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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, March 16th, 2023 - 7:00AM
Expires
Fri, March 17th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. Turnagain Pass received close to a foot of new snow in the past 24 hours combined with strong winds at upper elevations. Wind slab avalanches 1-2′ deep are likely to be triggered by a person and possible to release naturally. In areas sheltered from the wind new snow avalanches are possible. Keep an eye out for shooting cracks and firmer hollow feeling snow on the surface to identify unstable areas.

Below 2500′ the avalanche danger is MODERATE. Wind slabs and avalanches within the recent storm snow are still possible. Areas that received lower snow totals, like near Girdwood, will have shallower wind slabs and avalanches in sheltered areas are unlikely.

Special Announcements

Transmission inspection using snowmachines:  Chugach Electric Association will be inspecting the transmission lines along the non-motorized side of the Turnagain Pass (between Tincan and Johnson Pass) using snowmachines on one day between Thursday 3/16 and Tuesday 3/21.

Turnagain Pass Avalanche Awareness Day – this Saturday!
On March 18th swing by the Turnagain Pass moto lot on your way to or from your backcountry ride or ski!! Test your beacon skills, chow down on hot dogs, and bring your questions for CNFAIC forecasters. The Alaska Avalanche School will be there along with a chance to demo snowmachines from Alaska Mining and Diving Supply and Anchorage Yamaha and Polaris. More details HERE

Thu, March 16th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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. 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 have been reported with the recent storm, but visibility was horrible yesterday.

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 ended up surprising us with how much snow fell in the Turnagain Pass area, with almost a foot of new snow and 0.8″ of water over the past 24 hours. Unfortunately, the storm left Girdwood pretty dry with only 0.15″ of water. From what we saw in the field yesterday the light and dry new snow was not bonding very well to the old snow surface, which means surface avalanches at the interface with the old snow are likely today. Triggering an avalanche will be most likely be in areas with active or recent wind loading, like along upper elevation ridgelines and cross loaded gullies. Winds have been averaging 20 mph and gusting up to 50 mph at upper elevations which could build wind slabs 1-2′ deep in Turnagain Pass.

With almost a foot of new snow it is also possible to trigger a storm snow avalanche in areas that are sheltered from the wind. Due to the low density of the new snow we didn’t see evidence that the new snow was acting like a slab yesterday afternoon, but an additional 3-5″ of snow fell since we left the field which could be enough to make the new snow more cohesive. In the below treeline elevation band there were some pockets of surface hoar that were buried by this recent storm which could make storm slabs more reactive in these isolated areas. In addition to these surface slab avalanches, dry loose avalanches (sluffs) are very likely on any steep slope and they could entrain a lot of new snow on larger slopes.

To evaluate the potential for surface avalanches it is important to get off the beaten track and use test slopes, hand pits, or other means to check how well the new snow is bonding to the old surface. Any shooting cracks or stiffer feeling new snow on the surface are an indication that the new snow is acting like a slab and the potential to trigger an avalanche is higher. On steeper southern aspects there was an icy surface crust buried by this new snow which could make wind slabs and storm slabs release more easily on this slippery bed surface. In areas with lingering persistent weak layers, like near Johnson Pass and Pete’s, the new snow could reactivate these buried weak layers and extra caution is recommended to evaluate the snowpack conditions before entering avalanche terrain (see additional concern).

Low visibility storm day on Tincan yesterday, with mostly light winds near treeline. Photo 3.15.23

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 a long dry spell we are starting to get into a stormy weather pattern and the addition of more weight on the snowpack could start activating older buried weak layers in areas with a thinner snowpack. The areas that have shown signs of harboring buried weak layers in the past few weeks are concentrated along the southern boundary of our forecast zone near Johnson Pass and Pete’s. These area tend to have a thinner snowpack, more similar to Summit Lake, therefore weak layers can persist longer and be easier to trigger with the weight of a skier or rider. We recommend extra caution in these areas, especially once the next round of snowfall and strong winds start on Friday.

Weather
Thu, March 16th, 2023

Yesterday: Moderate to heavy snowfall throughout the day, with the most precipitation falling in Turnagain Pass and Portage. Over the past 24 hours Turnagain Pass has received almost a foot of new snow with 0.8″ of water and Portage received closer to 6″ of new snow with 0.5″ of water. Girdwood looks like it was left out of this storm a bit with only 0.1″ of water reported at their mid elevation weather station. Winds also increased during the day yesterday with averages of 20 mph at upper elevations over the past 24 hours and gusts up to 50 mph. Temperatures remained below freezing and snow fell down to sea level.

Today: A few inches of additional snowfall are possible today with mostly cloudy skies. Temperatures will remain in the teens to mid 20s. Winds are expected to remain in the 10-20 mph range with gusts possible up to 30+ mph. Thursday is kind of a lull between the pulse of precipitation on Tuesday night into Wednesday and another one that is expected to impact the forecast area Friday through Saturday.

Tomorrow: Heavy snowfall should start again during the day on Friday. Current estimates are for 12-18″ of new snow in Turnagain Pass and 18-24″ in Girdwood from Friday morning to Saturday morning. Snowfall will be heavier in areas closer to Prince William Sound like Portage and Placer. Winds are expected to increase to 40-60 mph with stronger gusts starting on Friday afternoon when the snowfall intensity is ramping up. Snow line might creep up to about 700′ on Friday evening as the temperatures increase slightly with the next low pressure system.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 24 11 0.8 78
Summit Lake (1400′) 22 3 0.2 40
Alyeska Mid (1700′) 24 2 0.1 64
Bear Valley – Portage (132′) 27 6 0.5 12

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 NE 20 47
Seattle Ridge (2400′) 18 E 9 18
Observations
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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.