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Issued
Fri, February 17th, 2023 - 7:00AM
Expires
Sat, February 18th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger has increased to CONSIDERABLE above 2,500′ due to strong easterly winds over the region with several inches of new snow. Fresh wind slab avalanches around a foot deep are likely developing now and should be easy to trigger through the day. Watch for new wind slabs in the mid and lower elevations as well where a MODERATE danger exists. Fresh pieces of cornices could fall today as well as smaller loose snow avalanches. Additionally, there is still a chance a larger avalanche could be triggered in an older weak layer buried 1.5-2.5′ deep.

Special Announcements

National Weather Service:  A Winter Weather Advisory was issued yesterday for blowing snow from Girdwood to Seward and extends until 9am this morning.

Fri, February 17th, 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. 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 are known of in our forecast zone from yesterday. The last avalanches were skier triggered sluffs on Monday and small sluffs during Tuesday’s 4-6+” of snowfall. The last slab avalanche was almost a week ago when a skier triggered a slab around 2′ deep on a lower elevation rollover on Eddie’s Ridge.

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

After a quiet day yesterday, last night the first of two fast moving storm systems moved in. This event looks to be more blow than snow and should peak this morning before moving out this afternoon. Only 2-4″ of snow is expected (~6″ in Portage/Placer). The next pulse should be tomorrow with similar snow amounts. The winds will the big player for changing avalanche conditions however. Overnight, ridgetop winds picked up into the 15-25mph range from the east with gusts in the 40’s, which should remain until midday when winds make a 180 and blow 10-20mph from the west by this evening. That said, wind slabs will be the primary concern.

The upper elevations will be the most likely place for wind slabs to develop. Exposed areas in the treeline and below treeline band are also suspect. These are likely to be around a foot deep and fairly soft. If you are headed out today, watching for where winds are actively loading slopes, or did so earlier in the day, will our best tool to avoid triggering a fresh wind slab. Also, feeling for stiffer snow over softer snow and looking for any cracks that shoot out from you will be key. Expect any fresh wind slab to be touchy, simply by definition. There is also a chance wind slabs could be forming on a new crop of surface hoar, making them even easier to trigger. All in all, the good news is the avalanche concerns now are mainly focused on the top of the snowpack, we call these surface instabilities. They are much easier to see and assess than the hidden buried weak layers we have been dealing with all season.

 

CNFAIC intern Megan Guinn holds up a chunk of soft surface snow from the Valentine’s Day storm with a new crop of surface hoar sitting on top yesterday. 2.16.23.

 

If the video below doesn’t load in your browser, you can click here to view it.

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

There are a couple older layers in the snowpack that we are continuing to monitor. These are the storm interface from Feb 5 (1.5-2.5′ deep) and the 1/10 buried surface hoar (2-5′ deep). With new snow and wind today, a wind slab or other surface avalanche could possibly step down to the Feb 5 interface and create a larger avalanche. Something we’ll be watching for. We have not seen any avalanche activity on the 1/10 buried surface hoar for almost a month, it would be highly unlikely an avalanche would release on this layer at this point.

The last known avalanche breaking in one of these older layers was on Saturday (6 days ago) when a skier triggered a ~2′ deep slab at 1,600′ on a lower steep roll on Eddies’s Ridge. This avalanche likely failed at the Feb 5 storm interface where at these lower elevations (2,000′ and below) the bed surface is a crust formed on 1/25.

 

Our snowpit from the front side (SE face) of Seattle Ridge yesterday. We did not find any weak layers in the top 3 feet of the snowpack. Good news. This includes the Feb 5 storm interface (~2.5′ deep). 2.16.23.

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
Fri, February 17th, 2023

Yesterday:  Overcast skies were over the region yesterday. Ridgetop winds were light and easterly (5-10mph) during the day with temperatures in the teens to 20’sF. Overnight a system moved in increasing easterly winds into the 15-25mph range with gusts in the 40’sF along ridgetops. Snowfall began this morning with around an inch falling as of 6am.

Today:  Moderate snowfall should continue until midday with an additional 1-3″ expected. Ridgetop winds are 15-25mph from the east and forecast to turn westerly this afternoon and decrease to the 10-20mph range. Temperature are in the teens 20’sF at the upper and mid elevations. A break in storms is slated for tonight.

Tomorrow:  A second pulse of snow and wind will be over the area tomorrow. Snow amounts look to be 2-4″ in general with up to 6+” in the Placer/Placer valleys. Ridgetop winds switch back to the east and look to blow in the 20-30mph range with stronger gusts. A break in weather and some clearing skies are on tap for Sunday and Monday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25 1 0.1 70
Summit Lake (1400′) 23 trace trace 36
Alyeska Mid (1700′) 25 1 0.1 7 2
Bear Valley – Portage (132′) 29 3 0.2

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 ENE 15 46
Seattle Ridge (2400′) 19 SE 10 29
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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.