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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, November 21st, 2023 - 7:00AM
Expires
Wed, November 22nd, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′. A slow pattern shift is underway, but the weather should remain quiet during the day today and our main concern is the lingering possibility of triggering a large avalanche on two different layers of weak snow. One is a layer of surface hoar buried about a foot deep, and the other is a layer of facets buried at the ground- now 3-4′ deep. Both of these layers become more problematic at higher elevations and both require careful snowpack assessment before committing to steep terrain. The danger is LOW below 1000′.

ROOF AVALANCHES: Be on the lookout for increased roof avalanche activity as this storm moves in, bringing warmer temperatures and (sadly) rain to lower elevations. Keep an eye on children and pets, be mindful of where you park your vehicles, and avoid walking below loaded roofs whenever possible.

Special Announcements

Chugach State Park: We will begin issuing weekend avalanche outlook summaries for this area starting Dec. 1.

Danger Rating Outlook – This season we’re introducing an avalanche danger ‘outlook’ for the next day. This tool aims to help assess the danger trend and hopefully help plan your outings more effectively.

Tue, November 21st, 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
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
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
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

The most recent human-triggered avalanche in our forecast zone was in the Goldpan area behind Magnum Ridge on Saturday, Nov. 18. This likely failed on a layer of surface hoar that was buried on Nov. 13, and was roughly a foot deep and 50 feet wide. We also continue to receive reports of glide activity from Girdwood to Summit.

Avalanche Problem 1
  • 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

The weather pattern is slowly starting to change, with temperatures climbing since last night and precipitation expected to start to trickle in this afternoon. However, this change is coming on slow, and it is looking like we won’t start to see major impacts until tonight and tomorrow. For now, that means our main concern is the lingering possibility of triggering a large avalanche on one of two buried weak layers. This includes a layer of surface hoar that was buried on Nov. 13 and is roughly a foot deep, and a layer of faceted snow on the ground, now 3-4′ deep. Both layers are more problematic at higher elevations, and both are becoming more stubborn to trigger as we get further out from our last precipitation event- which was now 6 days ago.

The two layers have similar, but slightly different distribution. We are fairly certain that the layer of facets on the ground exists on most slopes above around 3500′ elevation, and is weakest in steep, rocky terrain. The Nov. 13 surface hoar layer is a bit trickier. Despite being widespread before it got buried, it seems to only be reactive on isolated slopes for now. It was likely the culprit for an avalanche in Goldpan just three days ago, but we have not been able to find it in any of the snowpits we have dug since it got buried. This complicates things if you are considering getting into steep terrain at and above treeline, and it requires diligent snowpack assessment before getting into the steeps. As always, if you want to avoid the problem you can simply stick to gentler slopes.

The older snow that was buried by the Nov. 9 Superstorm was easy to spot in this snowpit in PMS bowl on Sunday. 11.19.2023

Predicted snowfall totals for today (top) and tomorrow (bottom). Let’s hope that rain line stays low! Graphic courtesy of NWS Anchorage. 11.21.2023

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

We continue to see glide avalanche activity from Girdwood to Summit Lake. Glide avalanches are very large and difficult to predict, so be sure to limit the amount of time you spend traveling under glide cracks.

Weather
Tue, November 21st, 2023

Yesterday: Temperatures remained cold during the day yesterday, hovering in the single digits above and below 0 F. Skies were mostly clear, with scattered clouds building through the day and valley fog hanging around in lower elevations. Winds were light out of the north in Girdwood and Turnagain Pass, with stronger winds of 10-25 mph and gusting close to 40 mph in the Summit and Chugach State park areas. No measurable precipitation was recorded.

Today: A shift is underway as temperatures have risen to the high teens to mid 20’s F this morning, and will continue to climb into the mid 20’s to 30 F by this evening. Skies will be mostly cloudy, and chances for precipitation begin to pick up this afternoon, with only a trace expected during the day. This should start out with snow to sea level, but the rain line will start to climb as precipitation picks up tonight into tomorrow. Winds should stay light out of the east at 5-10 mph with gusts of 10-15 mph during the day and through tonight. Overnight temperatures should drop slightly to the low to mid 20’s F.

Tomorrow: Weather will pick up tomorrow, with 8-10” snow at upper elevations in Portage and Placer, 3-4” in Girdwood,Turnagain Pass, and Seward, 1-2” in Summit, and only a trace in Chugach State Park. The rain line will continue to creep up as the storm progresses, making it up to around 2000’ during the day and reaching as high as 3000-4000’ as precipitation continues Wednesday night into Thanksgiving day. Strong winds will ramp up through the day, with sustained easterly winds at 30 mph and gusts to 40 mph by tomorrow afternoon. High temperatures should be in the upper 20’s to low 30’s F, with overnight temperatures hovering around 30 F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 12 0 0 31
Summit Lake (1400′) 4 0 0 21
Alyeska Mid (1700′) 14 0 0 30
Bear Valley – Portage (132′) 5 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 NE 8 20
Seattle Ridge (2400′) 17 var 3 10
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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.