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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, December 25th, 2022 - 7:00AM
Expires
Mon, December 26th, 2022 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Avalanche danger is expected to rise to CONSIDERABLE above 1000′ today as strong easterly winds pick up ahead of an approaching storm. It will be likely a person can trigger wind slab avalanches up to a foot deep, and it is possible some of these avalanches could step down to deeper weak layers buried 2-4′ deep.

The danger is MODERATE below 1000′. Avalanche danger is expected to continue to rise this evening into tomorrow as heavy snowfall begins.

 

*Roof Avalanches: With warming temperatures and a good chance of rain in Girdwood tonight, it will become likely that we will see some roof avalanches. Keep an eye on kids and pets playing outside, and be mindful of where you park your vehicles.

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Sun, December 25th, 2022
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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

There was a natural avalanche cycle with the Northeast winds that reached into the Girdwood Valley on Friday. Prior to that, the last known avalanches were following the last significant storm- which was 10 days ago.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • 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.
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

 

Dashing through the snow on skis and snow machines
Over the hills we go, assessing snowpack things

Winds will start to blow, and make conditions ripe
Fresh wind slabs on weak facets, an avalanche in sight!

Oh, warning signs, big red lights, are on the way today
Weak snow mid pack is kind of whack, there’s not much more to say

Winds will blow, then lots of snow, danger’s up today
What fun it is to ski and ride in low angle terrain!

 

It’s looking like our forecast area is going to get a solid storm event for Christmas, starting with strong winds during the day and heavy precipitation starting this evening through tomorrow. Easterly winds are expected to blow 15-25 mph with gusts of 30-40 mph today, and although we should only see light snowfall during the day, avalanche danger will be on the rise. Fresh wind slabs will be forming on top of weak, faceted snow, making human triggered avalanches likely and natural avalanches possible. The most likely places to trigger an avalanche will be in steep terrain just below ridgelines, in gullies, and on convex rolls. These fresh wind slabs will often give warning signs of unstable snow like shooting cracks and collapsing. Keep in mind, an avalanche failing near the surface has the possibility of stepping down to trigger a larger avalanche on weak layers buried deeper in the snowpack (see problem 2 below for more on this).

If you plan on getting out to enjoy the holidays, be aware of increasing danger through today into tomorrow. For today, the best way to stay out of trouble will be to stick to lower angle terrain, and find slopes that are sheltered from the winds. Isn’t it nice when the most stable slopes also happen to have the best conditions for riding and skiing?

The current surfaces that are about to be buried are likely to produce avalanches. This snowpit in the Crow Creek drainage shows some of the weakest snow in the advisory area, but there is faceted snow on the surface and in the middle of the snowpack across our advisory area. 12.24.2022

She’s a beaut Clark! Active weather is on the way, and avalanche danger will be on the rise starting today. Snowfall graphic courtesy of NWS Anchorage. 12.25.2022

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

For the past week, we have been saying that the weak layers buried in the middle of the snowpack are going to be a major concern when we get a significant loading event. Well, that loading event begins today. Wind slabs forming on top of these weak layers will start to add stress today, and the likelihood of triggering a bigger avalanche will really start to rise once the precipitation picks up this evening. This will be the first loading event the snowpack has seen since the last big cycle 10 days ago, and it will be a good test to see how these layers have evolved. Based on what we’ve seen in our most problematic parts of the forecast zone (Crow Creek, Notch, Lynx Creek), it is looking like we will see some bigger avalanches as this storm unfolds.

Click here to view the video below if it does not load in your browser.

Weather
Sun, December 25th, 2022

Yesterday: Temperatures bottomed out in the single digits below 0 F yesterday morning, before slowly starting to climb through the day. Skies were mostly sunny with light northeasterly winds and no measurable precipitation.

Today: Easterly winds are exepected to pick up ahead of the incoming storm, with sustained speeds of 15-25 mph and gusts of 30-40. It is looking like there may be a brief lull in the wind this afternoon, before they ramp up even stronger overnight. Temperatures are already on the rise, with weather stations showing temps in the mid 20’s F– a warming trend of 20-25 degrees F over the past 12 hours. We might see an inch or two of snow during the day, but precipitation is expected to pick up tonight, bringing 0.8-1″ of snow water equivalent (SWE) by tomorrow morning. Precipitation will favor Turnagain Pass compared to Girdwood, and totals will be closer to 1-1.5″ for Portage and Placer. The rain level is expected to rise up to around 1900′ tonight before dropping back down to 1000′ tomorrow, with temperatures hovering in the mid to upper 20’s F.

Tomorrow: Winds will peak early tomorrow morning, with sustained speeds expected to reach 30-45 mph and gusts of 40-60 mph. Heavy precipitation will continue through the day tomorrow, with another 0.8-1″ SWE expected for Girdwood and Turnagain Pass, and another 1-1.5″ expected for Portage and Placer. Temperatures should stay in the upper 20’s to low 30’s F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 14 0 0 39
Summit Lake (1400′) 4 0 0 N/A
Alyeska Mid (1700′) 14 tr tr 39
Bear Valley – Portage (132′) 2 tr tr –

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 ENE 10 44
Seattle Ridge (2400′) 13 ESE 4 20
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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.
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