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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, December 6th, 2022 - 7:00AM
Expires
Wed, December 7th, 2022 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 2500′ today as another round of light snow and moderate winds  will make it possible to trigger a wind slab avalanche 6-12″ deep.  The most dangerous terrain will be steep slopes below ridgelines, convexities, and wind-loaded gullies. The danger is expected to remain LOW below 2500′. Be on the lookout for changing conditions, and anticipate increasing danger as the storm develops later in the day through tonight.

Special Announcements

Forecaster Chat #1: Come join us at Alaska Mountaineering and Hiking on Thursday, December 15th for our first forecaster chat with John Sykes. Admission is free! Start time roughly 7pm, exact time TBD. The forecaster chat will focus on how to streamline submitting high quality observations and a discussion on decision-making in unusual conditions.

Tue, December 6th, 2022
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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 last known avalanches were small wind slab avalanches that released 10 days ago during the 11/25-26 NW outflow wind event.

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

Another round of snow is on the way today, which will hopefully bring 1-2″ during the day and 8-10″ to the area by tomorrow morning. It is looking like precipitation will start to pick up later in the afternoon, with the heaviest snowfall this evening and tonight. Southeast winds are expected to increase as the precipitation starts, with average speeds of 10-15 mph and gusts of 25-30 mph by this afternoon. For today, this makes fresh wind slab avalanches the main concern. Over the past week, we have seen a healthy variety of surfaces, including facets, surface hoar, crusts, and firm wind slabs. As the weather picks up today, it will be possible for a person to trigger an avalanche 6-12″ deep where the wind is building fresh slabs on these old surfaces.

The most concerning terrain will be steep, higher elevation terrain below ridgelines, convexities, and in gullies. Safe travel will require identifying and avoiding slopes with stiffer wind slabs on the surface. You can quickly identify these as you travel by taking a minute to step off the skin track and poke around. If you notice that stiff snow sitting on top of soft snow, or any warning signs like shooting cracks, collapsing, or fresh avalanche activity, head to protected terrain where the wind isn’t building fresh slabs. That’s most likely where the best skiing will be found too.

Andy Moderow identifying a buried crust, with about 3″ of facets on the snow surface. That soft snow that has been faceting on the surface for the past week will be a layer of concern once it gets buried by the approaching storm. Photo: Peter Wadsworth, 12.04.2022

Fingers crossed we end up on the high end of that forecast! Graphic courtesy of NWS Anchorage.

Looks like I’m not the only one that was excited to see there was snow down to sea level at the Portage side of the Whittier tunnel last night. Photo from the Bear Valley (Portage) webcam, 12.05.2022

Weather
Tue, December 6th, 2022

Yesterday: We ended up on the low end of the snow forecast yesterday, picking up only a trace to 2″ in the past 24 hours. Winds were 5-15 mph out of the south and east, with gusts of 15-20 mph. Skies were mostly cloudy and high temperatures were in the low to mid 30’s F. Most of the weather stations were showing the coldest temperatures yesterday morning, with temperatures in the 20’s F.

Today: Skies will be mostly cloudy, with chances for snow increasing later in the day. We should get 1-2″ snow during the day, with precipitation picking up as the sun goes down. Easterly winds are expected to increase to 10-15 mph with gusts of 25-30 mph by this afternoon. High temperatures should hover in the low 30’s F, with lows dropping into the 20’s tonight. The snow line is expected to drop to sea level as the storm picks up.

Tomorrow: Snowfall will slowly taper off during the day tomorrow, with another 6-10″ expected tonight. Unfortunately the northwest winds are expected to pick up as this low pressure moves out of the area, with average speeds around 15-20 mph and gusts around 30 mph by tomorrow afternoon. Skies will be mostly cloudy and temperatures will continue to drop into the high teens to low 20’s.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31 1 0.1 24
Summit Lake (1400′) 27 2 0.2 16
Alyeska Mid (1700′) 31 1 0.15 19
Bear Valley (132′) 28 tr 0.07 –

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27 E 5 20
Seattle Ridge (2400′) 27 ESE 9 15
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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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