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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, November 22nd, 2023 - 7:00AM
Expires
Thu, November 23rd, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

Today’s avalanche danger is CONSIDERABLE above 2500′. Precipitation and strong winds starting this morning will increase the likelihood of natural and human triggered avalanches. Wind slabs 1-2′ deep along ridgelines and convex features in exposed terrain are likely. At higher elevations larger persistent avalanches 3-4′ deep releasing on a layer of weak snow at the ground are possible.  Below 2500′ the avalanche danger is MODERATE. Wind slabs and wet loose avalanches are possible.

Roof Avalanches: As the snowfall turns to liquid today roof avalanches will become a significant hazard. After the early November superstorm there are a lot of heavily laden roofs in Girdwood and elsewhere that could drop their snow load today. Be cautious about where you park, walk, and watch for dogs or kids that unknowingly stray under snow laden eaves.

Seward Area: The SE flow direction of this storm system tends to favor the Seward area, so higher precipitation and snow totals are expected there. Avalanche conditions will become more dangerous rapidly as winds and precipitation intensity increase.

Wed, November 22nd, 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
Thu, November 23rd, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Thu, November 23rd, 2023
Alpine
Above 2,500'
2 - Moderate
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 avalanches were reported yesterday. The most recent avalanche activity was a human triggered avalanche in Goldpan Bowl on the skiers side of Turnagain pass on Nov 18th. During the cold snap over the weekend there were also several glide avalanches that released in Girdwood Valley, Turnagain Pass, and Summit Lake area.

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

Temperatures and wind speeds are on the rise today as a storm system starts to impacts our forecast region this morning. We are expecting about 0.5″ of water or 4-6″ of snowfall today, with rain line creeping up to about 2000′ throughout the day. In coastal areas like Portage the precipitation amounts are expected to be doubled, with about 1″ of water and 10-12″ of snowfall at upper elevations. Strong winds averaging 30-50 mph with gusts of 60+mph will accompany the precipitation.

Wind slabs will be the primary concern today. In addition to the new snowfall, there is plenty of soft snow left on the surface for winds to transport into fresh wind slabs 1-2′ deep. You are most likely to find these along ridgelines and convex features near treeline and above. Using small test slopes and stepping of the beaten path to feel the snow surface are the best way to see how reactive wind slabs are before entering more consequential terrain.

As the temperatures increase and precipitation turns liquid at lower elevations wet loose avalanches are also likely. These tend to be relatively small and are unlikely to bury a person, but can push you off balance in steeper terrain.

Snowfall totals from Wednesday at 3am through Thursday at 3am. Graphic from NWS Anchorage

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

At upper elevations (above ~3500′) there is a layer of weak faceted snow at the ground which caused a large human triggered avalanche earlier this month in Goldpan bowl. The incoming storm system will add some stress to this persistent weak layer which will make it more likely for a person to trigger a large 3-4′ deep avalanche. Given the weather and visibility today you would need to be a brave soul to venture into this elevation band, but if you do it is worthwhile to carefully evaluate the snowpack before committing to steeper terrain. To avoid this problem we recommend sticking to lower elevation terrain or choosing lower angle slopes.

Venturing into alpine terrain yesterday we found highly variable snow depths with thin areas harboring weak snow at the ground.

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Glide avalanches have been releasing naturally over the past week. These avalanches are very destructive and unpredictable, so we recommend avoiding spending time underneath glide cracks.

Weather
Wed, November 22nd, 2023

Yesterday: Weather was mostly calm yesterday, with light winds averaging 5 mph and gusting to 15 mph. Light snow showers throughout the day lead to 0.1″ SWE and about 1″ of snowfall. Cloud cover ranged from obscured to broken and temperatures were in the mid to upper 20s.

Today: Things are going to get a little wet and wild later today, with rain line expected to increase to 2000′ and strong SE winds averaging 30-50 mph with gusts of 60+ mph. About 0.5″ of SWE is expected during the day today, which should be roughly 4-6″ of snowfall above 2000′. In more coastal areas near Portage and Whittier about 1″ of SWE is expected which translates to roughly 10-12″ of snow at upper elevations. Temperatures will remain around 32 F overnight with light precipitation continuing.

Tomorrow: Warm temperatures and light precipitation will continue on Thanksgiving, with about 0.2″ of SWE which should be 2-4 inches of snowfall at upper elevations. Wind speeds will decrease tomorrow to averages of 10-20 mph out of the south with gusts up to 30 mph.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 1 0.1 31
Summit Lake (1400′) 20 0 0 20
Alyeska Mid (1700′) 27 1 0.1 28
Bear Valley – Portage (132′) 22 1 0.1

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25 ENE 5 17
Seattle Ridge (2400′) 28 SE 4 13
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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.