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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, December 24th, 2023 - 7:00AM
Expires
Mon, December 25th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche Warning
Issued: December 24, 2023 6:00 am
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
Avalanche risk The Bottom Line

Avalanche danger remains HIGH today following another intense round of precipitation last night. The storm is passing this morning, but very dangerous avalanche conditions exist through today. While natural avalanches will become less likely as the weather calms down, conditions will be primed for people to trigger very large avalanches within the 1-3 feet of new snow from the past two days, or possibly failing on deeper weak layers now 5-6′ deep. Travel in or below avalanche terrain is not recommended.

Roof Avalanches: With rain falling at lower elevations overnight, roofs have been shedding large amounts of snow. Keep an eye on children and pets playing outside, and be careful where you park your cars and how you enter and exit buildings.

We have extended our Avalanche Warning through the National Weather Service. With all of this new snow, very dangerous avalanche conditions will continue through the day despite relatively quiet weather.

Special Announcements

State of Alaska DOT&PF Avalanche Closure Notification: There will be intermittent traffic delays Sunday December 24, 2023 (today) on the Seward Highway and Portage Glacier Road for Avalanche Hazard Reduction work. From mileposts 83 to 89 on the Seward Highway, between Portage and Girdwood. From mileposts 3 to 5 on the Portage Glacier Road, between Portage Corner and Whittier Tunnel. Motorists should expect delays of up to 45 minutes between 9:00 am and 2:00pm. Updates will be posted on the 511 system (511.alaska.gov).

Hatcher Pass Avalanche Center has issued a Sunday morning forecast, strong winds have been impacting that region. See it at hpavalanche.org!

Sun, December 24th, 2023
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
Avalanche risk
Mon, December 25th, 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
Mon, December 25th, 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

Visibility was poor and backcountry traffic was limited yesterday, so we do not have a clear picture of the extent of avalanche activity since the storm picked up Friday night. We did receive observations of fresh glide activity in the Summit and Girdwood areas yesterday.

Fresh glide avalanche on Gilpatricks South, near the Devil’s Pass trailhead. Photo: Trevor Clayton, 12.23. 2023

Fresh glide avalanche on the east side of Raggedtop in Girdwood. 12.23.2023

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

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

Two consecutive days of stormy weather have brought another round of impressive snow totals to the area. Last night saw rain levels creep up to around 1000-1500 feet, but upper elevations have picked up 2-3 feet of snow over the past 48 hours, including 12-20″ in the past 12 hours. Here are some storm totals for the area:

  • Girdwood: 15-24″ snow/2.6″ Snow water equivalent (SWE)
  • Turnagain Pass: 20-30″ snow/3.2″ SWE
  • Summit: 4-9″ snow/0.9″ SWE
  • Seward: 13-15″ snow/1.5″ SWE

The very beginning of this storm saw the coldest temperatures, which has lead to an upside-down setup of dense snow sitting on very light snow. To make things just a little more interesting, the 2-3′ of snow that has fallen over the weekend has buried a widespread layer of surface hoar that was seen in the Turnagain Pass, Summit, and Seward zones immediately prior to the storm. This storm event is also adding a lot of stress to the questionable weak layers associated with the Thanksgiving crust (more on this in Problem 2 below).

All of this makes for a really dangerous setup. While Girdwood and Seward may see another 3-6″ snow today, things are quieting down for the most part weather-wise today. However, avalanche conditions will remain very dangerous through today while the snowpack adjusts to this intense loading event. Through today it will be very likely that a person can trigger a very big avalanche. Avoid traveling on or below steep slopes, and keep in mind that with these impressive storm totals that a large avalanche failing at upper elevations has the potential to run far into low-angle runout zones at lower elevations. It sounds like parking at the pass is a nightmare right now anyways.

The second round of intense snowfall arriving in Girdwood yesterday afternoon. 12.23.2023

Storm totals for the Snotel site on Center Ridge at Turnagain Pass. Snow totals at higher elevations are closer to 30″ or more. 12.24.2023

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

We’ve been paying close attention to weak snow surrounding the Thanksgiving crust layer, which is now buried around 5-6 feet deep. The 2-3″ SWE that we have received over the past two days is adding a lot of stress to this layer, and if the layer can still produce big avalanches, this is the time we will see them happen. There is a lot of uncertainty with this layer, and this will be a good test. Since we are already concerned with the much more likely avalanches failing within the 1-3 feet of new snow on the ground, for today, the Thanksgiving crust is more of an afterthought. But it is one more reason to give steep terrain a very wide berth today.

Additional Concern
  • Glide Avalanches
    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.
More info at Avalanche.org

Yesterday we saw continued glide avalanche activity for a cycle that has lasted for about a week now. These avalanches are very large and destructive, and although it’s impossible to predict when a single slope might fail, all of this recent activity suggests that glide avalanches will be likely today. One more big and exciting avalanche problem to avoid today.

Weather
Sun, December 24th, 2023

Yesterday: There was a brief lull in the storm for a few hours during the day yesterday but a second intense pulse of precipitation arrived right around sunset, bringing 1.5-2” SWE overnight. The rain line crept up to between 1000 and 1500 feet at times during the storm, and weather stations (all of which are in mid to low elevations) recorded around 8-12” snow. We likely received around 15-20” snow above 2000 feet over the past 24 hours. Ridgetop winds were strong out of the east at 20-50 mph with gusts of 60-80 mph. High temperatures were in the upper 20’s to low 30’s F, with lows in the high teens to low 30’s F.

Today: The most intense part of the storm has passed, and we should see fairly quiet weather today. Lingering snow showers could bring another 3-5” to Girdwood, with slightly less in Turnagain Pass and a little bit more in Seward. The rain/snow line is on its way back to sea level as the snow trickles in today. Winds will be strongest this morning, dropping slightly through the day with sustained speeds of 10-20 mph and gusts around 20-30 mph out of the east. Temperatures will be dropping starting this morning, with highs in the low to mid 20’s F and lows in the single digits to mid teens F.

Tomorrow: Temperatures are going to continue dropping into tomorrow with clearing skies as the low pressure system that has been bringing all of the stormy weather lately passes to the north. We should see highs in the single digits tomorrow with partly to mostly sunny skies and lingering valley clouds possible. Easterly winds will bump up to 15-25 mph tonight before turning westerly and calming tomorrow to 5-10 mph. No precipitation is expected.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 11 2.1 90
Summit Lake (1400′) 29 6 0.8 n/a
Alyeska Mid (1700′) 30 10 1.8 82
Bear Valley – Portage (132′) 36 n/a n/a
Grouse Ck – Seward (700′) 34 6 1.1 56

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 NE 28 75
Seattle Ridge (2400′) 26 n/a n/a n/a
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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.