ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Fri, February 12th, 2021 - 7:00AM
Expires
Sat, February 13th, 2021 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line
A CONSIDERABLE avalanche danger remains on wind loaded slopes above 1,000′. An easterly wind event pummeled the mountains yesterday and touchy wind slabs are expected to be found today. Fresh slabs 1-2′ thick will be likely for a person to trigger. These may be stiff enough to allow a person onto them before releasing and they may also be triggered remotely (from the side or the top). Watch for visible signs of wind loading and cracks that shoot out from you. Cautious route-finding and conservative decision-making are essential.
The avalanche danger is MODERATE below 1,000′. Wind slabs may have formed on low elevation steep slopes and if so, they will be possible to trigger. For example, in the Placer Valley.
SUMMIT LAKE/SNUG/LOST LAKE: Strong winds impacted these zones as well. Human triggered wind slabs are likely and extra caution is recommended.
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.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing
avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and
natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question.
Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to
hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are
relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure,
or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.
The easterly wind event over the region yesterday has just begun to quiet down early this morning. Snowfall amounts with the winds were meager, only a trace to a few inches around the region. For the past 24-hours the mountains around Girdwood, Portage Valley, south through Turnagain Pass and Summit Lake to Seward all saw strong wind with significant snow transport. The east winds are slated to remain in the moderate range today (15-25mph), which may be enough to still find remaining snow to move around. All that said, wind slabs are likely to be found in many exposed places.
If you are headed out in hopes the skies clear and some soft snow still remains, be on guard for very touchy wind slabs. These are likely to be found not only in the usual spots along the lee side of ridgelines, but cross-loaded in gullies and even lower on the slopes and on rollovers. Slabs should be in the 1-2′ range in the higher elevations while they could be quite shallow in the mid and lower elevations. Watching for the typical wind slab signs will be key:
Cracks in the snow that shoot out from you (as in the photo below), check.
Stiff and hollow feeling snow (hard snow over soft snow), check
Collapsing or whumpfing in the snowpack under you, check.
Noticeable wind pillows and drifts, smooth rounded surfaces, check.
Along with these clues, be aware slabs could be quite hard and allow a person onto them before releasing. This can make escaping off the side much more difficult. They also could be triggered remotely, from the top/side or below. Most new slabs are likely sitting on weak faceted snow and/or buried surface hoar. This is what is making them not only more touchy and easy to trigger, but can keep them from stabilizing quickly. We have to go into today treating these like they are prime to release. Sticking to slopes 30 degrees or less, with nothing steeper above us, is great way to have fun without worrying about triggering an avalanche.
Additionally, a wind slab could step down to an older layer of buried surface hoar and/or facets, creating a larger avalanche. This older persistent weak layer formed in late January is still on our radar. Something to consider especially in larger terrain.
Cornices: Cornices are getting bigger with each wind event and may be easier to break off today with the added stress. Remember that a cornice fall may trigger a wind slab on the slope below. Give them extra space as they can break farther back than expected and limit exposure under them.
A crack that shot out from Andrew Schauer’s ski yesterday on a quick tour up in the Girdwood Valley. 2.11.21.
Video above is thanks to Graham Predeger who was in Portage Valley yesterday (link). This is a notorious gap wind location near Portage Lake (Portage Pass). Winds are funneled through the tight gap as they make their way between the Gulf and the mainland. 2.11.21.
Additional Concern
Glide Avalanches
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. They 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.
Thanks to all the folks keeping us abreast of the new glide cracks. They are starting to open in more and more places in Girdwood Valley, Summit Lake and Turnagain (Tincan, Eddies, Seattle Ridge). The last known release was Tuesday on Penguin Ridge. Avoid time under glide cracks as they can avalanche at anytime and are completely unpredictable.
Weather
Fri, February 12th, 2021
Yesterday: Cloudy skies, light snowfall and strong winds were over the region yesterday. Only a trace to a few inches of snow fell with Turnagain Pass seeing up to 3 inches. Ridgetop east winds averaged 40-50mph with gusts in the 80’s for the whole of yesterday and just early this morning have quieted to half that. Temperatures are on the warm side with sea level in the mid 30’sF, mid elevations in the 20’sF and ridgetops in the teens.
Today: Skies are forecast to slowly clear out through the day, yet the easterly winds remain to some degree. These are expected to average 15-25mph along ridgelines at times before finally abating tonight. No precipitation is expected. Temperatures should remain mild, 20’s at most mid elevations and the teens along the ridgetops.
Tomorrow: Mostly clear skies and cooler temperatures are on tap for tomorrow. Ridgetop winds look to be in the 10-15mph range from the east. Temperatures should drop to the single digits in valley bottoms with an inversion in place while upper elevations remain in the teens. There looks to be slight chance for snow in the models for early next week.
Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff.
Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations.
Riding area questions contact: mailroom_r10_chugach@fs.fed.us
This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area.
This advisory does not apply to highways, railroads or operating ski areas.
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A crack that shot out from Andrew Schauer’s ski yesterday on a quick tour up in the Girdwood Valley. 2.11.21.