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Issued
Sat, December 9th, 2023 - 7:00AM
Expires
Sun, December 10th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is expected to rise to CONSIDERABLE today in the upper elevations (above 2,500′) due to increasing winds and light snowfall. Naturally occurring wind slab avalanches will be possible on the steeper slopes above the trees where winds are the strongest. In the mid elevations the danger is MODERATE where winds are not as strong, yet may be enough to form shallow slabs that people could trigger. Watching, and avoiding, slopes with active wind loading is recommended.

Special Announcements

Headed to the Chugach State Park, Summit or Seward this weekend?
Be sure to check the Weekend Avalanche Outlook products, new this season, which we will be posting Friday evenings through the season. Located under the Forecast tab.

Hatcher Pass: Check out HPAC’s Saturday morning forecast HERE.

Sat, December 9th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Sun, December 10th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sun, December 10th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Recent Avalanches

We received a report of an avalanche on the north side of Tincan Ridge yesterday, running into Todd’s Bowl. It was reported to have been recent and triggered by the winds. We also received a photo of a large slab avalanche on the backside of Seattle Ridge (between Main and Zero Bowls). This slab could be older, but it’s a deeper slab that may have released on the Thanksgiving crust. More on that in our ‘Additional Concerns’ below.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

After a break in storms yesterday, another shot for a few inches of snow and wind is slated for today. Plus… an even larger storm is on the horizon for tomorrow into Monday. An active weather pattern is setting up for Southcentral over the next week and with that will come increasing avalanche danger. 

For today, ridgetop winds are on the rise from the east (10-20mph with gusts as high as 40mph) and light snowfall should add up to 2-6″ by tonight. With a surface that has already seen varying degrees of wind effect, another round of wind slabs will be adding to the equation. Winds today should be enough that in exposed areas above treeline, and especially along the higher ridgelines, natural wind slab avalanches may occur. These are likely to be in the foot deep category and if they are fresh, should be easy for us to trigger. With low visibility conditions expected, sheltered areas in the mid elevation trees will be the best bet for not only avoiding these avalanches but also finding the softest snow.

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

As mentioned above, there was a large avalanche seen on the backside of Seattle Ridge on Thursday, 12/7, (photo below) . The slab appears to be 2-3′ deep and has the character of an avalanche releasing on a buried weak layer, as opposed to a typical wind slab for example. This is the first avalanche we have seen that looks like it could have released on the Thanksgiving crust. We do not know when it occurred, but best guesses are sometime in the last week.

Whether this avalanche is a precursor of things to come, we don’t know yet. However, we are keeping a close eye on the buried Thanksgiving crust. We have seen in snow pits that facets are developing on top of the crust. As time goes on these facets can continue to grow and become a weak layer, making the ingredients for a scary avalanche setup: lots of snow on top (big slab), weak layer (facets), over a uniform crust (bed surface).

What we believe is a natural slab avalanche between Main and Zero Bowl on the backside of Seattle Ridge. Thank you to Travis Smith for the photo taken Thursday, 12.07.23.

Weather
Sat, December 9th, 2023

Yesterday:  Partly sunny skies were over the region yesterday after 4-8″ of light snow fell the evening before. Ridgetop winds were from the northwest in the 10-15mph range with gusts near 30mph. Temperatures were cold – single digits.

Today:  Clouds have built in overnight and today a weak weather system moves in. Ridgetop winds have turned easterly and should increase through the day to the 15-25mph range with gusts near 40mph in some areas. Between 2-6″ of snow is expected through today before tapering off tonight.

Tomorrow:  A more powerful storm is forecast for Sunday into Monday. Strong east winds with 12-18″ of snow could fall by Monday morning. The rain/snow line looks to climb to around 500′ with this event. A stormy weather pattern is setting up for the remainder of the work week.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 8 0 0 50
Summit Lake (1400′) 12 0 0 26
Alyeska Mid (1700′) 13 0 0 35
Bear Valley – Portage (132′) 15 0 0
Grouse Creek (700′) 9 0 0 26

 

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 4 W->E 12 35
Seattle Ridge (2400′) 8 SE 7 16
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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.