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

The avalanche danger is MODERATE above 1000′ today. Human triggered avalanches 1-2′ deep are possible in areas with lingering wind slabs from the Valentine’s Day storm. In areas closer to Prince William Sound that saw much heavier precipitation on Tuesday lingering storm snow avalanches up to 2′ deep are possible, even in areas sheltered from the wind. Larger avalanches releasing on buried weak layers 2-4′ deep are possible in isolated areas where these weak layers are well preserved. We recommend careful evaluation of the snowpack before entering avalanche terrain. Below 1000′ the avalanche danger is LOW. 

WINTER WEATHER ADVISORY: The NWS in Anchorage has issued a winter weather advisory for Western Prince William Sound from 10 pm on Thursday to 9 am Friday. Strong winds and heavy snowfall are expected overnight with 4-6″ of snowfall in the core forecast area and 8-12″ in areas closer to Prince William Sound. Avalanche danger will increase rapidly with the onset of this storm.

Thu, February 16th, 2023
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

No new avalanches were reported yesterday. We saw one area in PMS bowl on Magnum where a chunk of cornice fell off and entrained loose snow below. Otherwise we did not see any recent avalanche activity during the period of good visibility yesterday morning.

Cornices have grown quite large along Magnum ridge and a sizable chunk fell off during the last wind storm and entrained some loose snow below. Photo 2.15.23

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

The Valentine’s Day storm strongly favored areas closer to Prince William Sound, with reports of up to 2′ of new snow in more coastal areas while Turnagain Pass only received 4-6″. Moderate easterly winds during the storm formed wind slabs 1-2’+ deep at upper elevations on Tuesday. Yesterday we saw calm winds and cold temperatures which should give these lingering wind slabs some time to become less sensitive to human triggers. It is still possible for human triggered wind slabs on specific terrain features today, including upper elevation ridgelines or cross loaded gully features. To identify wind slabs keep an eye out for stiff or hollow feeling snow and look for shooting cracks in wind loaded areas to get a sense of how reactive this avalanche problem is today.

The cold temperatures during the last storm have left a fair bit of low density snow on the surface which will make dry loose avalanches (aka sluffs) likely today on steeper terrain features. Be aware of the terrain below you and have a plan to manage your sluff if you are travelling in steeper terrain today. Cornices have also grown quite large and it is possible to knock off a big chunk if you are travelling along a corniced ridgeline today.

Calm weather yesterday gave the snowpack some time to adjust to the new snowfall from the Valentine’s Day storm. Photo 2.15.23

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

There are a few different persistent weak layers in the snowpack that we are continuing to monitor, but we have not seen any avalanche activity on these layers this week. The most recent avalanche observation was from Eddies on Saturday which likely released on facets above the 1/25 melt freeze crust. This weak layer extends up to roughly 2000′ and has shown signs of propagation in stability tests. In areas that have not seen much snowfall in the past few weeks the layer is still near the surface and could become more reactive with new snowfall. In areas near Prince William Sound this layer is up to 4′ deep with a strong snowpack on top.

At higher elevations the crust does not exist but there are a few layers of surface hoar and facets in the upper snowpack that could cause wind slabs or storm snow avalanches to linger longer than normal. On Magnum yesterday we found a layer of facets about 16″ deep that was somewhat reactive in our snowpit tests and is worth keeping track of as a potential future weak layer. In addition to these surface weak layers we are still tracking the 1/10 buried surface hoar that is widespread across the forecast area. In most locations this layer has likely already collapsed or bonded to the layers above and below and is not showing signs of instability in snowpit tests. However, in a few isolated locations that are protected by forest and off the beaten path we have found propagation on this layer of buried surface hoar. We were surprised to get propagation in two extended column tests on this layer in the treeline elevation band on Cornbiscuit yesterday. The buried surface hoar crystals were very large and well preserved in this area but it is a reminder that there is always some uncertainty with a buried weak layer like this.

Layer of facets 16″ down that is worth keeping track of for future storms. Photo 2.15.23

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

For areas with a thinner snowpack (Silvertip, Summit Lake), the weak layers near the bottom of the snowpack are still a factor in our terrain choices. This includes the weak snow around the Thanksgiving crust, as well as faceted snow at the bottom of the snowpack. Luckily these layers do not seem to be a factor in our core advisory area, but they are still a concern around the periphery.

Weather
Thu, February 16th, 2023

Yesterday: Mostly sunny in the morning but a wall of low level clouds moved into Turnagain Pass around noon leading to overcast cloud cover. Temperatures were in the teens at upper elevations and low 20s F at road level. Winds were calm at all elevations and no new precipitation fell.

Today: Another calm day before a storm enters the forecast area later this evening. During the day expect calm to light winds averaging 0-10 mph and mostly cloudy skies. Snowfall and stronger winds are expected to start around 10 pm this evening. Wind speeds should reach 20-40 mph overnight with gusts of 60+ mph. By Friday morning snowfall totals could add up to 4-6″ for Turnagain Pass, Girdwood, and Summit Lake. Areas closet to Prince William Sounds like Portage and Whittier could see 8-12″ of more of new snow. Snowline is expected to remain at sea level throughout the storm.

Tomorrow: Snowfall is expected to end rapidly during the day on Friday and give way to clearing skies. Wind speeds will remain high Friday morning with averages of 20-40 mph out of the east. Around midday winds will switch to the west and decrease to 15-30 mph. Friday afternoon and evening should a brief period of clearing skies before another pulse of snowfall arrives overnight Friday or Saturday morning.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22 1 0.1 71
Summit Lake (1400′) 19 0 0 37
Alyeska Mid (1700′) 21 0 0 73
Bear Valley – Portage (132′) 23 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14 E 3 12
Seattle Ridge (2400′) 17 SE 0* 4*

* The wind sensor at Seattle Ridge weather station looks like it has been rimed up for much of the past 24 hours.

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.