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Issued
Thu, November 30th, 2023 - 7:00AM
Expires
Fri, December 1st, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger will remain HIGH at the upper elevations (above treeline, ~2,500′) and CONSIDERABLE below due to continued snowfall. New snow avalanches around 1-2 feet deep are expected to occur naturally on slopes in the high elevations and may also occur below 2,500′. At all elevations, it will be likely for a person to trigger an avalanche on slopes steeper than 30 degrees. This is the kind of day to simply avoid traveling in avalanche terrain above treeline. If choosing to travel in the trees, expert level snowpack assessment is recommended.

Special Announcements

Chugach State Park:  Wind slab avalanches have been seen in Anchorage’s Front Range this week. Strong winds occurred in this region again yesterday.

New avalanche outlook products:  Starting this Friday, Dec. 1, we will begin issuing Weekend Avalanche Outlooks for Chugach State Park, the Summit Lake/Central Kenai zone, and the Seward/Southern Kenai zone. These products will be published at 5pm on Fridays and will provide avalanche information for these zones.

Thu, November 30th, 2023
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Fri, December 1st, 2023
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
Fri, December 1st, 2023
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

We do not yet know the extent of avalanche activity associated with the storm that began late Tuesday night (11/28) and peaked yesterday. We can bet there were widespread natural avalanches in the mountains. How big they were and where is still to be determined once skies begin to clear.

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

The storm that pounded Turnagain Pass yesterday persisted overnight and looks to finally move out this afternoon. Anywhere from 4″ to 2′ of snow has fallen so far with another 2-8″ expected through today. There was a high degree of variability in new snow depths across the area, see the snowfall totals below. Ridgetop winds have backed off this morning yet should still blow 10-20mph with gusts near 30 from the east.

Snowfall totals (estimated at the mid-elevations, around treeline)

Turnagain Pass:  20-22″ (1.8″ water equivalent)
Portage Valley:  6-8″ (0.72 water equivalent)
Girdwood Valley: 6-10″ (0.63 water equivalent)
Summit Lake:  2-3″ (0.2 water equivalent)

With all this new snow and wind there are two main types of avalanches expected. These are storm slabs (slab avalanches composed of the new snow in areas that are sheltered from the winds) and wind slabs (slab avalanches in places the winds have deposited snow).

Storm Slabs – in the trees and elsewhere:  The forecast shows a rise in temperature today that could bring the rain/snow line to 1,000′ (the road level at Turnagain Pass). If this warm up occurs it could keep the new snow unstable by creating denser snow over lighter snow, especially in treed areas. We don’t know how the new is stabilizing yet. Therefore, being extra careful of where we are, and knowing if we are venturing into avalanche terrain, will be key to avoiding a run-in with an avalanche. This is the kind of day we recommend to be conservative and not poke the dragon. Watching for recent avalanches, shooting cracks, and heavier snow over lighter snow will be a sign the snowpack is unstable and to stick to the lower angle slopes. 

Wind Slabs – above treeline:  Although the easterly winds have died down, they are still blowing, and forecast to continue, 10-20mph with gusts near 30mph along the ridgelines. At the higher elevations we can expect naturally occurring wind slabs to keep releasing through today, making them very easy to trigger by a person. With low visibility, wind, snowfall, and the threat of natural avalanches, we don’t recommend traveling in the higher terrain.

 

Snowfall totals for today. Note that the graphic is a 24-hour total from 3am today until 3am tomorrow. We’ve already seen a few inches fall between 3-6am. These are produced by the NWS and found HERE. 

 

Turnagain Pass at 5am this morning. The Pass looks snowy again! Image from the RWIS webcams found HERE.

 

Weather
Thu, November 30th, 2023

Yesterday:  Stormy weather was over the region yesterday with rain up to 500-700′ at times and heavy snow above this. Between 0.5-2″ of water equivalent was recorded (4-24″ of snow depending on location). Turnagain Pass was favored with around 20″ of snow. Easterly ridgetop winds gusted in the 50’smph with sustained winds 20-30mph.

Today:  Snow showers are forecast to continue through today with warming temperatures. The rain/snow line may rise to 1,000′ midday before cooling off tonight. Easterly ridgetop winds should average 10-20mph with gusts near 30 through today before turning southerly tonight.

Tomorrow:  A break in weather is slated for tomorrow, Friday. Skies should begin clearing in the morning with no precipitation expected. Winds look to be moderate along the ridgelines from the east (10-20mph).

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 20 1.8 48
Summit Lake (1400′) 31 2 0.2 22
Alyeska Mid (1700′) 31 6 0.63 32
Bear Valley – Portage (132′) 34 rain 0.72

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 NE 20 54
Seattle Ridge (2400′) 25 SE 12* 24*

*Seattle Ridge weather station anemometer (wind sensor) is rimed and therefore wind speeds notes are only from  yesterday morning.

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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.