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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, December 27th, 2021 - 7:00AM
Expires
Tue, December 28th, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Today’s avalanche danger is MODERATE. Westerly winds picked up yesterday evening and are expected to continue to create a fresh round of wind slabs through the day. Combined with the above-freezing temperatures since yesterday morning, it is possible a person could trigger an avalanche near the surface up to a foot deep. It is also possible to trigger a deeper avalanche on weak snow buried 2-5′ deep that has been giving us trouble for over a month now.

Roof avalanches:  Continued above freezing temperatures may cause roofs to shed their snow. Keep an eye on children and pets, and be mindful of where you park your car.

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Mon, December 27th, 2021
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'
Avalanche risk
Moderate (2)
Below Treeline
Below 1,000'
Avalanche risk
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

Seward Highway: Multiple small natural avalanches released during yesterday’s warm up. They ran about halfway down the paths, stopping well above the highway.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.

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.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Winds have returned to the area, and will once again be at work making a fresh round of wind slabs to look out for today. Weather stations are showing sustained westerly winds of 15-25 mph since yesterday evening, with gusts at 30-50 mph near Girdwood and Turnagain, and as high as 72 mph closer to Summit Lake. We are expecting to see similar speeds through the day today. There is limited snow for transport after the strong winds from last week, which will limit the size of new wind slabs forming today. Expect to find these in the typical locations- below ridgelines, on the downhill side of convexities, and in gullies. Clear warning signs like shooting cracks or collapsing are all the info you need to avoid steep terrain. Keep in mind that a relatively small wind slab triggered near the surface has the potential to step down to a deeper weak layer, creating a very large avalanche. More on this in problem 2.

The continued warmup is the wildcard today, and is likely increasing the chances of triggering an avalanche. There were several small natural avalanches yesterday along the Seward Highway near Girdwood, which were most likely a result of continued above-freezing temperatures. With similar warm weather today, we may see some similar activity. There is some uncertainty as to just how tender these surface problems will be today. Whenever we are dealing with a higher level of uncertainty, the safest bet is to increase your safety margins by sticking to lower angle terrain, traveling one at a time, and watching your partners from safe spots out of the way of avalanche paths.

Cornice Fall: Strong winds and above-freezing temperatures will combine to push cornices towards their breaking point today. Be sure to give them plenty of space while traveling at ridgetops, and limit time spent traveling below them. A chunk of cornice breaking off is a big load to drop on the snowpack, and would likely trigger an avalanche near the surface, or possibly deeper.

Loose Wet Avalanches:  Continued warm temperatures may increase the likelihood of loose wet avalanches. While it is unlikely these will bury a person, they can be dangerous if they drag you into terrain traps like trees, alders, rocks, ravines, etc. They also have the potential to trigger a larger avalanche on deeply buried weak layers.

Charts from the Sunburst weather station (3800′ el.). Over 24 hours of above freezing temperatures and continued strong westerly winds may cause issues today. 12.27.2021

 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

The deep persistent weak layer of facets from early November is still giving us cause for concern today. In most places, this layer is buried deep enough that the current warm blast is not having a big effect. This may not necessarily be the case towards the southern end of our forecast area and the Summit Lake zone, where the weak layer is closer to the surface. None of this really changes the way we have been approaching this layer. The potential for a large and unpredictable avalanche requires dialing terrain choices back and avoiding big steep slopes. Keep in mind these avalanches can be triggered remotely from above, below, or adjacent to a steep slope.

In the long run, the current warm spell will actually do a lot of good in helping us put this layer to rest. We are anticipating this layer is going to become really difficult to trigger once temperatures drop again and the snowpack locks up. For now, it is still a concern. Stay tuned in the coming days for more on this.

Weather
Mon, December 27th, 2021

Yesterday: Temperatures started out at 20-30 F, warming to the mid 30’s to 40F overnight. Winds were blowing 5-10 mph out of the west during the day, picking up to 15-20 mph overnight with gusts of 30-50 mph. We dodged the rain bullet in our advisory area, with brief periods of light drizzle that did not amount to any measurable precipitation. Skies were mostly cloudy during the day, clearing overnight.

Today: Continued warm temperatures are expected today, with daytime temperatures hanging in the upper 30’s F under partly cloudy skies. Westerly winds are expected to continue at 10-20 mph with gusts of 30-40 mph. No precipitation is expected today. Overnight lows should drop below freezing to the mid 20’s to 30 F.

Tomorrow: Temperatures will creep back up into the mid 30’s with mostly clear skies until early afternoon when clouds start moving in. Chances of snow increase in the evening, with 1-4″ possible tomorrow night. Rain level should drop back down to 100-200′ before it starts snowing. Winds are expected to settle down mid morning, dropping to 5-10 mph out of the west.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34 0 0 67
Summit Lake (1400′) 30 0 0 21*
Alyeska Mid (1700′) 31 0 0 40

*Estimate. Snow depth sensor currently not reporting.

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 36 W 12 47
Seattle Ridge (2400′) 31 E 7 21
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Riding Areas
Updated Wed, December 01st, 2021

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

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Open
Open as of Dec 1st.
Placer River
Open
Open as of Dec 1st. Limited parking due to Portage curve construction.
Skookum Drainage
Open
Open as of Dec 1st. Limited parking due to Portage curve construction.
Turnagain Pass
Open
Open as of Saturday, Nov 27. Be aware of early season hazards (alders/creeks) and open water.
Twentymile
Open
Open as of Dec 1st.
Seward District
Carter Lake
Open
Open as of Dec 1st.
Lost Lake Trail
Open
Open as of Dec 1st.
Primrose Trail
Open
Open as of Dec 1st.
Resurrection Pass Trail
Open
Open as of Dec 1st.
Snug Harbor
Open
Open as of Dec 1st.
South Fork Snow River Corridor
Open
Open as of Dec 1st.
Summit Lake
Open
Open as of Dec 1st.

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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.