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Issued
Fri, December 16th, 2022 - 7:00AM
Expires
Sat, December 17th, 2022 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE at elevations above 1,000′. The snowfall and winds have moved out of the Turnagain forecast zone and our main avalanche issue will be triggering a lingering wind slab avalanche. Wind slabs could be up to 2′ deep and will be found on windloaded slopes and in cross-loaded gullies. Additionally, sluffs on steep slopes will be possible in the soft snow. The danger is LOW below 1,000′.

GIRDWOOD VALLEY:  Triggering a slab avalanche breaking under the storm snow may be possible due to an unusual avalanche seen yesterday. Until we know more, watch for unstable snow in sheltered zones as well a wind slab that could release larger than anticipated.

SUMMIT LAKE:  A much shallower snowpack exists in the Summit Lake area. Weak snow has been found under the recent storm snow and triggering a slab avalanche breaking in a buried weak layer is a concern. The danger here could be more on the CONSIDERABLE side; careful route-finding and a conservative mindset are key.

Special Announcements

Chugach State Park:  Several observations have come in describing large wind slabs releasing yesterday. Winds may persist in the Front Range today keeping avalanche conditions dangerous.

Fri, December 16th, 2022
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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

A large storm slab avalanche was seen in Girdwood yesterday morning, on the lower east face of Raggedtop. There was also a slide reported on the Goat Mtn shoulder (southerly facing) that was a deeper slab and may have broke in a weak layer under the storm snow (this is the unusual slab seen in the Girdwood Valley). Otherwise, several shallow slab avalanches and sluffs were seen yesterday morning composed of the 6-8″ of new snow from Wednesday night.

 

Natural slab avalanche that occurred sometime overnight on Dec 14th (Wednesday night) and seen by several folks. This photo was from H Johnston, taken from the chairlift at Alyeska. 12.15.22.

 

Avalanches up Crow Creek drainage in Girdwood on the southerly face of Goat Mtn. The slab on the left appears deeper, which is suspect for a breaking in a buried weak layer, as opposed to just the storm snow avalanche type seen on the right. Thanks to George Creighton for the photo. 12.15.22.

 

Very shallow, but wide, avalanche in Main Bowl on the backside of Seattle Ridge. Likely released also overnight on Wednesday with 6-8″ of new snow and some wind. 12.15.22.

 

Evidence of 2 older wind slab avalanches on the SW face of Sunburst. These are thought to have occurred several days ago during the storm on Sunday night (12/11). Photo by Sarah Heck on 12.15.22.

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

Around midnight last night the NW outflow winds finally slowed down. For today and into the weekend, it looks as though clear and cold conditions will prevail. The winds should also stay relatively light and variable. That’s good news for the snowpack to start stabilizing.

For today, wind slabs should be the most likely kind of avalanche to trigger. From our field day on Seattle Ridge yesterday, the wind slabs already seemed to be bonding, but this may not be the case for all wind slabs. Slabs could be soft or quite stiff depending on how much wind the area saw. They could also allow a person out onto them before releasing. Watching for wind loaded slopes and any signs of cracking around us will be key. Additionally, having an escape plan in case a wind slab releases and making sure our partners are spotting us will good ways to stack the odds in our favor.

Loose Snow Avalanches:  Sluffs on steeper slopes should be expected within the loose soft surface snow.

 

A look at Seattle Ridge yesterday with winds transporting snow in the upper Girdwood area in the background. 12.15.22.

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

After seeing the photo on the Goat Shoulder (pictured above) and hearing reports of some weakness near/above the Thanksgiving crust in the Girdwood Valley, there could be some funny business below all the storm snow from this month. From our data points so far, it appears this issue could be just in the Girdwood Valley.

On another note, outside our forecast zone to the south in Summit Lake, we know weak faceted snow exists under the storm snow. Folks traveling in the Summit Lake zone or places the snowpack is much thinner than Turnagain should be on guard for the potential for a slab breaking deeper in the pack. An extra cautious mindset is required for these shallow areas.

Weather
Fri, December 16th, 2022

Yesterday:  High clouds were over the region with sunshine poking through. Ridgetop winds were NW to west averaging ~10-15mph with gusts in the 40’s+ in favored areas. Temperatures were in 20’s to teens in most locations.

Today:  A mostly clear sky day is on tap with lighter winds. Ridgetops should see averages ~5mph with some gusts in the 20’s in favored locations. Temperatures are cooling off and should be in the teens most areas to even the single digits in some valley bottoms.

Tomorrow:  Mostly sunny skies are expected for the weekend. Ridgetop winds look to be light and variable with temperatures cooling to the single digits at all elevations.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 0 0 42
Summit Lake (1400′) 15 0 0 33
Alyeska Mid (1700′) 20 0 0 48
Bear Valley (132′) 20 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 W 10 30
Seattle Ridge (2400′) 19 W 6 17
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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.