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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, March 18th, 2023 - 7:00AM
Expires
Sun, March 19th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche Warning
Issued: March 18, 2023 6:00 am
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
Avalanche risk The Bottom Line

The avalanche danger remains HIGH at all elevations. Two strong storms within 48 hours are rapidly loading the snowpack and creating very unstable avalanche conditions. Large to very large natural and human triggered avalanches 2-4’+ deep are almost certain. We recommend avoiding avalanche terrain today and being aware of any overhead slopes that could release naturally and run out down to lower elevations.

SUMMIT LAKE / LOST LAKE / SNUG HARBOR: Heavy snowfall across the Kenai Peninsula is creating dangerous avalanche conditions. Large natural avalanches are likely today and we recommend avoiding all avalanche terrain. In areas with a thin and weak snowpack, like Summit Lake, very large avalanches could release on buried weak layers and runout much further than normal.

* Avalanche Warning and NWS Winter Storm Warning in effect

Special Announcements

Turnagain Pass Avalanche Awareness Day – POSTPONED, new date TBD!
Road conditions, avalanche conditions, and parking lots are all too sketchy to safely host an avalanche awareness event today. We are working with our co-sponsors of the event to reschedule and will post on the forecast and social media once we line up a new date.

Sat, March 18th, 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
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 very limited yesterday, so the only avalanche observed was a small storm slab immediately adjacent to the Tincan skin track. This avalanche appears to have failed 8-12″ deep at the interface of the moist new storm snow that started Friday morning and the colder, drier new snow that was on the surface after Wednesday’s storm. We saw lots of shooting cracks and collapsing when we stepped of the skin track yesterday, which indicates that this type of avalanche activity was likely widespread.

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

Friday’s storm did not disappoint, dropping almost 2″ of water (17″ snow) in Girdwood and 1.5″ in Turnagain pass (13″ snow). The precipitation gauge in Portage is down, but it is safe to assume that they received well over 2″ of water in the last 24 hours. The new snow that fell during the day yesterday was dense and moist and was not bonding well with the existing snow surface. Storm slabs were very touchy on Tincan yesterday and the new snow has continued to fall since we checked on conditions in the afternoon. Today we are expecting another 1.5-2.0″ of water which equates to roughly 16-20″ of new snow. Warmer temperatures will come along with today’s storm and snow line is expected to rise to 700-1,000′. In addition to the rapid loading of new snow, the winds have been averaging almost 30 mph over the past 24 hours with gusts up to 60 mph. Today we are expecting the wind speeds to increase slightly to averages of 40-60 mph and gusts possible to 80+ mph.

We strongly recommend avoiding avalanche terrain today and being very aware of any slopes overhead that could release naturally and runout down to the valley bottom. The snowpack needs time to adjust to this very rapid loading and natural or human triggered avalanches 2-4’+ deep are almost certain today. Avalanches releasing in the new snow have the potential to step down to deeper weak layers and create a much larger avalanche that could entrain a lot of new snow in the track. On southern aspects, like repeat offender, there is an icy crust (Pi Day Crust) underneath the new snow from this week that could make avalanches runout farther than normal. 

Snowfall forecast courtesy of NWS Anchorage showing 12-18″ of new snow from Saturday morning to Sunday morning. Graphic 3.17.23

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • 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.

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

For the last two weeks we have listed lingering persistent weak layers as an additional concern in the forecast, but today we are upgrading them to problem 2 because the rapid loading we are experiencing means these layers could become active again and very large natural avalanches on buried weak layers 3-4’+ deep are possible. Areas with a thinner overall snowpack like near Johnson Pass or on the far southern end of Turnagain Pass are most likely to have avalanche activity on deeply buried weak layers. For today the message remains the same, avoid all avalanche terrain, but keep in mind that persistent weak layers take a longer time to stabilize after a loading event so areas with a thinner snowpack could remain unstable for longer once the storm ends.

Weather
Sat, March 18th, 2023

Yesterday: Very heavy snowfall and strong winds throughout the day. Over the past 24 hours 1.8″ of water fell in Girdwood with 17″ of snow and 1.4″ of water with 13″ of snow in Turnagain Pass. The precipitation gauge in Portage was not working this morning, but it is safe to assume that the area near Prince William Sound received upwards of 2″ of water with over 2′ of snow. Cloud cover was obscured throughout the day and temperatures reached 34 degrees at sea level with a wintery mix falling at the lowest elevations in the afternoon. The warmer temperatures caused the snowfall to be much more dense and moist than the prior storm on Wednesday.

Today: Over the next 24 hours we are expecting another 16-20″ of snowfall with 1.5-2.0″ of water. Strong winds will accompany the snowfall with averages reaching 40-60 mph in the afternoon and gusts of 80 mph + possible. Snow line is expected to rise to approximately 1000′ was the temperatures warm up with the onset of the heavy snowfall. Cloud cover is expected to remain obscured throughout the day.

Tomorrow: Lingering snow showers are expected on Sunday but with just a few inches of new snowfall. Winds will also back off and average 15-20 mph at upper elevations. Mostly cloudy conditions are expected to persist but there will be a chance of clouds breaking up and visibility improving.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 13 1.4 89
Summit Lake (1400′) 25 9 0.6 48
Alyeska Mid (1700′) 26 17 1.8 82
Bear Valley – Portage (132′) 32

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18 ENE 27 61
Seattle Ridge (2400′) 22 SE 13 38
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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.