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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 26th, 2024 - 7:00AM
Expires
Tue, February 27th, 2024 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger will be CONSIDERABLE today and could increase to HIGH overnight as a storm brings between 1 to 2 feet of new snow during the nighttime hours. In advance of the heavy snowfall today, strong winds and a few inches of snow should create new wind slabs in the higher elevations. These could release naturally and be easily triggered by a person.

It’s one of those days to pay close attention to changing weather. Be sure to get tomorrow’s forecast as dangerous human-triggered avalanche conditions are likely to exist.

 

Special Announcements

Winter Weather Advisory:  The National Weather service as issued a Winter Weather Advisory for Turnagain Pass from noon today (Monday) through 6am tomorrow (Tuesday). Screenshot of the NWS advisory regarding Seward Highway road conditions:

Mon, February 26th, 2024
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
Tue, February 27th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Tue, February 27th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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 were no known human triggered avalanches during yesterday’s clear skies. There were also no known natural avalanches from the winds that were affecting many upper elevations.

The Seward and Lost Lake area, south of the forecast zone, did see natural avalanches with the outflow winds.

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 sunny day yesterday, another storm is moving in. This one looks to be a quick hitter; ramping up through today, peaking overnight, and exiting tomorrow. With such cold air in place and a relatively cool storm, snow is expected to fall to sea level with snow densities being 5-8% (light powder snow). Snow should start falling around noon with 1-2+’ expected by tomorrow. Ridgetop easterly winds will be increasing through today and by tonight averaging 25-35 mph with gusts 50-70 mph.

Today:  Before the snow begins to really accumulate, wind slab avalanches will again be the primary concern. By sunset (6:14pm) only a few inches of new snow is expected, but ridgetop winds should be enough to form fresh wind slabs that could release naturally along the higher peaks and exposed areas in the mid elevations. Paying close attention to past and current wind effect will be key for those getting out. Watch for active loading onto slopes as well as surface clues such as stiffer snow over softer snow and cracks that shoot out from you.

Visibility is likely to be hampered today, so sticking to the sheltered mid elevation treed terrain with nothing steep above us will be the safer bet.

Tonight:  Dangerous avalanche conditions expected due to heavy snowfall and strong easterly winds. Natural avalanches will be likely anywhere over a foot of snow falls in a short amount of time. These avalanches could run further than expected.

Tomorrow:  Check tomorrow’s forecast! A lot of new snow overnight + clearing skies during the day = easily triggered avalanches. If skies open up and the sun comes out there could be natural avalanches due to solar warming.

 

The NWS’s snowfall forecast from today through 3am tomorrow morning. Add another few inches to these numbers as light snowfall should extend after 3am into Tuesday. Find these graphics HERE.

Snowfall totals expected by the end of the storm tomorrow.
Girdwood area:  8-12″
Portage and Placer Valleys:  20-30″
Turnagain Pass:  14-24″
Summit Pass:  5-8″
Seward area:  8-12″

Finally some sunny skies for folks to get out and enjoy the mountains. Note the strong winds along Carpathian Pk in the back of the photo. Thanks to Graham Predeger for this image from Skookum drainage 2.25.24. 

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 is a layer of old facets (formed in January) buried 1.5-3′ deep. This layer is still on our radar because it could produce a dangerous avalanche with no previous signs of instability. For the most part the weak layer does not appear to be as unstable in the Turnagain Pass area as it is in Summit lake. The snowfall event tonight should test the layer somewhat so we’ll be watching for any new snow avalanches that ‘step down’ and break at the faceted interface. Elevation bands that are most suspect are between 2,000 and 3,000′ in the Summit Pass area that sits to the south of the Turnagain Pass forecast zone.

Weather
Mon, February 26th, 2024

Yesterday:  Sunny skies, strong ridgetop winds, and chilly temperatures were over the region yesterday. Northwesterly outflow winds were strongest in the first part of the day, averaging 15-25 mph with gusts near 50mph. Temperatures were generally in the teens F and rose to the 20s F in the lower/mid elevations with direct sun.

Today:  A quick hitting storm is approaching that will peak tonight. Heavy snowfall is expected to begin around noon and by tomorrow morning  1-2′ of new snow is expected. Snow should fall to sea level with cool temperatures associated with the storm. Ridgetop winds are easterly and will pick up through the day peaking tonight with averages 25-35 mph gusting 50-70 mph.

Tomorrow:  The storm should be tapering off tomorrow morning and skies could break up by the afternoon. Another 2-6″ of snowfall is expected from 3am tomorrow till around noon. Ridgetop winds should decrease dramatically through the day and become light and variable by the evening. Clearing skies and moderate northwest winds are expected for Wednesday and into Thursday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 15 0 0 89
Summit Lake (1400′) 11 0 0 48
Alyeska Mid (1700′) 17 0 0 94
Bear Valley – Portage (132′) 19 0 0 –
Grouse Ck (700′) 15 0 0 69

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8 NW 8 28
Seattle Ridge (2400′) 13 NW 5 20
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.
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