpack settle out and to give these layers more time to adjust. 

In the Summit Lake zone there is much more potential for triggering a slab on facets near the ground. Several natural avalanches were seen yesterday on the far Northern end of Summit Pass that may have stepped down into older layers of the snow pack. We have a lot of observations over the last week that demonstrate propagation potential within older weak layers (facet/crust mid-pack and near the ground.) New snow in Summit Lake will be adding stress to these layers and human triggered avalanches are a concern. Look out for whumpfing, cracking, and recent avalanches. Please keep in mind that there is no snowpack info from Johnson Pass and Lynx Creek, which often have a similar snowpack structure as Summit Lake.

SE aspect where storm slabs stepped down to an older deep layer in the zone between Summit and Johnson Pass near the Hope Wye. 

 

If you head to Lost Lake be aware of triggering a slab 2-3′ in an older weak layer of the snowpack. This picture was taken yesterday and the timing of this avalanche is unknown and may be a natural avalanche from a different day. Photo and observation courtesy of Iron (iii) Oxide.

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 we know about are on Sunburst’s SW face under the weather station, SW face of Tincan Proper, Gold Pan area (behind Cornbiscuit/Magnum) and Moose Mt in Summit Lake and crack that did release in the Johnson Pass area. These cracks can release at any moment. They are not associated with human triggers and the best way to manage the hazard is to avoid being on or beneath slopes with cracks. 

Mountain Weather

Yesterday: Skies were broken to overcast. Snow showers were intermitent with a few inches to trace of new snow across the region. Ridgetop winds were light from the East. Temperatures were in the single digits to teens near ridgetops and low 20F’s near sea level.

Today: Expect snow showers throughout the day with 4-8 inches of snow possible. Ridgetop winds are expected to be 10-20 mph with gusts in the mid 20’s mph. Temperatures will increase from the teens to mid 20F’s in the mid and upper elevations.  

Tomorrow: Temperatures may reach low 30F’s tomorrow morning near sea level. More winter weather is expected as another low tracks into our region. Snow showers will continue tomorrow and into Sunday.

*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′) 16    0.2 2   72  
Summit Lake (1400′) 8    0.1 1   18  
Alyeska Mid (1700′) 18   0.1   2   37  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11   NE   8   31  
Seattle Ridge (2400′) *NA   *NA     *NA     *NA