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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, March 23rd, 2023 - 7:00AM
Expires
Fri, March 24th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger remains HIGH above 1000′. Heavy snowfall and strong winds until yesterday evening will make human triggered avalanches very likely at upper elevations. The high level of uncertainty about the current snowpack conditions and the potential for large to very large human triggered avalanches are the primary reason for keeping the danger elevated. Very large human triggered avalanches releasing underneath all the storm snow from this week 3-6′ deep are possible. We recommend avoiding avalanche terrain and giving the snowpack another day to adjust to all the new snowfall.

Below 1000′ the avalanche danger is CONSIDERABLE. A conservative mindset and careful evaluation of snowpack conditions is recommended before entering avalanche terrain. Avalanches releasing at upper elevations could run down into lower elevations so it is important to be aware of overhead avalanche paths.

Special Announcements

AK DOT&PF:  There will be intermittent traffic delays today, March 23, on the Seward Highway for Avalanche Hazard Reduction Work from mileposts 88 to 85 on the Seward Highway, South of Girdwood and near mile Post 5 and Bear Valley on the Portage Glacier Highway.  Motorists should expect delays of 45 minutes or longer between 9:00 am and 2:00 pm. More info on road closures at 511.alaska.gov.

Turnagain Pass Avalanche Awareness Day – Rescheduled for this Saturday.
Cross your fingers the weather and parking lot conditions allow for us to hold the 2023 Turnagain Pass Avalanche Awareness Day on Saturday March 25th! Check back Friday (Mar 24) after 5pm in case we need to cancel due to unsafe conditions and/or lack of parking.

* Parking: As of about noon yesterday there was limited plowing in the Turnagain Pass lots, with a small amount of space available at Tincan, Eddies, Cornbiscuit, and Bertha Creek. The motorized lot had some trucks getting in but the snow was fairly deep. Center Ridge, Sunburst, and Johnson Pass all looked like they were impassable with a normal car/truck. Plows were very active yesterday so this may have improved since we last laid eyes on it.

Thu, March 23rd, 2023
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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. Dangerous avalanche conditions. 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

No recent avalanches were reported yesterday, but the visibility made it impossible to see and the parking situation made access an issue.

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

Yesterday’s storm brought 12-20″+ of snowfall to the forecast area with Portage and areas near Prince William Sound likely receiving the most snow, Turnagain Pass in second, and Girdwood trailing behind a bit. Strong winds accompanied the snowfall with averages of 20-40 mph during the day yesterday and gusts up to 70 mph. All this new snow and wind created a fresh round of surface snow instabilities that are very likely for human triggering today. We recommend avoiding avalanche terrain and giving the snowpack time to adjust to the new load.

The most likely avalanche problems to encounter today are lingering wind and storm slabs which could be 2-4′ deep. Wind slabs will be most likely in open areas at and above treeline, especially along upper elevation ridgelines and cross loaded gullies. To identify areas with wind slabs look for firmer or hollow feeling snow on the surface that is producing shooting cracks or collapsing on small test features. Storm snow avalanches in sheltered areas are still possible today, however the likelihood will decrease throughout the day as the new snow has some time to settle and the interface with the old snow surface can bond better. If the weather forecast is wrong and the sun comes out today natural avalanches releasing at the interface of the old snow surface about 2′ deep are possible on sunny slopes where the new snow will quickly warm up and gain enough strength to act like a slab.

In the treeline and alpine elevation bands dry loose avalanches (sluffs) are very likely which could be enough to push you into an area you don’t want to be or cause you to fall into a tree well. All the new snow and wind will have added significant mass to cornices which might be more likely to fail under the weight of a person or if they are receiving direct sunlight.

Unstable result in an extended column test with the new snow falling so quickly that it doesn’t have time to bond with the old snow surface (about 2′ deep in this location). Photo 3.22.23

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

Over the past week Turnagain Pass has received 7″ of water which is roughly equivalent to 7′ of new snowfall, with Girdwood getting slightly less and areas near Portage and Placer likely getting significantly more. All that snow fell on top of a weak surface of facets on shaded northern and lower angle slopes or on a sun crust on steeper southern exposures. During breaks in the weather over the past week we have observed several very large natural avalanches that appear to have released back on that old weak surface and were able to propagate widely across terrain features (see obs from Lynx, Magnum and Summit Lake). The latest installment of our storm series brought roughly 2.5″ of water or 2-3′ of snowfall over the past 48 hours, which is a lot of weight to add to the snowpack over a short period. Due to the lack of visibility yesterday we have very little information about how the snowpack responded to the latest load so there is a lot of uncertainty in the forecast today.

The worst case scenario for most of the forecast region is that the new snow load stressed the snowpack to the point where human triggered avalanches are likely to release all the way back to the facets or sun crust that were buried last Wednesday. These avalanches would likely be 3-6’+ deep and could propagate widely to create very large avalanches. Based on our uncertainty and the consequences of that worst case scenario we are recommending that people continue to avoid avalanche terrain today and give the snowpack some time to adjust to all the recent loading.

Weather
Thu, March 23rd, 2023

Yesterday: Heavy snowfall until about 9 pm with 16-20″ falling in Turnagain Pass, 12-16″ in Girdwood, and likely closer to 24″+ in Portage and Placer but our normal precipitation sensor is down so we have limited information for that area. Snowline remained at sea level but the new snow was moist to wet below roughly 1000′. Winds were averaging 20-40 mph during the day with gusts close to 70 mph, but then quickly backed off starting around 6pm Wednesday night and have been averaging 5-10 mph with gusts to 15 mph since then.

Today: Some lingering snow showers are possible today but no significant snowfall is expected. Winds will remain light in the 5-10 mph range. Mostly cloudy sky cover to start the day with a trend toward broken skies in afternoon and evening. Temperatures will be in the low to mid 20s during the day and then drop into the teens as the skies start to clear in the evening.

Tomorrow: Continued snow showers are expected Friday with potential for 1-3″ of new snow. Winds will increase slightly to the 10-20 mph range and are expected to increase further late Friday evening. Mostly cloudy skies are expected again with the potential for clearing in the evening. Temperatures will decrease slightly ranging from the the teens to low 20s at upper elevations.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 18 1.6 116
Summit Lake (1400′) 24 3 0.2 49
Alyeska Mid (1700′) 28 13 1.4 102
Bear Valley – Portage (132′) 33 1.1

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 ENE 18 67
Seattle Ridge (2400′) 24 SE 11 31
Observations
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
11/13/23 Turnagain Avalanche: Tincan Trees
11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
11/11/23 Turnagain Observation: Taylor Pass – Sunburst
11/10/23 Turnagain Observation: Tincan
11/10/23 Turnagain Observation: Tincan Trees
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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.